Vitamin Metabolism - an overview (2023)

Metabolism of Vitamin D

Vitamin D is not a true vitamin, because nutritional supplementation is not required in humans who have adequate sun exposure. When exposed to ultraviolet irradiation, the cutaneous precursorof vitamin D, 7-dehydrocholesterol, undergoes photochemical cleavage of the carbon bond between carbons 9 and 10 of the steroid ring (Fig. 29.15). The resultant product, previtamin D, is thermally labile and over a period of 48 hours undergoes a temperature-dependent molecular rearrangement that results in the production of vitamin D. Alternatively, this thermally labile product can isomerize to two biologically inert products, luminosterol and tachysterol. This alternative photoisomerization prevents production of excessive amounts of vitamin D with prolonged sun exposure. The degree of skin pigmentation, which increases in response to solar exposure, also regulates the conversion of 7-dehydrocholesterol to vitamin D by blocking the penetration of ultraviolet rays.

The alternative source of vitamin D is dietary. The elderly, the institutionalized, and those living in northern climates likely obtain most of their vitamin D from dietary sources. However, with increasing avoidance of sun exposure by the general population, ensuring adequate dietary intake of vitamin D has become important for the population at large. Vitamin D deficiency is prevalent and has been shown to contribute significantly to osteopenia and fracture risk. The major dietary sources of vitamin D are fortified dairy products, although the lack of monitoring of this supplementation results in marked variation in the amount of vitamin D provided.¹⁸³ Other dietary sources include egg yolks, fish oils, and fortified cereal products. Vitamin D provided by plant sources is in the form of vitamin D₂, whereas that provided by animal sources is in the form of vitamin D₃ (seeFig. 29.15). These two forms have equivalent biologic potencies and are activated equally efficiently by the hydroxylases in humans. While vitamin D₃ has been shown to be more effective at increasing 25-hydroxyvitamin D levels,¹⁸⁴ this effect is dependent upon vitamin D–binding protein (VDBP) genotype and concentration.¹⁸⁵

Vitamin D is absorbed into the lymphatics and enters the circulation bound primarily to VDBP, although a fraction of vitamin D circulates bound to albumin. The human VDBP is a 52-KDa α-globulin synthesized in the liver. The protein has a high affinity for 25(OH)D but also binds vitamin D and 1,25(OH)₂D. Approximately 88% of 25(OH)D circulates bound to the VDBP, 0.03% is free, and the rest circulates bound to albumin. In contrast, 85% of the circulating 1,25(OH)₂D₃ binds to the VDBP, 0.4% is free, and the rest binds to albumin. Mice lacking VDBP have increased susceptibility to 1,25(OH)₂D₃ toxicity as well as to dietary vitamin D deficiency.¹⁸⁶ Vitamin D–binding protein polymorphisms have been postulated to be the basis for difference in vitamin D levels in several groups, including African Americans and Finns.¹⁸⁷ Measurements of free 25(OH) vitamin D across multiple populations, however, suggest that the monoclonal antibody used in the former study can bring misleading results.¹⁸⁸

Introduction

The word vitamin is used to describe any of a heterogeneous group of organic molecules that are needed in small quantities for normal growth, reproduction, and homeostasis, but that the human body is unable to synthesize in adequate amounts. The group includes the fat-soluble vitamins (A, D, E, and K) and the water-soluble vitamins (B-complex and C). Vitamins are generally needed in catalytic quantities and do not function as structural elements in the cell. Other organic compounds are not synthesized in the body but are required for maintenance of normal metabolism, such as essential fatty acids (Chapter 16) and essential amino acids (Chapter 3). These substances are needed in relatively large quantities, serve as nonregenerated substrates in metabolic reactions, and are used primarily as structural components in lipids and proteins. Vitamins discussed in other chapters include vitamin D (Chapter 35) and vitamin K (Chapter 34). All the B vitamins function as cofactors or precursors for cofactors in enzyme-catalyzed reactions and are discussed in appropriate chapters. Additional properties and less-well-defined actions are reviewed here.

Vitamin deficiency is caused by nutritional inadequacy, or may result from malabsorption, effects of pharmacological agents, and abnormalities of vitamin metabolism or utilization in the metabolic pathways. Thus, in biliary obstruction or pancreatic disease, the fat-soluble vitamins are poorly absorbed despite adequate dietary intake, because of steatorrhea. Absorption, transport, activation, and utilization of vitamins require the participation of enzymes or other proteins whose synthesis is under genetic control. Dysfunction or absence of one of these proteins can produce a disease that is clinically indistinguishable from one caused by dietary deficiency. In vitamin-dependent or vitamin-responsive disorders, use of pharmacological doses of the vitamin can overcome the blockage sufficiently for normal function to occur.

Vitamin deficiency can result from treatment with certain drugs. Thus, destruction of intestinal microorganisms by antibiotic therapy can produce symptoms of vitamin K deficiency. Isoniazid, used to treat tuberculosis, is a competitive inhibitor of pyridoxal kinase, which is needed to produce pyridoxal phosphate. Isoniazid can produce symptoms of pyridoxine deficiency. To prevent this, pyridoxine is often incorporated into isoniazid tablets. Methotrexate and related folate antagonists act by competitively inhibiting dihydrofolate reductase (Chapter 25).

Excessive intake of vitamins A and D produces hypervitaminosis. Vitamin D toxicosis was discussed in Chapter 35; vitamin A toxicosis is discussed later in this chapter. The toxicity of high doses of vitamin B₆ is also covered later in the chapter.

Physiological age-related changes in the elderly can affect nutritional status. Decreased active intestinal transport and atrophic gastritis impair the absorption of vitamins and other nutrients. Reduced exposure to sunlight can lead to decreased vitamin D synthesis. Many drugs may impair both appetite and absorption of nutrients. Some examples of unfavorable drug–nutrient interactions are drugs that inhibit stomach acid production (e.g., omeprazole); drugs that reduce vitamin B₁₂ absorption; anticonvulsant drugs (e.g., barbiturates, phenytoin, primidone) that act by inducing hepatic microsomal enzymes which accelerate inactivation of vitamin D metabolites and aggravate osteoporosis (Chapter 35); interference with folate metabolism by antifolate drugs (methotrexate) used in the treatment of some neoplastic diseases; and vitamin B₆ metabolism affected by isoniazid, hydralazine, and D-penicillamine. Examples of negative impacts of vitamins on drug action are vitamin B₆-dependent action of peripheral conversion of L-3,4-dihydroxyphenylalanine (L-dopa) to L-dopamine, which is mediated by aromatic L-amino acid decarboxylase and prevents L-dopa’s transport across the blood–brain barrier; also, ingestion of large amounts of vitamin K-rich foods or supplements, and action of warfarin on anticoagulation (Chapter 34). L-dopa is the metabolic precursor of L-dopamine and is used in the treatment of Parkinson’s disease (Chapter 30). Thus, L-dopa is administered along with a peripherally acting inhibitor of aromatic L-amino acid decarboxylase (e.g., carbidopa).

Vitamin D Metabolism

Vitamin D has become a focus of nutritional interest because of its importance not only as a major determinant of bone health but also as an important factor in other biologic processes.⁴ Vitamin D exists in two forms: vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol). Vitamin D₃, the naturally occurring form, is a normal by-product of cholesterol synthesis when 7-dehydrocholesterol is exposed to ultraviolet light. This process depends on exposure to sunshine and thus is subject to limitations. In northern climates or in individuals who are not exposed to 30 minutes per day of direct sunlight, vitamin D synthesis will likely be inadequate to sustain metabolic needs. Such individuals would include large sections of the adult population, who work indoors under fluorescent lighting.

After synthesis in the skin, vitamin D₃ is converted to 25-hydroxyvitamin D, which is abbreviated 25(OH)D, in the liver. This substance is then subsequently converted into 1,25-dihydroxyvitamin D—abbreviated 1,25(OH)₂D—which is the biologically active metabolite. Normal production of 1,25(OH)₂ D is stimulated by low circulating serum of calcium or phosphate or by increased PTH levels. This rise in 1,25(OH)₂D increases calcium absorption from the intestinal tract, thus raising serum calcium levels. In addition, 1,25(OH)₂ D increases reabsorption in the distal tubules of the kidney, again resulting in increased serum calcium levels.⁵

Inadequate vitamin D levels during skeletal development result in osteomalacia or rickets. In the skeletally mature individual, vitamin D deficiency is associated with osteoporosis. This deficient state is a common cause of osteoporosis in adults. For those with severe deficiency, 50,000 IU weekly of vitamin D for 3 months may be given.⁴ Otherwise, 1000 IU daily should be sufficient.⁵ The effect of vitamin D and calcium supplementation on bone is to substantially reduce the risk of osteoporotic fracture by up to 40%.⁴ These effects, which are noted only in the elderly or postmenopausal patient, are associated with an increase in bone mineral density of 1% to 2%. For the surgeon commonly treating elderly patients, knowledge of the patient's bone health is critical for optimizing outcomes, particularly in patients undergoing spinal fusion procedures. When a patient's candidacy for elective spinal fusion is being considered, procedures determining the patient's vitamin D [1,25(OH)₂D] level should be considered as part of a thorough preoperative work-up. Changes in bone density occur slowly, even with aggressive supplementation therapy, a fact that should be kept in mind by the physician or surgeon treating an osteoporotic patient who may be a candidate for surgery.

Drug-induced vitamin deficiencies

Vitamin metabolism and requirements in patients with renal disease and renal failure are described in Chapter 25, Vitamin D in Kidney Disease, and Chapter 26, Vitamin Metabolism and Requirements in Chronic Kidney Disease and Kidney Failure. Current guidelines do not recommend the routine prescription of micronutrient supplementation to patients with CKD1–5D albeit this does not preclude the need for vitamin supplementation in some patients [28]. Thiamine deficiency may be aggravated or caused by chronic alcoholism. The literature, at present, does not suggest that specific short- or long-term drug therapies may cause vitamin B₁ deficiency. However, thiamine deficiency may occur in severely ill patients who undergo parenteral nutrition [29]. Thiamine is a coenzyme for pyruvate dehydrogenase, and thiamine deficiency may cause the acute onset of unexplained, severe lactic acidosis [29]. Riboflavin deficiency can be caused or aggravated by long-term administration of chlorpromazine or amitriptyline. Pyridoxine deficiency may be caused by long-term treatment with isoniazid. It is recommended that vitamin B₆ supplements (10–15mg/d) be prescribed for the entire period of time that isoniazid is taken. Vitamin B₆ deficiency may also be caused by hydralazine and penicillamine [15]. High doses of pyridoxine hydrochloride reduce the serum levels of anticonvulsants and may reduce the clinical seizure control. Chronic vitamin B₁₂ deficiency may develop during long-term treatment with colchicine, metformin, or cimetidine, other H₂-blockers and proton pump inhibitors, especially if the dietary intake is marginal [30,31].

Several drugs antagonize folic acid and may cause megaloblastic anemia. These include phenytoin, phenobarbital, sulfasalazine, triamterene, trimethoprim, trimetrexate, and methotrexate [32]. On the other hand, folate supplementation may interact with these medicines and reduce their clinical efficacy. Daily dosages of folate of more than 5mg reduce the plasma levels of phenytoin and phenobarbital and may reduce their therapeutic efficacy. A risk for the development of niacin deficiency may exist when treatment with isoniazid is prescribed. During such treatment, concurrent administration of niacin (100mg/d) may be advisable. Retinoids and possibly retinol increase the blood cyclosporine levels [33]. Vitamin A supplements should be avoided in renal patients. In addition to Coumadin, there is a number of drugs that can cause vitamin K deficiency and that may induce or enhance severe bleeding. This has been described particularly with the administration of moxalactam, cefotetan, cefamandole, cefoperazone, and other cephalosporins that contain the methylthiotetrazole side chain. Vitamin K supplements should be administered concurrently with these antibiotics [34]. Weaker antivitamin K effects have been shown with tetracycline and cholestyramine. Ingestion of megadoses of vitamin E can cause vitamin K deficiency and should be avoided.

Drug-induced osteomalacia can be due to chronic intake of anticonvulsants, isoniazid and possibly cimetidine. Anticonvulsant therapy with phenytoin, phenobarbital, or carbamazepine results in reduced levels of 24,25-dihydroxyvitamin D₃, and this may play a role in the anticonvulsant-induced osteomalacia [35]. In patients with CKD, this drug-induced risk for osteomalacia may be additive to the increased risk of renal bone disease. Patients undergoing chronic dialysis therapy are often prescribed 1,25-dihydroxycholecalciferol or analogs. However, in patients with moderate CKD not receiving 1,25-dihydroxycholecalciferol, vitamin D supplements should be given concurrently with the previous drug treatments. Moreover, it is recommended to prescribe vitamin D supplementation (cholecalciferol or ergocalciferol) to maintain normal 25(OH)-vitamin D levels to patients with CKD1–5D [28].

Vitamin Metabolism

Along with the intestine, the liver is responsible for the metabolism of the fat-soluble vitamins A, D, E, and K. These vitamins are obtained exogenously and absorbed in the intestine. Their adequate intestinal absorption is critically dependent on adequate fatty acid micellization, which requires bile acids.

Vitamin A is from the retinoid family and is involved in normal vision, embryonic development, and adult gene regulation. Storage of vitamin A is solely in the liver and occurs in the hepatic stellate cells. Overingestion of vitamin A can result in hepatic toxicity. Vitamin D is involved in calcium and phosphorus homeostasis. One of vitamin D’s activation steps (25-hydroxylation) occurs in the liver. Vitamin E is a potent antioxidant and protects membranes from lipid peroxidation and free radical formation. Finally, vitamin K is a critical cofactor in the posttranslational γ-carboxylation of the hepatically synthesized coagulation factors II, VII, IX, and X, as well as of protein C and protein S, the so-called vitamin K–dependent cofactors. Cholestasis syndromes can result in the inadequate absorption of these vitamins secondary to poor micellization in the intestine. The associated vitamin deficiency syndromes, such as metabolic bone disease (vitamin D deficiency), neurologic disorders (vitamin E deficiency), and coagulopathy (vitamin K deficiency), can subsequently occur.

The liver is also involved in the uptake, storage, and metabolism of a number of water-soluble vitamins, including thiamine, riboflavin, vitamin B₆, vitamin B₁₂, folate, biotin, and pantothenic acid. The liver is responsible for converting some of these water-soluble vitamins to active coenzymes, transforming some to storage metabolites and using some for enterohepatic circulation (e.g., vitamin B₁₂).

Vitamin D

Conflicting guidelines are proposed by different professional organizations regarding the optimal dosing of vitamin D. The AAP Committee on Nutrition issued guidelines establishing the amount of recommended vitamin D as 400 IU/day for infants. The recommendation applies to infants receiving human milk and those who are consuming less than 1 quart of infant formula per day and is based in part on the risk of rickets in exclusively breastfed infants who do not receive supplementation with 400 IU of vitamin D per day. This level of supplementation is sufficient to meet a target plasma 25-hydroxyvitamin D concentration of 50 mmol/L in most infants (Abrams and Committee on Nutrition, 2013; McCarthy et al., 2013). However, the Endocrine Society recommends that infants may require up to 1000 IU/day to meet a target plasma 25-hydroxyvitamin D concentration of 75 mmol/L for nonskeletal health benefits (Nehra et al., 2013).

VITAMIN METABOLISM

Although few measurement data are available concerning infection-induced changes in vitamin metabolism, the consensus is that the use or metabolism of most vitamins is accelerated.^16,155 Scattered reports suggest that infectious diseases in humans may be followed by classic scurvy, beriberi, pellagra, or xerophthalmia.¹²⁹

More recent attention has focused on vitamin A, which previously was termed the anti-infection vitamin. Declining concentrations of plasma vitamin A during episodes of childhood infections are accompanied or perhaps caused by a marked urinary loss of this vitamin.^1,142 Not only does the heightened vitamin A deficiency induced by infection contribute to the subsequent development of ocular and conjunctival pathologies, but also subclinical deficiencies of vitamin A and their associated immunologic dysfunctions can heighten the mortality associated with childhood infections,¹³⁰ as shown most dramatically in measles.¹³⁸ Depressed plasma concentrations of several other vitamins have been reported.¹⁰ In addition to the urinary losses of vitamin A,^1,142 increased excretion of urinary riboflavin and vitamin C may occur in conjunction with negative nitrogen balance.¹⁶

Vitamins are known to participate in metabolic processes activated during host defensive mechanisms.⁸ The rapid synthesis of steroid hormones by the adrenal cortex is accompanied by a decline in the adrenal content of vitamin C. The B group vitamins, vitamin C, and folate all participate in the metabolism of activated phagocytic cells.

Controversy continues to exist over whether massive daily doses of vitamin C can suppress or prevent the common cold and other viral respiratory infections. More than 2 decades ago, the American Academy of Pediatrics Committee on Drugs² failed to find sufficient scientific evidence to support Pauling's claim, but new data subsequently were introduced.⁶³ Concentrations of vitamin C in neutrophils decline during infectious diseases. Vitamin C is recognized for its importance in the locomotive activity of phagocytic cells and its contributions to the immune system.³

Intestinal parasites, such as tapeworms, may take up sufficient vitamin B₁₂ from the succus entericus to diminish vitamin B₁₂ absorption and lead to the development of megaloblastic anemia. The intestinal absorption of fat-soluble vitamins and folate also may be impaired for a time in children with enteric infections or parasitic diseases.⁸³

The antioxidant role of several vitamins (A, C, and E) has been recognized more recently. Antioxidants act to reduce oxidative stress within the body by serving as scavengers of singlet oxygen radicals. The antioxidant functions of several other provitamin-A carotenoids are of still greater importance. They include beta carotene, lycopene (the red pigment found in tomatoes and other brightly colored fruits), lutein (from spinach), and zeoxanthin (from kale and other dark green collard greens). Lycopene is more abundant in plasma than is beta carotene and twice as powerful in quenching free oxygen radicals. Lutein and zeoxanthin are retinal pigments, with lutein being found throughout the retina and zeoxanthin being concentrated in the macula. Antioxidants are thought to protect cell membranes, DNA, and arteries to augment the immune system and the function of natural killer cells and to help prevent heart attacks, macular degeneration, and various cancers, especially prostate cancer. Little is known about the probable role of antioxidants during episodes of infectious diseases of children.

More recent studies on a small sample of adults have suggested that vitamin D is decreased in patients with HIV infection, and that vitamin D levels correlate directly with the CD4 count and inversely with mortality in HIV-infected patients.¹⁵⁴ Vitamin C seems to be decreased in patients with HIV infection, whereas vitamin E concentrations are similar to those in healthy subjects.¹⁴³ Oxidative damage is known to play an important role in inflammation, but the exact mechanism by which vitamin C is involved in the antioxidant host defense is unclear. One study that looked at Klebsiella pneumoniae infection showed that vitamin C does not prevent lipid and protein oxidation. Another study suggests that vitamin C is protective against oxidative stress, at least partly by decreasing lipid peroxidation.⁷² Vitamin C seems to protect against infections in mouse models.⁵²

Liver is affected by iron-induced oxidative stress

Iron-induced injury to the hepatocytes stimulates the Kupffer cells and the endothelial cells to secrete inflammatory cytokines, while the hepatic stellate cells (HSCs) mediate the wound healing mechanism that involves secretion of several profibrogenic cytokines and the ECM. Normally, this is a well-regulated fibrogenic mechanism, but chronic stimulation of this response, as found in iron-excess conditions, such as hereditary hemochromatosis, derails the regulation and leads to pathological liver fibrosis and cirrhosis.⁶¹ In the iron-loaded Kupffer cells and hepatocytes, ROS induces the production of several cytokines, such as tumor necrosis factor-α (TNF-α) that is inflammatory as well as apoptotic.⁶² While AP-1 is involved in the regulation of procollagen,⁵¹ NF-kB not only stimulates apoptosis but also accelerates the pathological progression by stimulating profibrotic and proinflammatory cytokines, such as TNFα and interleukin (IL)-6.²⁴ While these factors sensitize the hepatocytes to lipopolysaccharide and TNFα toxicity, these also activate the resident and infiltrating immune cells and the HSCs. Also, the haemochromatosis patients demonstrate increased hepatic expression of transforming growth factor beta (TGF-β), the most potent profibrogenic cytokine with pleotropic effects that further promotes an inflammatory environment in the liver.⁶³ In rat HSCs, free radicals increase the expression of TGF-β,⁶⁴ which stimulates ROS production in fibroblasts by the activation of NADPH oxidase and the alteration of complex IV in the mitochondrial respiratory chain.^65,66 In addition, ROS can directly induce HSC proliferation and lipid peroxidation causing excessive production of α-smooth muscle actin (SMA) and collagen, along with quantitatively and qualitatively altered ECM proteins. This demonstrates a pivotal of HSCs in the development of liver fibrosis, and its progression to cirrhosis, a stage marked by portal hypertension and distortion of liver architecture.⁵⁰

Thus, excess-iron mediated oxidative stress modulates several processes in the liver. When the hepatic iron reaches 60μmol/g dry weight of liver, the HSCs display early signs of activation, such as αSMA expression, a key event that marks the initiation of liver fibrosis. When the iron levels exceed 250μmol/g dry weight of liver, cirrhosis is inevitable.⁶⁷

In conditions, such as ALD, NAFLD, NASH, low to moderate levels of iron play a significant role in the progression of the underlying disease due to cumulative effects. In ALD, liver damage is directly related to ROS abundance due to alcohol metabolism. Acetaldehyde, an intermediate of alcohol metabolism forms adducts with DNA as well as its derivative MDA to form MDA-acetaldehyde adducts. When these products are recognized by the Kupffer cells, endothelial cells, and HSCs, cytokine production is induced and an inflammatory response is triggered.⁶⁰ Under chronic alcohol exposure, when the cytochrome P450 (CYP2E1) is activated for ethanol degradation, it causes ROS release. Thereby, oxidative stress and inflammation are the main elements that drive the pathogenesis of ALD. To add to the damage, reduction in serum hepcidin levels increases intestinal iron absorption by twofold in the alcoholics,⁶⁸ which further increases oxidative stress and exacerbates the condition. Thus, hereditary hemochromatosis, in combination with excessive alcohol consumption can cause more liver damage than either condition alone^69,70; it accelerates cirrhosis and enhances the predisposition to hepatocellular carcinoma.

In case of NAFLD, the pathogenesis is determined by the disruption in synthesis, beta-oxidation, and export of fatty acids, which leads to free fatty acids in circulation and excessive accumulation of triglycerides in the liver.³⁶ While insulin resistance, visceral obesity, and dyslipidemia are the primary events that initiate the development of NAFLD, a cascade of inflammatory events mediates its progression to NASH and fibrosis. Moreover, increased hepatic iron, as observed in a third of NAFLD patients, potentiates further increase in oxidative stress, alters insulin signaling and lipid metabolism to enhance steatosis and accelerates the progression to NASH. Interestingly, ectopic expression of hepcidin in adipose tissue has been reported in obese individuals.⁷¹ Both, steatosis and iron can separately exert oxidative stress, and when combined, the pathology is exacerbated. Not surprisingly, hereditary hemochromatosis patients with NASH exhibit higher serum alanine transaminase levels and a higher fibrosis grade compared to patients with either condition alone.^72,73

THYROID HORMONES

Thyroid hormones have broad effects on development and metabolism; these effects include changes in oxygen consumption, protein, carbohydrate, lipid, and vitamin metabolism.

The thyroid gland secretes the active thyroid hormones, T₃ and T₄. T₄ is more abundant in the circulation but T₃ is more active in the periphery. T₃ is directly secreted by the thyroid and is also produced by peripheral deiodination of T₄. Peripheral production of T₃ can be inhibited by a variety of drugs, including propylthiouricil (PTU), dexamethasone, propranolol, iodinated contrast material, and amiodarone.

TSH is a glycoprotein hormone secreted by the anterior pituitary. TSH acts on the thyroid gland via cell surface receptors to stimulate cell growth, iodine uptake and organification, and thyroid hormone synthesis and release. Thyrotropin-releasing hormone (TRH) is a tripeptide synthesized in the hypothalamus by neurons in the supraoptic and supraventricular nuclei. TRH reaches the anterior pituitary via the pituitary portal venous system. It acts via cell surface receptors on pituitary TSH-secreting cells and prolactin-secreting cells to cause synthesis and secretion of TSH and prolactin. TRH secretion is pulsatile in its secretion and shows a circadian rhythm causing a TSH peak in the early hours of the morning.

The synthesis and secretion of thyroid hormones by the thyroid gland are regulated by the hypothalamus and pituitary via a feedback loop. TRH stimulates secretion of TSH by the anterior pituitary. TSH acts on the thyroid to stimulate many steps in thyroid hormone synthesis. In turn, thyroid hormones exert negative feedback control over TSH and TRH secretion. High levels of thyroid hormones inhibit TRH and TSH secretion, and low levels stimulate TSH secretion. High levels of thyroid hormones also inhibit the actions of TRH on TSH-secreting cells of the anterior pituitary gland. Acute and chronic disease, dopamine and dopamine agonists, somatostatin, and glucocorticoids also inhibit TSH secretion.

Thyroid hormone synthesis occurs in the colloid of the thyroid follicle and requires multiple steps in its production: the uptake of iodide by active transport, thyroglobulin (TG) biosynthesis, oxidation and binding of iodide to TG, and coupling of two iodotyrosines into iodothyronines. If any of these steps are blocked, thyroid hormone synthesis does not occur. This occurs in genetic disorders called dyshormonogenesis and is one of the less common causes of congenital hypothyroidism.

Thyroid hormones are stored in TG in the colloid of thyroid follicles. The hormones remain bound to TG until secreted. TG is a glycoprotein secreted by the follicular cells. TG serum levels and their increase after TSH stimulation constitute a useful index of the functional state of the gland. TG is absent in athyreosis. It should be absent after treatment for thyroid cancer that requires that all thyroid tissue be ablated. A rise in the TG after thyroid ablation indicates that there is residual thyroid tissue, which is a risk for cancer recurrence.

Thyroid hormones are bound to plasma proteins. The major serum thyroid hormone-binding proteins are thyroid-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin. TBG binds 75% of serum T₄, whereas TBPA and albumin bind only 20% and 5%, respectively. Deficiencies and excesses of thyroid binding proteins produce changes in the values of serum total thyroid hormones, because the assays for total hormones measure both free and bound T₄ and T₃. Measurement of free hormone or an estimate of the binding capacity such as T₃ uptake (T₃U) helps to differentiate between pathologically abnormal thyroid hormone levels and abnormalities in the binding proteins that are variants of normal.

Iodine is essential in the production of thyroid hormone. Dietary sources of iodine include milk, meat, vitamin preparations, some medications such as cough syrups, and iodized salt. The recommended daily intake of iodine is 100 μg for adults and adolescents, 60 μg to 100 μg for children, and 30 to 40 μg for infants less than one year. Although it is rare in North America, iodine deficiency is the leading cause of hypothyroidism worldwide. Its effects are most pronounced if iodine deficiency is present early in life, when it results in intellectual impairment.

3.7 Role and Regulation of CYP2A5, CYP3A, and CYP4 Isozymes in Liver Disease

CYP4 enzyme family members have multiple functions in human biochemistry and physiology through not only the metabolism of potent signaling eicosanoids, but also their functional role in peroxisome-mediated fatty acids oxidation, vitamin, and steroid metabolism. These CYP4A enzymes also play pathophysiological roles in liver disease, hypertension, shock and sepsis, ischemic stroke adrenoleukodystrophy, Refsum disease, Bietti's crystalline dystrophy, and hyperkeratotic skin disease (See chapter “Cytochrome P450 ω-hydroxylases in inflammation and cancer” by Johnson et al.). For instance, CYP4A isozyme can be important in producing ROS and NASH, as shown in mice exposed to MCD, which was shown to elevate CYP2E1 mRNA and activity along with NASH-like inflammation (Chalasani et al., 2003; Weltman et al., 1996). In this case, CYP2E1 may be important in promoting NASH-related inflammatory changes in the pericentral regions. However, MCD-exposed Cyp2e1-null mice still developed NASH with lipid peroxidation, despite the absence of CYP2E1 (Leclercq et al., 2000; Robertson, Leclercq, Farrell, & Robertson, 2001). In this model, CYP4A becomes a major player in producing ROS and NADPH-dependent lipid peroxidation, which can cause liver injury. In fact, treatment with a specific antibody to CYP4A in Cyp2e1-null mice prevented the ROS production and lipid peroxidation, although treatment with the same CYP4A antibody did not prevent lipid peroxidation in wild-type mice. Similar to CYP2E1, CYP4A can metabolize various long-chain fatty acids at ω and ω-1 positions to produce shorter chain fatty acids (Hardwick, 2008). Through uncoupling during its catalytic cycle, CYP4A-mediated metabolism can produce ROS (Hardwick et al., 2013). In addition, it can produce dicarboxylic acids of long-chain fatty acids, which can inhibit the mitochondrial ETC, increased oxidative stress, and toxicity (Hardwick, 2008; Hardwick et al., 2013). One recent study showed that CYP4A, elevated in db/db mice, seems to play a major role in promoting high-fat-induced insulin resistance, ER stress, and apoptosis since inhibition of CYP4A with a specific inhibitor (HET0016) or intravenous injection of a small hairpin RNA specific to CYP4A mRNA efficiently blocked insulin resistance, ER stress, and apoptosis in diabetic db/db mice (Park et al., 2014).

Compared to CYP2E1, there are few studies on the role of human CYP4 family members in either NAFLD or AFLD. In humans with NAFLD, a fourfold increase in CYP4A11, which metabolizes arachidonic acid to 20-hydroxyeicosatetraenoic acid (HETE) (Nakamura et al., 2008), was observed with a slight increase in CYP2E1 during steatosis and a decrease of CYP2E1 in patients with NASH (Fisher et al., 2009). Because vitamin E improves liver histology in patients with NAFLD and that CYP4F2 is the major enzyme metabolizing vitamin E, participants in PIVENS and TONIC clinical trials were genotyped for CYP4F2 variants (V433M and W12G) (Athinarayanan et al., 2014). The results showed a significant decrease in plasma α-tocopherol in patients with CYP4F2 V433M genotype, but CYP4F2 polymorphisms likely play a minor or moderate role in the overall pharmacokinetics of vitamin E used as a therapeutic agent. A recent publication indicated that 20-HETE impairs endothelial insulin signaling by inducing the phosphorylation of IRS-1 (Li, Wong, et al., 2014; Li, Zhao, et al., 2014) with activation of SREBP-1α that induces the expression of mouse hepatic CYP4A genes, possibly leading to increased production of 20-HETE (Horton et al., 2003). These data indicate an important role of CYP4A/CYP4F produced 20-HETE in the regulation of insulin signaling in mice. However, the interplay of CYP4A11 and CYP4F2 P450s in the regulation of the fasting and feeding response in the progression of NAFLD needs further studies to identify the precise role of 20-HETE in insulin resistance and activation of AMPK by cellular stress.

Several reports indicated the differential expression of cytochrome P450 omega-hydroxylase isoforms in the clinic-pathological features of liver cirrhosis and cancer. The human CYP4F2 metabolizes the potent chemotactic eicosanoid leukotriene B4 to 20-hydroxy-leukotriene B4, which has less potent capabilities in recruiting immune cells. The induction of mouse CYP4A during hepatic steatosis along with fatty acid-induced uncoupling of the catalytic cycle can produce ROS. Increased ROS production and decreased levels of 20-hydroxy-leukotriene B4 due to suppressed CYP4F may be an important mechanism for providing the third hit, which promotes the progression of steatosis to steatohepatitis and eventually liver fibrosis, cirrhosis, and hepatocarcinogenesis. Decreased activity of CYP4F2 in the metabolism of arachidonic acid to 20-HETE due to Val433Met (1297C/T) substitution was strongly associated with rapid hepatic cirrhosis development (OR=6.0, CI=0.28, p=0.222) (Vavilin et al., 2013). In contrast, the potent vasoconstrictive 20-HETE, which has strong mitogenic and angiogenic properties, is increased in tumors of liver, brain, kidney, and ovary with increased expression of CYP4A/4F genes compared to those in normal tissues (Alexanian, Miller, Roman, & Sorokin, 2012). Similarly, increased expression of CYP4A11, CYP4F2, and CYP4F3 isoforms were significantly expressed in pancreatic ductal adenocarcinoma (Gandhi et al., 2013), suggesting that 20-HETE, which increases expression of HIF and its downstream target vascular endothelial growth factor (VEGF), promotes blood vessel sprouting and metastasis by activation of metalloproteinases (MMPs) (Yu et al., 2011). Thus, selective inhibitors of 20-HETE synthesis by CYP4 omega hydroxylase have been demonstrated to reduce proliferation, angiogenesis, and invasion in lung, renal, and brain cancers (Edson & Rettie, 2013). Consistently, other reports indicated the utility of selective inhibitors of 20-HETE formation as potential therapeutic agents to inhibit tumor progression. In fact, the administration of HET0016 inhibited both 9L gliosarcoma and U251 glioma cell proliferation and tumor growth in a dose-dependent manner (Guo, Roman, Falck, Edwards, & Scicli, 2005), leading to increased mean survival time of the animals (Guo et al., 2006). Although these results and other reports suggest a promising role of 20-HETE antagonist as a therapeutic agent in the treatment of cancer, the development of isoform-selective antagonist may show increased efficacy without adverse drug reactions that may be present. Many of the presently used antagonists inhibit CYP4-mediated formation of 20-HETE in human microsomes with an IC50 value of less than 100nM (Sato et al., 2001) although various CYP4A/4F isoforms can be differentially inhibited by broad-spectrum pan-CYP4 inhibitors (Miyata et al., 2001; Nakano, Kelly, & Rettie, 2009). These results suggest that careful cautions should be considered when using these pan-CYP4A inhibitors to define the role of 20-HETE CYP4A isoforms in the pathophysiological progression of disease. Thus, future efforts need to focus on the development of selective inducers and inhibitors of specific CYP4 subfamily members, and identification of major CYP4 isoforms in these widely diverse diseases.

Alcohol intake or nonalcoholic molecules can increase the levels of CYP3A and CYP2B, although the degree of their elevation is lower than that of CYP2E1 (Johansson et al., 1988; Niemelä et al., 2000). Since CYP3A is responsible for the metabolism of many drugs, it is likely that metabolic activation of some drugs by CYP3A may be directly related to drug disposition (Yin, Tomlinson, & Chow, 2010) or drug-induced cytotoxicity (Hosomi, Fukami, Iwamura, Nakajima, & Yokoi, 2011; Hosomi et al., 2010), especially after alcohol intake, as reported (Wolf et al., 2007). Examples of fat accumulation and DILI include APAP, isoniazid, valproate, tamoxifene, troglitazone, tacrin, rifampicin, and many others. The reactive metabolites of these drugs may be responsible for stimulating DILI (Jaeschke et al., 2012; Pessayre et al., 2012; Stachlewitz et al., 1997; Yuan & Kaplowitz, 2013). Alternatively, metabolism of these substrates may increase oxidative stress, which can activate the cell-death-associated JNK and/or p38K, leading to mitochondria-dependent apoptosis, as demonstrated with APAP (Bae et al., 2001) and troglitazone (Bae & Song, 2003). Furthermore, the levels of acetaldehyde and lipid peroxidation-protein adducts seem to correlate with the induced levels of CYP3A and CYP2E1 in alcohol or high-fat exposed rats, suggesting an important role of CYP3A in protein adducts formation (Niemelä et al., 1998).

Additive or synergistic interactions between alcohol and smoking can lead to increased hepatotoxicity and carcinogenesis in experimental animal models and human cases (Kuper et al., 2000; Purohit et al., 2013; Seitz & Cho, 2009). Chronic alcohol intake is known to increase the levels of CYP2A5, which can metabolize nicotine, a major ingredient of tobacco (Lu, Zhuge, Wu, & Cederbaum, 2011; Niemelä et al., 2000). In mice, alcohol feeding induces CYP2A5 in a CYP2E1-dependent manner (Lu et al., 2011), possibly through the CYP2E1-ROS-Nrf2 axis (Lu, Zhang, & Cederbaum, 2012). Elevated levels of hepatic CYP2A6 (the human ortholog of the mouse CYP2A5) were also observed in some patients with ALD or cirrhosis than the control, despite the small sample size (Lu et al., 2011). In the mouse model, ethanol-mediated CYP2A5 induction was dependent on the presence of CYP2E1, while ethanol induction of CYP2E1 was not CYP2A5 dependent. Ethanol-mediated CYP2A5 induction was not observed in Cyp2e1-null mice despite ethanol feeding. However, CYP2A5 induction was markedly elevated in the Cyp2e1 knockin mice after treatment with ethanol but not with the dextrose-control. Furthermore, CYP2E1-dependent ROS was needed for CYP2A5 induction through activation of the NRF2 (Lu et al., 2012). Since CYP2A5 can also metabolize many cancer causing agents, such as aflatoxin B1 and nitrosamines, the induction of CYP2A5 in rodents by alcohol (Lu et al., 2011) or HFD (Choi et al., 2013) and CYP2A6 in alcoholic individuals is likely to contribute to increased oxidative stress and hepatic injury in ALD and NALD.