Effects of nicotinic acid on glucose tolerance and glucose incorporation into adipose tissue in hypertriglyceridaemia

Synthetic analogues of 2-oxo acids discriminate metabolic contribution of the 2-oxoglutarate and 2-oxoadipate dehydrogenases in mammalian cells and tissues

The biological significance of the DHTKD1-encoded 2-oxoadipate dehydrogenase (OADH) remains obscure due to its catalytic redundancy with the ubiquitous OGDH-encoded 2-oxoglutarate dehydrogenase (OGDH). In this work, metabolic contributions of OADH and OGDH are discriminated by exposure of cells/tissues with different DHTKD1 expression to the synthesized phosphonate analogues of homologous 2-oxodicarboxylates. The saccharopine pathway intermediates and phosphorylated sugars are abundant when cellular expressions of DHTKD1 and OGDH are comparable, while nicotinate and non-phosphorylated sugars are when DHTKD1 expression is order(s) of magnitude lower than that of OGDH. Using succinyl, glutaryl and adipoyl phosphonates on the enzyme preparations from tissues with varied DHTKD1 expression reveals the contributions of OADH and OGDH to oxidation of 2-oxoadipate and 2-oxoglutarate in vitro. In the phosphonates-treated cells with the high and low DHTKD1 expression, adipate or glutarate, correspondingly, are the most affected metabolites. The marker of fatty acid β-oxidation, adipate, is mostly decreased by the shorter, OGDH-preferring, phosphonate, in agreement with the known OGDH dependence of β-oxidation. The longest, OADH-preferring, phosphonate mostly affects the glutarate level. Coupled decreases in sugars and nicotinate upon the OADH inhibition link the perturbation in glucose homeostasis, known in OADH mutants, to the nicotinate-dependent NAD metabolism.

Diverse therapeutic efficacies and more diverse mechanisms of nicotinamide

BACKGROUND

Nicotinamide (NAM) is a form of vitamin B3 that, when administered at near-gram doses, has been shown or suggested to be therapeutically effective against many diseases and conditions. The target conditions are incredibly diverse ranging from skin disorders such as bullous pemphigoid to schizophrenia and depression and even AIDS. Similar diversity is expected for the underlying mechanisms. In a large portion of the conditions, NAM conversion to nicotinamide adenine dinucleotide (NAD⁺) may be a major factor in its efficacy. The augmentation of cellular NAD⁺ level not only modulates mitochondrial production of ATP and superoxide, but also activates many enzymes. Activated sirtuin proteins, a family of NAD⁺-dependent deacetylases, play important roles in many of NAM's effects such as an increase in mitochondrial quality and cell viability countering neuronal damages and metabolic diseases. Meanwhile, certain observed effects are mediated by NAM itself. However, our understanding on the mechanisms of NAM's effects is limited to those involving certain key proteins and may even be inaccurate in some proposed cases.

AIM OF REVIEW

This review details the conditions that NAM has been shown to or is expected to effectively treat in humans and animals and evaluates the proposed underlying molecular mechanisms, with the intention of promoting wider, safe therapeutic application of NAM.

KEY SCIENTIFIC CONCEPTS OF REVIEW

NAM, by itself or through altering metabolic balance of NAD⁺ and tryptophan, modulates mitochondrial function and activities of many molecules and thereby positively affects cell viability and metabolic functions. And, NAM administration appears to be quite safe with limited possibility of side effects which are related to NAM's metabolites.

The NAD ratio redox paradox: why does too much reductive power cause oxidative stress?

The reductive power provided by nicotinamide adenine dinucleotides is invaluable for several cellular processes. It drives metabolic reactions, enzymatic activity, regulates genetic expression and allows for the maintenance of a normal cell redox status. Therefore, the balance between the oxidized (NAD(+)) and the reduced (NADH) forms is critical for the cell's proper function and ultimately, for its survival. Being intimately associated with the cells' metabolism, it is expected that alterations to the NAD(+)/NADH ratio are to be found in situations of metabolic diseases, as is the case of diabetes. NAD(+) is a necessary cofactor for several enzymes' activity, many of which are related to metabolism. Therefore, a decrease in the NAD(+)/NADH ratio causes these enzymes to decrease in activity (reductive stress), resulting in an altered metabolic situation that might be the first insult toward several pathologies, such as diabetes. Here, we review the importance of nicotinamide adenine dinucleotides in the liver cell and its fluctuations in a state of type 2 diabetes mellitus.

Does nicotinic acid (niacin) lower blood pressure?

Nicotinic acid (niacin) is a well-established treatment for dyslipidaemia - an important cardiovascular disease (CVD) risk factor. However, niacin may also reduce blood pressure (BP), which is another important CVD risk factor. This review examines the limited publicly available data on niacin's BP effects. Acute administration of immediate-release niacin may lower BP because of niacin's acute vasodilatory effects. Although not always supported by clinical trial data, the package insert of a prescription, extended-release niacin describes niacin-induced acute hypotension. From a chronic standpoint, larger studies, such as the Coronary Drug Project, suggest that niacin may lower BP when administered over a longer period of time. Post hoc analyses of some of the more recent niacin clinical trials also support a more chronic, dose-dependent, BP-lowering effect of niacin. Because laropiprant [a prostaglandin D(2) (PGD(2)) type 1 (DP1) receptor antagonist] does not attenuate niacin's BP-lowering effects, it is unlikely that any chronic lowering of BP by niacin is due to dilation of dermal vessels through activation of the DP1 receptor by PGD(2.) Further research is warranted to evaluate the extent and mechanisms of niacin's effects on BP.

Effects of niacin on glucose control in patients with dyslipidemia

Niacin (nicotinic acid), the most effective available pharmacotherapy for increasing high-density lipoprotein cholesterol, also lowers triglycerides and hence may be useful, alone or in combination with hydroxymethylglutaryl coenzyme A reductase inhibitors (statins), to offset residual cardiovascular risk in patients with mixed or diabetic dyslipidemia. We conducted a review of published consensus guidelines since 2000 and an English-language PubMed search of prospective, randomized controlled trials and open-label studies from January 1, 1990, through December 31, 2007, concerning the effects of niacin, alone or in combination with statins, on glycemic regulation in dyslipidemic patients (with or without diabetes mellitus). For search terms, we used the title words niacin or nicotinic acid and key words including diabetes, diabetic, dyslipidemia, glucose, glycemic, HbA1c, hemoglobin, hyperglycemia, human, insulin, postprandial, and safety. Retrospective and observational studies, case reports, and case studies were excluded. On the basis of our analysis, the effects of niacin (< or =2.5 g/d), alone or in combination with statins, on fasting glucose (an increase of 4%-5%) and hemoglobin A1c levels (an increase of < or =0.3%) are modest, transient or reversible, and typically amenable to adjustments in oral hypoglycemic regimens without discontinuing niacin. Niacin therapy was infrequently associated with incident diabetes or the need for new insulin prescriptions. Studies showed important clinical benefits of niacin or niacin-statin regimens despite modest effects on glucose control. On a population basis, significant reductions in incidences of cardiovascular events and the degree of atherosclerotic progression associated with long-term niacin (or niacin-statin) therapy in patients with diabetic dyslipidemia outweigh the typically mild effects of this therapy on glycemic regulation. Consensus guidelines recommend monitoring glycemic control after initiating niacin treatment or increasing its dosage.

Pyridine nucleotides in glucose metabolism and diabetes: a review

Nicotinamide adenine dinucleotide (NAD) and its derivatives NADH, NADP and NADPH have regulatory functions in the generation of triose phosphates and pyruvate from glucose. In many studies of the influence of the diabetic state on relationships between pyridine nucleotide and glucose metabolism, the focus has been on the sorbitol pathway. Less attention has been paid to other aspects of the role of pyridine nucleotides in pyruvate formation from glucose, in particular the effects of the NAD precursors nicotinamide and nicotinic acid on glucose metabolism. This paper reviews current knowledge of the involvement of pyridine nucleotides and their precursors in glucose catabolism in the normal and diabetic state. Reference is also made to the following three current hypotheses for mechanisms underlying diabetic microangiopathy: 1. Chronic glucose overutilization, caused by hyperglycemia, in tissues which lack insulin receptors and therefore are freely permeable to glucose. 2. Enhancement of sorbitol pathway activity with an ensuing decrease in the ratio of NAD/NADH. 3. Enhanced utilization of both glucose and pyridine nucleotides in formation of triose phosphates and pyruvate. Therapy with NAD precursors like nicotinamide might have corrective effects on these proposed biochemical aberrations, thereby retarding progression of microangiopathy.

Vitamin supplementation therapy in the elderly

Vitamin supplementation in large dosages is increasingly common in the older population. Often, such supplementation is used in an attempt to improve an individual's health status. There have been claims that the effects of vitamins halt the normal aging process or prevent and cure disease. However, several recent studies have failed to demonstrate the efficacy of vitamin supplementation in preventing several types of cancer. In moderate dosages, supplementation with vitamin E (tocopherols) shows promise as a lipid antioxidant, and may reduce the risk of coronary heart disease. However, before vitamin E becomes an accepted medical therapy, further long term studies must be undertaken to examine the safety and efficacy of such therapy. An adequate intake of vitamins should be ensured by adherence to a well balanced diet. However, the elderly are prone to circumstances that may prevent them from eating a balanced diet. In addition, there are several age-related medical conditions that may predispose individuals to dietary and vitamin deficiencies. To prevent vitamin deficiency diseases and their associated morbidity, modest vitamin supplementation may be necessary. However, supplementation should be reserved for individuals with documented deficiency or who are at risk of developing such deficiencies, especially those who are homebound or institutionalised. Vitamins taken in large dosages should be considered as drugs. These medicines, which are obtainable over-the-counter, should be carefully regulated to prevent toxicity.

Effects of nicotinic acid treatment on glyceride formation and lipolysis in adipose tissue of hyperlipidemic patients

Thirty-one weight-stable patients with different types of hyperlipoproteinemia were treated daily with 4 g nicotinic acid for 6 weeks. Effects of this therapy on adipose tissue metabolism were evaluated. By using biopsy specimens of subcutaneous adipose tissue, fatty acid and glucose incorporation into adipose tissue glycerides were measured in vitro as well as glycerol and fatty acid release, which allowed us to estimate adipose tissue lipolysis. The amount of fatty acids produced by lipolysis and thereafter utilized within adipose tissue without being released (fatty acid retention) was estimated. Fatty acid and glucose incorporation into adipose tissue, glycerol release and fatty acid retention values increased, but serum triglyceride levels decreased (all P < 0.001) after nicotinic acid treatment. The change in fatty acid incorporation was positively correlated with changes in glucose incorporation into adipose tissue (r = 0.53, P < 0.01) and fatty acid retention (r = 0.76, P < 0.001). Although adipose tissue lipolysis, measured as glycerol release, increased, the lipolyzed fatty acids were retained in adipose tissue, suggesting an enhanced synthesis of glycerides both from exogenous and endogenous sources. The increase in fatty acid incorporation into adipose tissue indicates that the decrease in serum triglyceride levels produced by nicotinic acid treatment may partly be due to the fact that this drug promotes incorporation of fatty acids, derived from lipoprotein-carried triglycerides in the blood, into adipose tissue glycerides.