Lipotoxicity of beta-cells in obesity and in other causes of fatty acid spillover.

Abstract

A recently identified function of leptin is to protect nonadipose tissues from the nonoxidative metabolic products of long-chain fatty acids (FAs) during periods of overnutrition by increasing the ␤-oxidative metabolism of surplus FAs and reducing lipogenesis. When this protective system fails, harmful products of nonoxidative metabolism such as ceramide increase in nonadipose tissues, including the pancreatic islets and heart, and cause nitric oxide-mediated lipotoxicity and lipoapoptosis. The triacylglycerol content in nonadipocytes provides a useful index of overall nonoxidative metabolism. In normal animal tissue, triacylglycerol is maintained within a narrow range; even when the caloric intake is excessive, compensatory FAinduced upregulation of oxidation prevents overaccumulation. However, if leptin is deficient or if leptin receptors (Ob-R) are nonfunctional, this autoregulatory system does not operate, and triacylglycerol content rises in nonadipose tissues. This provides a source of excess FAs that enter potentially toxic pathways of nonoxidative metabolism leading to apoptosis of certain tissues. FA overload in skeletal muscle causes insulin resistance; in myocardium, it impairs cardiac function; and in pancreatic islets, it causes ␤-cell dysfunction, apoptosis, and diabetes. All abnormalities in these tissues can be blocked by troglitazone, an inhibitor of FA accumulation. Diabetes 50 (Suppl. 1):S118-S121, 2001T he evolution of the adipocyte may well constitute one of the most transforming events in phylogenetic development. As proposed in the "thrifty gene" hypothesis of Neel (1), the ability to preload calories and to store them in the form of triacylglycerol for use during famine and/or exertion improved survival for many species. Recent work from this lab (2) suggests that the adipocytes have an additional function, namely, to protect nonadipocytes, which have only a limited capacity to store triacylglycerol, from excessive accumulation of lipids and their entry into nonoxidative metabolic pathways. This protection requires the permissive action of the adipocyte hormone leptin (3), which, as will be explained below, reduces lipogenesis and increases oxidation in nonadipocytes during overnutrition, dissipating the unneeded energy of the fatty acid (FA) excess as heat (4). This limits FA entry into potentially toxic nonoxidative metabolic pathways (5). LIPOTOXIC DISEASESLipotoxicity refers to the tissue disease that may occur when FA spillover in excess of the oxidative needs of those tissues enhances metabolic flux into harmful pathways of nonoxidative metabolism. Although lipotoxicity is not currently recognized as a clinical entity in obese humans, human disorders of FA metabolism exhibit clinical syndromes similar to those of the obese rodent. It seems reasonable, therefore, to group them under the rubric of lipotoxic diseases. The disorders in which lipotoxicity due to FA overflow is suspected include congenital generalized lipodystrophy (CGL), obesity due to mutations in the leptin or leptin r...

Adiponectin counteracts cytokine- and fatty acid-induced apoptosis in the pancreatic beta-cell line INS-1

“…NF-κB activation represents a crucial event in cytokine-regulation of expression of several chemokines and cytokines [8], Fas expression [9], iNOS and COX-2 expression [10,11], which are all regarded as essential factors for causing dysfunction and destruction of insulin-producing cells thus leading to the development of insulin-dependent diabetes. Importantly, a ceramide-dependent NF-κB-mediated up-regulation of iNOS expression was recently shown in response to palmitic acid, indicating a potential involvement of this transcription factor also in the regulation of beta-cell lipotoxicity and pathogenesis of Type 2 diabetes [6].…”

“…In the case of Type 2 diabetes, which is associated with peripheral insulin resistance and beta-cell dysfunction, recent evidence suggests a reduced beta-cell mass as a contributor to the pathogenesis of this chronic disease [4]. Moreover, fatty acid-induced apoptosis of pancreatic beta cells has been regarded as a critical determinant in switching from obesity and insulin resistance to Type 2 diabetes [5,6].…”

“…Recent work has identified a novel function of leptin consisting of the prevention of islet lipotoxicity by increasing β-oxidative metabolism of fatty acids and reducing lipogenesis [5,6]. This protective action of leptin against beta-cell lipoapoptosis was accompanied by a reduction in ceramide synthesis and iNOS mRNA expression as well as prevention of fatty-acidinduced suppression of Bcl-2 expression thus inhibiting apoptotic death [4,5,6,12].…”

“…This protective action of leptin against beta-cell lipoapoptosis was accompanied by a reduction in ceramide synthesis and iNOS mRNA expression as well as prevention of fatty-acidinduced suppression of Bcl-2 expression thus inhibiting apoptotic death [4,5,6,12]. Further, the effects of leptin on fatty acid metabolism (in muscle) are mediated through activation of 5'-AMP-activated protein kinase (AMPK) and subsequent down-regulation of acetyl coenzyme A carboxylase (ACC) [13,14].…”

Anti-CD38 autoantibodies: Characterisation in new-onset Type I diabetes and latent autoimmune diabetes of the adult (LADA) and comparison with other islet autoantibodies

“…Whether this damage occurs by autoimmune mechanisms, as in Type I (insulin-dependent) diabetes mellitus [1], or by other less defined phenomena (e.g., glucose toxicity [2], lipotoxicity [3], prolonged excessive insulin secretion [4]), as in Type II (non-insulin-dependent) diabetes mellitus [5], the final result is impaired insulin secretion [6]. A number of autoantibodies (aAbs) marking the immune-mediated destruction of the beta cell typical of Type I diabetes have been described since the identification of islet cell antibodies (ICA) [7].…”