S J DUNMORE and J E P BROWN Adipokines, b-cell failure and type 216:1 T37–T45 Thematic Review 2 diabetes The role of adipokines in b-cell failure of type 2 diabetes Simon J Dunmore and James E P Brown1 Diabetes and Metabolic Disease Research Group, Research Institute in Healthcare Science, University of Correspondence should be Wolverhampton, Wolverhampton WV1 1LY, UK addressed to S J Dunmore 1Aston Research Centre for Healthy Ageing, School of Life and Health Sciences, Aston University, Birmingham B4 Email 7ET, UK [email protected] Abstract b-Cell failure coupled with insulin resistance is a key factor in the development of type 2 diabetes. Changes in circulating levels of adipokines, factors released from adipose tissue, form a significant link between excessive adiposity in obesity and both aforementioned factors. In this review, we consider the published evidence for the role of individual adipokines on the function, proliferation, death and failure of b-cells, focusing on those reported to have the most significant effects (leptin, adiponectin, tumour necrosis factor a, resistin, visfatin, dipeptidyl peptidase IV and apelin). It is apparent that some adipokines have beneficial effects whereas others have detrimental properties; the overall contribution to b-cell failure of changed concentrations of adipokines in the blood of obese pre-diabetic subjects will be highly dependent on the balance between these effects and the interactions between the adipokines, which act on the b-cell via a number of intersecting intracellular signalling pathways. We emphasise the importance, and comparative dearth, of studies into the combined effects of adipokines on b-cells. Journal of Endocrinology Journal of Endocrinology (2013) 216, T37–T45 Introduction Pancreatic b-cell failure, alongside insulin resistance, can caused uptake of glucose into adipocytes via insulin be considered as one of the two key events that leads to the receptor binding and signalling and thus led to increased development of type 2 diabetes (Leahy et al. 2010). In the storage of triacylglycerol. What was also accepted was that early stages of the disease, insulin resistance is countered insulin resistance in adipose tissue contributed to b-cell by an increase in pancreatic b-cell mass and function failure (Weir et al. 2001), a phenomenon that highlighted (Chang-Chen et al. 2008) that can often delay diagnosis for the importance of the relationship between these two a period of years. The b-cell eventually succumbs to rising tissues. Our understanding of adipose tissue has grown insulin resistance via various mechanisms (including immensely over the past few decades so that we now know glucotoxicity, lipotoxicity and endoplasmic reticulum that adipose tissue is not an inert storage depot for excess stress) and hyperglycaemia ensues (Jonas et al. 2009). energy but an active endocrine organ with a wide, What is key in preventing the onset of type 2 diabetes biologically active secretome (Deng & Scherer 2010). under these conditions is the ability of the body to The molecules released from adipose tissue are com- maintain both pancreatic b-cell mass and function. monly referred to as adipokines (or adipocytokines). The list The relationship between the pancreas and adipose of molecules confirmed as adipokines grows every year, as tissue was for many years believed to be something of a both novel and existing molecules are seen to be secreted by one-way street. It was accepted that insulin secretion from adipose tissue or adipocytes. Most, but not all, adipokines are pancreatic islets in response to increased blood glucose peptides/proteins with hormone-like properties (some are http://www.joe.endocrinology-journals.org Ñ 2013 Society for Endocrinology Published by Bioscientifica Ltd. DOI: 10.1530/JOE-12-0278 Printed in Great Britain This paper is one of three papers that form part of a thematic review section on Adipokines. The Guest Editor for thisDownloaded section was from Simon Bioscientifica.com Dunmore, Diabetes at 09/27/2021 and 04:34:24PM Metabolic Disease Research Group, University of Wolverhampton, UK. via free access Thematic Review S J DUNMORE and J E P BROWN Adipokines, b-cell failure and type 216:1 T38 2 diabetes known cytokines), but in this context, non-esterified fatty stimulatory (Tanizawa et al. 1997) and null (Leclercq- acids (NEFAs; free fatty acids) can also be considered to be Meyer & Malaisse 1998) effects on insulin secretion being adipokines. It has been widely demonstrated that the reported. It is now generally accepted that leptin has a enlargement of adipose tissue depots seen in obesity can potent inhibitory effect on insulin secretion from pan- lead to dysregulated adipokine secretion, representing a creatic b-cells in vitro and in vivo and has the additional potential pathophysiological link between obesity and type effect of reducing pre-proinsulin gene expression (Laubner 2 diabetes (and vascular disease; Wellen & Hotamisligil et al. 2005). 2005), and so research into their precise role in metabolic Our own research on this adipokine using human disorders is ongoing. It is becoming increasingly clear islets showed evidence of a U-shaped response (Brown that adipokines form an important part of an ‘adipo-insular et al. 2002), with lower concentrations inhibiting insulin axis’ (see Fig. 1), dysregulation of which may contribute to release and higher levels having a relatively stimulatory b-cell failure and hence to type 2 diabetes. effect, which may provide an explanation for some of the conflicting reports. This type of response is also consistent with the cytokine receptor-like nature of the leptin Leptin: the ‘first’ adipokine receptor and signalling cascade. We also demonstrated With the discovery of the product of the OB gene, leptin, that leptin decreased uncoupling protein 2 (UCP2) in 1994 by Friedman and colleagues (Zhang et al. 1994), expression in these islets and this may contribute to an the first insights into adipose-derived hormonal factors effect of the adipokine in ameliorating b-cell apoptosis – that later became known as adipokines, and hence the further research in a clonal b-cell line from our group also bidirectional nature of the adipo-insular axis, were showed an effect of leptin to alter the B-cell lymphoma established. Leptin can claim to be the first adipokine to 2/Bcl2-associated X protein (BCL2/BAX) expression ratio, be associated with direct pancreatic effects, and certainly which may also explain the reduction in apoptosis the most studied of all the adipokines with respect to its induced by this factor (Brown & Dunmore 2007). By pancreatic effects. After initial clinical work suggested a contrast, a recent study by Chetboun et al. (2012) possible relationship between circulating leptin levels and found no effect of leptin (or adiponectin) on apoptosis, islet function (Ahren & Larsson 1997), basic laboratory although both adipokines caused a reactive oxygen species studies confirmed effects of leptin exposure on glucose- (ROS)-mediated increase in cell proliferation and leptin stimulated insulin secretion (GSIS) in ex vivo pancreatic (but not adiponectin)-inhibited GSIS. This potential role Journal of Endocrinology islets, although the initial few studies gave mixed results, of low levels of ROS in promoting b-cell mass may with inhibitory (Kulkarni et al. 1997, Brown et al. 2002), underline the importance of our aforementioned finding that leptin decreases UCP2 as this protein may also have a role in reducing ROS levels – generally thought to be beneficial to b-cells but possibly, in view of Chetboun Adiponectin Positive et al.’s hypothesis, decreasing proliferation. This under- regulation Visfatin IGF1 lines the need for further research into the role of UCP2 NEFA (acute) in the b-cell. It is apparent that leptin’s effects on the b-cell are Insulin direct and it is clear that pancreatic b-cells express the leptin receptor, LepR (Ob-R), in both the functional long Pancreatic form (LepRb) and truncated shorter forms (Kieffer et al. β-cell 1996). Mechanistically speaking, it appears that leptin can Leptin apelin activate multiple signalling pathways in the pancreatic Negative Adipose regulation TNFα tissue b-cell. The classical pathway associated with leptin resistin NEFA (chronic) binding its receptor is activation of the Jak/STAT pathway where receptor-associated kinase JAK2 phosphorylates Figure 1 tyrosine residues on LepRb, which subsequently recruits Diagrammatic representation of the interrelationship between adipose and phosphorylates STAT family members that can then tissue and the pancreatic b-cell: the ’adipo-insular axis’. Positive regulation translocate to the nucleus and regulate gene transcription may include stimulation of insulin synthesis and secretion and cell proliferation. Negative regulation can involve inhibition of insulin (Seufert 2004). Evidence also suggests that various synthesis and secretion and increased cell apoptosis and/or necrosis. members of the MAPK/ERK pathway are activated in http://www.joe.endocrinology-journals.org Ñ 2013 Society for Endocrinology Published by Bioscientifica Ltd. DOI: 10.1530/JOE-12-0278 Printed in Great Britain Downloaded from Bioscientifica.com at 09/27/2021 04:34:24PM via free access Thematic Review S J DUNMORE and J E P BROWN Adipokines, b-cell failure and type 216:1 T39 2 diabetes pancreatic b-cells by leptin, possibly through interaction have external carboxy-terminuses and cytoplasmic with shorter forms of the leptin receptor (Tanabe et al. amino-terminuses;