Identifying alterations in adipose tissue-derived islet GPCR peptide ligand mRNAs in obesity: implications for islet function

Abstract

In addition to acting as an energy reservoir, white adipose tissue is a vital endocrine organ involved in the modulation of cellular function and the maintenance of metabolic homeostasis through the synthesis and secretion of peptides, known as adipokines. It is known that some of these secretory peptides play important regulatory roles in glycaemic control by acting directly on islet β-cells or on insulin-sensitive tissues. Excess adiposity causes alterations in the circulating levels of some adipokines which, depending on their mode of action, can have pro-inflammatory, pro-diabetic or anti-inflammatory, anti-diabetic properties. Some adipokines that are known to act at β-cells have actions that are transduced by binding to G protein- coupled receptors (GPCRs). This large family of receptors represents ~35% of all current drug targets for the treatment of a wide range of diseases, including type 2 diabetes (T2D). Islets express ~300 GPCRs, yet only one islet GPCR is currently directly targeted for T2D treatment. This deficit represents a therapeutic gap that could be filled by the identification of adipose tissue-derived islet GPCR peptide ligands that increase insulin secretion and overall β-cell function. Thus, by defining their mechanisms of action, there is potential for the development of new pharmacotherapies for T2D. Therefore, this thesis describes experiments which aimed to compare the expression profiles of adipose tissue-derived islet GPCR peptide ligand mRNAs under lean and obese conditions, and to characterise the functional effects of a selected candidate of interest on islet cells. Visceral fat depots were retrieved from high-fat diet-induced and genetically obese mouse models, and from human participants. Fat pads were either processed as whole tissue, or mature adipocyte cells were separated from the stromal vascular fraction (SVF) which contains several other cell populations, including preadipocytes and macrophages. The expression levels of 155 islet GPCR peptide ligand mRNAs in whole adipose tissue or in isolated mature adipocytes were quantified using optimised RNA extraction and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) protocols. Comparisons between lean and obese states in mice models and humans revealed significant modifications in the expression levels of several adipokine mRNAs. As expected, mRNAs encoding the positive control genes, Lep and AdipoQ were quantifiable, with the expression of Lep mRNA increasing and that of AdipoQ mRNA decreasing in obesity. Expression of Ccl4 mRNA, encoding chemokine (C-C motif) ligand 4, was significantly upregulated in whole adipose tissue across all models of obesity compared to their lean counterparts. This coincided with elevated circulating Ccl4 peptide levels. This increase was not replicated in isolated mature adipocytes, indicating that the source of upregulated Ccl4 expression in obesity was the SVF of adipose tissue. Based on this significant increase in Ccl4 mRNA expression within visceral fat and its undetermined effects on β-cell function, Ccl4 was selected for further investigation in MIN6 β-cells and mouse islets. PRESTO-Tango β-arrestin reporter assays were performed to determine which GPCRs were activated by exogenous Ccl4. Experiments using HTLA cells expressing a protease-tagged β- arrestin and transfected with GPCR plasmids of interest indicated that 100ng/mL Ccl4 significantly activated Cxcr1 and Cxcr5, but it was not an agonist at the previously identified Ccl4-target GPCRs Ccr1, Ccr2, Ccr5, Ccr9 and Ackr2. RNA extraction and RT-qPCR experiments using MIN6 β-cells and primary islets from lean mice revealed the expression of Cxcr5 mRNA in mouse islets, but it was absent in MIN6 β-cells. The remaining putative Ccl4 receptors (Ccr1, Ccr2, Ccr5, Ccr9, Cxcr1 and Ackr2) were either absent or present at trace levels in mouse islets and MIN6 β-cells. Recombinant mouse Ccl4 protein was used for functional experiments at concentrations of 5, 10, 50 and 100ng/mL, based on previous reports of biological activities at these concentrations. Trypan blue exclusion testing was initially performed to assess the effect of exogenous Ccl4 on MIN6 β-cell viability and these experiments indicated that all concentrations (5-100ng/mL) were well-tolerated. Since β-cells have a low basal rate of apoptosis, cell death was induced by exposure to the saturated free fatty acid, palmitate, or by a cocktail of pro-inflammatory cytokines (interleukin-1β, tumour necrosis factor-α and interferon-γ). In MIN6 β-cells, Ccl4 demonstrated concentration-dependent protective effects against palmitate-induced and cytokine-induced apoptosis. Conversely, while palmitate and cytokines also increased apoptosis of mouse islets, Ccl4 did not protect islets from either inducer. Quantification of bromodeoxyuridine (BrdU) incorporation into β-cell DNA indicated that Ccl4 caused a concentration-dependent reduction in proliferation of MIN6 β-cells in response to 10% fetal bovine serum (FBS). In contrast, immunohistochemical quantification of Ki67-positive mouse islet β-cells showed no differences in β-cell proliferation between control- and Ccl4-treated islets. Whilst the number of β-cells and δ-cells were unaffected, α- cells were significantly depleted by Ccl4 treatment. Exogenous Ccl4 had no effect on nutrient- stimulated insulin secretion from both MIN6 β-cells and primary mouse islets. The 3T3-L1 preadipocyte cell line was used to assess potential Ccl4-mediated paracrine and/or autocrine signalling within adipose tissue. Ccl4 did not alter the mRNA expression of Pparγ, a master regulator of adipocyte differentiation, but did significantly downregulate the mRNA expression of the crucial adipogenic gene, adiponectin. Oil Red O staining and Western blotting were performed to assess lipid accumulation, and insulin and lipolytic signalling, respectively, and these experiments indicated that the observed Ccl4-induced decrease in adiponectin expression failed to correlate with any changes in adipocyte function. In summary, these data demonstrated anti-apoptotic and anti-proliferative actions of the adipokine, Ccl4, on MIN6 β-cells that were not replicated in mouse islets. The absence of any anti-apoptotic, insulin secretory and/or pro-proliferative effects of Ccl4 in islet β-cells suggests that it is unlikely to play a role in regulating β-cell function via crosstalk between adipose tissue and islets. The divergent functional effects highlight that whilst MIN6 cells are a useful primary β-cell surrogate for some studies, primary islets should always be used to confirm physiological relevance. On the other hand, significant α-cell depletion following Ccl4 treatment suggests a cell-specific function within the islets. Furthermore, Ccl4 impaired adiponectin mRNA expression in adipocytes, although, how adipocyte function is affected as a result requires further investigation. Collectively, these data have contributed increased understanding of the role of obesity in modifying the expression of adipose tissue-derived islet GPCR peptide ligands

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