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Dipeptidyl peptidase-4 (DPP-4), a ubiquitous proteolytic enzyme, inhibits insulin secretion from pancreatic beta cells by inactivating circulating incretin hormones GLP-1 and GIP. High circulating levels of DPP-4 is presumed to compromise insulin secretion in people with type 2 diabetes (T2D). Our group recently reported lipid induced DPP-4 expression in pancreatic beta cells, mediated by the TLR4-NFkB pathway. In the present study, we looked at the role of Vildagliptin on pancreatic DPP-4 inhibition, preservation of islet mass and restoration of insulin secretion. MIN6 mouse insulinoma cells incubated with palmitate and fetuin-A, a proinflammatory organokine associated with insulin resistance, showed activation of TLR4-NFkB pathway, which was rescued on Vildagliptin treatment. In addition, Vildagliptin, by suppressing palmitate-fetuin-A mediated DPP-4 expression in MIN6, prevented the secretion of IL-1beta and fetuin-A in the culture media. DPP-4 siRNA abrogated TLR4-NFkB pathway mediated islet cell inflammation. Vildagliptin also reduced palmitate-fetuin-A mediated intracellular lipid accumulation in MIN6 and isolated islets from high fat fed (HFD) mice as observed by Oil O Red staining with downregulation of CD36 and PPARgamma. Vildagliptin also preserved islet mass and rescued insulin secretory defect in HFD mice. Our results suggest that inhibition of DPP-4 by Vildagliptin protects pancreatic beta cells from the deleterious effects of lipid and fetuin-A, preserves insulin secretory functions and improves hyperglycemia. [Display omitted] • Vildagliptin downregulates DPP-4 and fetuin-A expression in MIN6 and palmitate treated pancreatic islets • DPP-4 inhibition in MIN6 cell line suppresses TLR4-NFkB activation • Vildagliptin lowers high fat and fetuin-A mediated lipid accumulation and improves insulin secretion in pancreatic beta cells [ABSTRACT FROM AUTHOR]

Adiponectin secreted from adipocytes is an anti-diabetic and anti-atherogenic adipokine. Adiponectin level is known to fall significantly in obesity induced type 2 diabetes which worsen insulin sensitivity because of aberrant lipid management. However, underlying mechanism of adiponectin decrease in obese diabetic condition is yet unclear. We report here that lowering of plasma adiponectin coincided with the higher Fetuin A (FetA) level in high fat diet (HFD) induced obese diabetic mice. Knock down of FetA gene (FetA KD ) elevated adiponectin level markedly in HFD mice, while reinforcement of FetA into FetA KD HFD mice reduced its level again. These results indicate FetA's involvement in the lowering of adiponectin level in obesity induced diabetic mice. Our findings to understand how FetA could affect adiponectin decrease demonstrated that FetA could enhance Wnt3a expression in the adipocyte of HFD mice. FetA addition to 3T3L1 adipocyte incubation elevated Wnt3a expression in a dose dependent manner. Overexpression of Wnt3a by FetA inhibited PPARγ and adiponectin. FetA failed to reduce PPARγ and adiponectin in Wnt3a gene knocked down 3T3L1` adipocytes. All these suggest that FetA mediate its inhibitory effect on adiponectin through Wnt3a–PPARγ pathway. Inhibition of adiponectin expression through FetA and Wnt3a significantly compromised with the activation of AMPK and its downstream signalling molecules which adversely affected lipid management causing loss of insulin sensitivity. Downregulation of adiponectin in inflamed adipocyte by FetA through the mediation of Wnt3a and PPARγ is a new report. [ABSTRACT FROM AUTHOR]

Obesity induced insulin resistance is primarily regulated by the inhibitory phosphorylation of peroxisome proliferator-activated receptor γ at serine 273 (PPARγS273) which has been shown to be regulated by MEK and ERK. An upstream regulatory molecule of this pathway could be a therapeutic option. Here we analyzed the involvement of Fetuin-A (FetA), a key hepato-adipokine implicated in insulin resistance, as an upstream regulator molecule for the regulation of PPARγ inhibitory phosphorylation. Mice fed with standard diet (SD), high fat diet (HFD) and HFD with FetA knockdown (HFD-FetAKD) were used to examine the role of FetA on PPARγS273 phosphorylation in adipocytes. The mechanism of regulation and its effect on skeletal muscle were studied using primary adipocytes, 3T3-L1 (preadipocyte) and C2C12 (myotube) cell lines. Increased FetA in HFD mice strongly correlated with augmentation of PPARγS273 phosphorylation in inflamed adipocytes while knockdown of FetA suppressed it. This effect of FetA was mediated through the activation of Ras which in turn activated MEK and ERK. On addressing how FetA could stimulate activation of Ras, we found that FetA triggered TNFα in inflamed adipocytes which induced Ras activation. The ensuing sharp fall in adiponectin level attenuated AMPK activation in skeletal muscle cells affecting mitochondrial ATP production. Our data reveal the essential role of FetA induced activation of Ras in regulating PPARγ inhibitory phosphorylation through Ras-MEK-ERK pathway which downregulates adiponectin disrupting skeletal muscle mitochondrial bioenergetics. Thus, FetA mediated PPARγ inactivation has adverse consequences upon adipocyte-myocyte crosstalk leading to disruption of energy homeostasis and loss of insulin sensitivity. • FetA regulates PPARγ inhibitory phosphorylation in obese insulin resistant mice. • Upregulation of pPPARγS273 by FetA through MEK-ERK is mediated by Ras. • TNFα mediated Ras activation induces FetA's stimulatory effect upon MEK-ERK. • PPARγ inactivation reduces adiponectin attenuating mitochondrial bioenergetics. [ABSTRACT FROM AUTHOR]