Increase In Ppar Gamma Inhibitory Phosphorylation By Fetuin-A Through The Activation Of Ras-Mek-Erk Pathway Causes Insulin Resistance

Obesity induced insulin resistance is primarily regulated by the inhibitory phosphorylation of peroxisome proliferator-activated receptor gamma at serine 273 (PPAR gamma S273) 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 PPAR gamma inhibitory phosphorylation. Mice fed with standard diet (SD), high fat diet (HFD) and HFD with FetA knockdown (HFD-FetA(KD)) were used to examine the role of FetA on PPAR gamma S273 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 PPAR gamma S273 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 TNF alpha 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 PPAR gamma inhibitory phosphorylation through Ras-MEK-ERK pathway which downregulates adiponectin disrupting skeletal muscle mitochondrial bioenergetics. Thus, FetA mediated PPAR gamma inactivation has adverse consequences upon adipocyte-myocyte crosstalk leading to disruption of energy homeostasis and loss of insulin sensitivity.