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. 2016 May 31;6:27002. doi: 10.1038/srep27002

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Copyright © 2016, Macmillan Publishers Limited

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The constitutively active GPR21 recruits and activates Gα_15/16, which facilitates the hydrolysis of PIP₂ into DAG and IP₃ through the action of PLC. Both DAG and IP₃ activate PKC, which signals to and activates the MAPK cascade. The MAPK JNK plays a crucial role in the development of insulin resistance as it promotes the phosphorylation of a key component of the insulin signalling cascade, IRS1, at Ser³⁰⁷, thereby preventing insulin-induced tyrosine phosphorylation of this protein. GPR21-induced activation of the MAPKs may contribute to the development of type 2 diabetes. Due to the potential of a native inverse agonist in serum, GPR21 signal transduction is believed to be tightly regulated under normal physiological conditions. However in a state of obesity, elevated GPR21 activity may become deleterious, potentially promoting the genesis of insulin resistance.