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This study investigates the role of protein kinase C (PKC) activation in modulating insulin signaling in human kidney fibroblast (293) cells. The findings reveal that PKC activation inhibits insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), a critical step in insulin signaling. This inhibition was observed both in vivo and in vitro when IRS-1 and the insulin receptor were isolated from PKC-activated cells. To determine whether serine phosphorylation contributes to this effect, researchers mutated serine 612 (S612) to alanine (S612A). Unlike the wild-type IRS-1, the mutant retained its ability to undergo insulin-induced tyrosine phosphorylation and associate with phosphatidylinositol 3-kinase (PI3K) even after PKC activation. Further analysis confirmed that PKC promotes the phosphorylation of IRS-1 at serine 612, which reduces its susceptibility to insulin receptor-mediated tyrosine phosphorylation. Notably, cytosolic extracts from the livers of obese (ob/ob) mice exhibited an eightfold increase in kinase activity targeting this phosphorylation site compared to lean littermates. These findings suggest that PKC-mediated serine phosphorylation of IRS-1 disrupts insulin signaling, providing a potential mechanistic link between PKC activation and insulin resistance in obesity-related metabolic dysfunction.