Insulin and epidermal growth factor stimulate phosphorylation of a 170-kDa protein in intact hepatocytes immunologically related to lipocortin 1

Lipocortin 1 is a steroid-induced, calcium-regulated membrane binding protein (M(r) = 39,000) which is a substrate for the epidermal growth factor receptor kinase in intact cells. Using a polyclonal antibody to human recombinant lipocortin 1, we have identified a 170-kDa phosphoprotein in freshly isolated rat hepatocytes which shares antigenic determinants with lipocortin 1. The protein was recognized by four different anti-lipocortin 1 antisera, and antibody binding was inhibited by a 100-fold molar excess of human recombinant lipocortin 1 over antibody. Based on Coomassie Blue staining, the 170-kDa lipocortin-related protein is abundant (~100 ng/10⁶ cells) in rat liver, while lipocortin 1 itself is found in very low amounts. Epidermal growth factor and insulin stimulated phosphorylation of this 170-kDa protein in intact rat hepatocytes. The increase in phosphorylation was more pronounced in hepatocytes from dexamethasone-treated animals. The phosphorylation occurred exclusively on serine residues and was maximal 30-60 min after hormone addition. The 170-kDa protein was localized in the cytoplasm in the absence of calcium, while increasing calcium concentration led to partial association with the membrane compartment in rat liver. This 170-kDa protein represents a new member of the class of proteins whose serine phosphorylation is regulated by insulin and EGF and may belong to the family of lipocortin-related molecules.