Insulin-Induced Mitogen-Activated Protein (MAP) Kinase Phosphatase-1 (MKP-1) Attenuates Insulin-Stimulated MAP Kinase Activity: A Mechanism for the Feedback Inhibition of Insulin Signaling
Insulin signaling involves the transient activation/inactivation of various proteins by a cycle of phosphorylation/dephosphorylation. This dynamic process is regulated by the action of protein kinases and protein phosphatases. One family of protein kinases that is important in insulin signaling is the mitogen-activated protein (MAP) kinases, whose action is reversed by specific MAP kinase phosphatases (MKPs). Insulin stimulation of Hirc B cells overexpressing the human insulin receptor resulted in increased MKP-1 mRNA levels. MKP-1 mRNA increased in a dose-dependent manner to a maximum of 3- to 4-fold over basal levels within 30 min, followed by a gradual return to basal. The mRNA induction did not require the continuous presence of insulin. The induction of MKP-1 protein synthesis followed MKP-1 mRNA induction; MKP-1 protein was maximally expressed after 120 min of insulin stimulation. MKP-1 mRNA induction by insulin required insulin receptor tyrosine kinase activity, since overexpression of an altered insulin receptor with impaired intrinsic tyrosine kinase activity prevented mRNA induction. Forskolin, (Bu)2-cAMP, 8-bromo-cAMP, and 8-(4-chlorophenylthio)-cAMP increased the MKP-1 mRNA content moderately above basal. These agents also augmented the insulin-stimulated expression of MKP-1 mRNA. However, in some cases the increase in MKP-1 mRNA expression was less than additive. Nevertheless, these results indicate that multiple signaling motifs might regulate MKP-1 expression and suggest another mechanism for the attenuation of insulin-stimulated MAP kinase activity by cAMP. Overexpression of MKP-1 in Hirc B cells inhibited both insulin-stimulated MAP kinase activity and MAP kinase-dependent gene transcription. The results of these studies led us to conclude that insulin regulates MKP-1 and strongly suggest that MKP-1 acts as a negative regulator of insulin signaling.