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 v arious proteins by a cycle of phosphorylation/dephosphorylation. This dynamic process is regulated by the action of protein kinases and prot ein phosphatases. One family of protein kinases that is important in i nsulin signaling is the mitogen-activated protein (MAP) kinases, whose action is reversed by specific MAP kinase phosphatases (MKPs). Insuli n stimulation of Hirc B cells overexpressing the human insulin recepto r resulted in increased MKP-1 mRNA levels. MKP-1 mRNA increased in a d ose-dependent manner to a maximum of 3- to 4-fold over basal levels wi thin 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 w as maximally expressed after 120 min of insulin stimulation. MKP-1 mRN A induction by insulin required insulin receptor tyrosine kinase activ ity, since overexpression of an altered insulin receptor with impaired intrinsic tyrosine kinase activity prevented mRNA induction. Forskoli n, (Bu)(2)-cAMP, 8-bromo-cAMP, and 8-(4-chlorophenyl-thio)-cAMP increa sed the MKP-1 mRNA content moderately above basal. These agents also a ugmented the insulin-stimulated expression of MKP-1 mRNA. However, in same cases the increase in MKP-1 mRNA expression was less than additiv e. 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. Overex pression of MKP-1 in Hirc B cells inhibited both insulin-stimulated MA P kinase activity and MAP kinase-dependent gene transcription. The res ults of these studies led us to conclude that insulin regulates MKP-1 and strongly suggest that MKP-1 acts as a negative regulator of insuli n signaling.