Regulation of an hepatic low-M(r) membrane-associated protein-tyrosine phosphatase

Use of an antisense strategy to dissect the signaling role of protein-tyrosine phosphatase alpha

The protein-tyrosine phosphatase PTPalpha has been proposed to play an important role in controlling the dephosphorylation of a number of key signaling proteins and in regulating insulin signaling. To examine the potential cellular functions and physiological substrates of PTPalpha, a potent phosphorothioate oligonucleotide-based antisense strategy was developed that specifically depleted endogenous PTPalpha from 3T3-L1 adipocytes. The antisense probe, alphaAS1, achieved PTPalpha depletion levels normally of >/=85% and which varied up to levels where PTPalpha was not detected at all. Elimination of PTPalpha by 85% inhibited c-Src activity by 80%. Abolishing PTPalpha to levels undetected did not alter the tyrosine dephosphorylation of the insulin receptor or insulin receptor substrate proteins. Moreover, the ability of insulin to activate ERK2 or to stimulate DNA synthesis was not altered by alphaAS1. It is concluded that endogenous PTPalpha is a key regulator of c-Src activity in 3T3-L1 adipocytes and that PTPalpha is not required for the dephosphorylation of the insulin receptor or the insulin receptor substrate proteins or for the regulation of several downstream insulin signaling events in 3T3-L1 adipocytes. Finally, the development of the antisense probe, alphaAS1, provides an important molecular tool of general applicability for further dissecting the roles and precise targets of endogenous PTPalpha.

A peptide-based protein-tyrosine phosphatase inhibitor specifically enhances insulin receptor function in intact cells

3S-peptide-I is a synthetic tris-sulfotyrosyl dodecapeptide corresponding to the major site of insulin receptor autophosphorylation that potently inhibits dephosphorylation of the insulin receptor in a cell-free system and in digitonin-permeabilized Chinese hamster ovary (CHO) cells overexpressing the human insulin receptors (CHO/HIRc cells) (Liotta, A. S., Kole, H. K., Fales, H. M., Roth, J., and Bernier, M. (1994) J. Biol. Chem. 269, 22996-23001). In the present study, we found that 3S-peptide-I was not capable of inhibiting dephosphorylation of the epidermal growth factor (EGF) receptors in digitonin-permeabilized CHO cells that overexpress human EGF receptors (CHO/EGF-R cells). Moreover, the addition of a N-stearyl derivative of 3S-peptide-I to intact CHO/HIRc cells caused a concentration-dependent increase in insulin-stimulated phosphorylation of the insulin receptor, with a maximum effect (approximately 2.7-fold) at 50 microM. In contrast, ligand-stimulated EGF receptor phosphorylation in CHO/EGF-R cells was not affected by the presence of stearyl 3S-peptide-I. Furthermore, treatment of CHO/HIRc cells with this N-stearyl peptide led to a significant enhancement of the insulin-induced association of phosphatidylinositol (PI) 3-kinase activity with insulin receptor substrate 1 and the activation of mitogen-activated protein kinase. However, stearyl 3S-peptide-I had no effect on the EGF-stimulated activation of PI-3-kinase and mitogen-activated protein kinase in CHO/EGF-R cells. These data indicate that this tris-sulfotyrosyl dodecapeptide selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor in intact cells.