Dissociation of PTPase levels from their modulation of insulin receptor signal transduction

Protein tyrosine phosphatases have been implicated in the regulation of rec eptor tyrosine kinase signalling pathways, including that of the insulin re ceptor. Here, cell density-dependent changes in PTPase expression have been exploited to investigate the relationship between cellular PTPase levels a nd the insulin receptor signal transduction pathway. Increasing cell densit y (20%, 50%, and >90%) in the rat McA-RH7777 hepatoma cell line resulted in increased protein expression of the receptor-like PTPase LAR (14-fold), an d the nonreceptor PTPases PTP1B (11-fold) and SHP2 (10-fold). Each of these PTPases has previously been implicated in regulating insulin receptor sign al transduction. Despite these marked increases, maximum insulin receptor a utophosphorylation as well as receptor expression actually increased 2-fold . MAP kinase also increased similar to 2-fold as a function of cell density and paralleled increases in expression levels. Neither sensitivity nor max imum responsiveness to insulin were decreased at increasing cell densities as assessed by activation of PI 3-kinase. Duration of response was also uni mpaired. These results suggest that expression levels of relevant PTPases a re not the primary determinant in their modulation of insulin receptor kina se activity. Restricted accessibility at the molecular level or involvement of accessory proteins may be more critical parameters. CELL SIGNAL 11;10:7 19-725, 1999. (C) 1999 Elsevier Science Inc.