EFFECT OF TUMOR-NECROSIS-FACTOR-ALPHA ON THE PHOSPHORYLATION OF TYROSINE KINASE RECEPTORS IS ASSOCIATED WITH DYNAMIC ALTERATIONS IN SPECIFIC PROTEIN-TYROSINE PHOSPHATASES

Tumor necrosis factor-alpha (TNF-alpha) can modulate the signalling ca pacity of tyrosine kinase receptors; in particular, TNF-alpha has been shown to mediate the insulin resistance associated with animal models of obesity and noninsulin-dependent diabetes mellitus. In order to de termine whether the effects of TNF-alpha might involve alterations in the expression of specific protein-tyrosine phosphatases (PTPases) tha t have been implicated in the regulation of growth factor receptor sig nalling, KRC-7 rat hepatoma cells were treated with TNF-alpha, and cha nges in overall tissue PTPase activity and the abundance of three majo r hepatic PTPases (LAR, PTP1B, and SH-PTP2) were measured in addition to effects of TNF-alpha on ligand-stimulated autophosphorylation of in sulin and epidermal growth factor (EGF) receptors and insulin-stimulat ed insulin receptor substrate-1 (IRS-1) phosphorylation. TNF-alpha cau sed a dose-dependent decrease in insulin-stimulated IRS-1 phosphorylat ion and EGF-stimulated receptor autophosphorylation to 47-50% of contr ol. Overall PTPase activity in the cytosol fraction did not change wit h TNF-alpha treatment, and PTPase activity in the particulate fraction was decreased by 55-66%, demonstrating that increases in total cellul ar PTPase activity did not account for the observed alterations in rec eptor signalling. However, immunoblot analysis showed that TNF-alpha t reatment resulted in a 2.5-fold increase in the abundance of SH-PTP2, a 49% decrease in the transmembrane PTPase LAR, and no evident change in the expression of PTP1B. These data suggest that at least part of t he TNF-alpha effect on pathways of reversible tyrosine phosphorylation may be exerted through the dynamic modulation of the expression of sp ecific PTPases. Since SH-PTP2 has been shown to interact directly with both the EGF receptor and IRS-1, increased abundance of this PTPase m ay mediate the TNF-alpha effect to inhibit signalling through these pr oteins. Furthermore, decreased abundance of the LAR PTPase, which has been implicated in the regulation of insulin receptor phosphorylation, may account for the less marked effect of TNF-alpha on the autophosph orylation state of the insulin receptor while postreceptor actions of insulin are inhibited. (C) 1997 WiIey-Liss.