Overexpression of protein-tyrosine phosphatase-1B in adipocytes inhibits insulin-stimulated phosphoinositide 3-kinase activity without altering glucose transport or Akt/protein kinase B activation

Previous studies suggested that protein-tyrosine phosphatase IB (PTP1B) ant agonizes insulin action by catalyzing dephosphorylation of the insulin rece ptor (IR) and/or other key proteins in the insulin signaling pathway. In ad ipose tissue and muscle of obese humans and rodents, PTP1B expression is in creased, which led to the hypothesis that PTP1B plays a role in the pathoge nesis of insulin resistance. Consistent with this, mice in which the PTP1B gene was disrupted exhibit increased insulin sensitivity. To test whether i ncreased expression of PTP1B in an insulin-sensitive cell type could contri bute to insulin resistance, we overexpressed wild-type PTP1B in 3T3L1 adipo cytes using adenovirus-mediated gene delivery. PTP1B expression was increas ed similar to 3-5-fold above endogenous levels at 16 h, similar to 14-fold at 40 h, and similar to 20-fold at 72 h post-transduction, Total protein-ty rosine phosphatase activity was increased by 50% at 16 h, 3-4-fold at 40 h, and 5-6-fold at 72 h post-transduction Compared with control cells, cells expressing high levels of PTP1B showed a 50-60% decrease in maximally insul in-stimulated tyrosyl phosphorylation of IR and insulin receptor substrate- 1 (IRS-1) and phosphoinositide 3-kinase (PI3K) activity associated with IRS -I or with phosphotyrosine. Akt phosphorylation and activity were unchanged . Phosphorylation of p42 and p44 MAP kinase (MAPK) was reduced similar to 3 2%. Overexpression of PTP1B had no effect on basal, submaximally or maximal ly (100 nM) insulin-stimulated glucose transport or on the EC,, for transpo rt. Our results suggest that: 1) insulin stimulation of glucose transport i n adipocytes requires less than or equal to 45% of maximal tyrosyl phosphor ylation of IR or IRS-1 and <50% of maximal activation of PI3K, 2) a novel P I3K-independent, pathway may play a role in insulin-induced glucose transpo rt in adipocytes, and 3) overexpression of PTP1B alone in adipocytes does n ot impair glucose transport.