Dynamics of protein-tyrosine phosphatases in rat adipocytes

Protein-tyrosine phosphatases (PTPases) play a key role in maintaining the steady-state tyrosine phosphorylation of the insulin receptor (Ht) and its substrate proteins such as insulin receptor substrate 1 (IRS-1), However, t he PTPase(s) that inactivate IR and IRS-1 under physiological conditions re main unidentified. Here, we analyze the subcellular distribution in rat adi pocytes of several PTPases thought to be involved in the counterregulation of insulin signaling. We found that the transmembrane enzymes, protein-tyro sine phosphatase (PTP)-alpha and leukocyte common antigen-related (ZAR), we re detected predominantly in the plasma membrane and to a lesser extent in the heavy microsomes, a distribution similar to that of insulin receptor, P TP-1B and IRS-1 were present in light microsomes and cytosol, whereas SHPTP 2/Syp was exclusively cytosolic. Insulin induced a redistribution of PTP-al pha from the plasma membrane to the heavy microsomes in a parallel fashion with the receptor. The distribution of PTP-1B in the light microsomes from resting adipocytes was similar to that of IRS-1 as determined by sucrose ve locity gradient fractionation. Analysis of the catalytic activity of partia lly purified rat adipocyte PTP-alpha and LAR and recombinant PTP-1B showed that all three PTPases dephosphorylate IR When a mix of IR/IRS-1 was used a s a substrate, PTP-1B was particularly effective in dephosphorylating IRS-1 . Considering that IR and IRS-1 can be dephosphorylated in internal membran e compartments from rat adipocytes (Kublaoui, B., Lee, J., and Pilch, P.F. (1995) J, Biol. Chem. 270, 59-65) and that PTP-alpha. and PTP-1B are the re spective PTPases in these fractions, we conclude that these PTPases are res ponsible for the counterregulation of insulin signaling there, whereas both LAR and PTP-alpha may act upon cell surface insulin receptors.