Role of cAMP in mediating effects of fasting on dephosphorylation of insulin receptor

We studied the effect of fasting on phosphotyrosine phosphatase (PTPase) activities in particulate (PF) and cytosolic (CF) fractions of rat adipocytes and liver. PTPase activity was assessed using [³²P]tyrosine insulin receptor (IR). In adipocytes, 48 h fasting significantly inhibited PTPase activity. Dephosphorylation of IR by PF and CF PTPases was reduced by 80 and 65%, respectively. Similar reductions of lesser magnitude were observed in fasted rat livers. The effect of fasting was completely reversed by either refeeding or by incubating 'fasted' adipocytes for 2 h in tissue culture medium containing 5 mM glucose. Neither 20 mM glucose nor the presence of insulin influenced phosphatase activity. Because fasting is accompanied by elevated protein kinase C (PKC) and adenosine 3',5'-cyclic monophosphate (cAMP) levels, we examined their influence on adipocyte PTPases. Neither activation (1 μM 12-O-tetradecanoylphorbol-13-acetate) nor inhibition (20 μM sphingosine) of PKC affected PTPase activity. In contrast, cAMP (2 mM) significantly inhibited PTPase activity (80% inhibition at 2 h), and its effect was prevented by a cAMP antagonist RpcAMP. Fasting- and cAMP-induced inhibition of PTPase activity was restored by incubating PF with trypsin (4 μg/ml for 5 min), which separated the putative inhibitors from the phosphatases. We conclude that fasting-induced inhibition of PTPases is mediated by elevated cAMP levels, most likely by activating phosphatase inhibitors.