Control of PHAS-I phosphorylation in 3T3-L1 adipocytes: Effects of inhibiting protein phosphatases and the p70(S6K) signalling pathway

PHAS-I is a recently discovered regulator of translation initiation. Non-phosphorylated PHAS-I binds and inhibits eukaryotic initiation factor-4E, the mRNA cap-binding protein that mediates a rate-limiting step in translation initiation. When PHAS-I is phosphorylated in response to insulin! the PHAS-I/eukaryotic initiation factor-4E complex dissociates. The present study was conducted to investigate mechanisms involved in the control of PHAS-I. Phosphorylation of PHAS-I was monitored by immunoblotting after subjecting extracts to polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. This was possible because phosphorylation markedly decreases the electrophoretic mobility of PHAS-I. Incubating 3T3-L1 adipocytes with rapamycin and wortmannin inhibited insulin-stimulated phosphorylation of PHAS-I at concentrations similar to those that inhibited activation of p70(S6K). Both agents increased the amount of PHAS-I that co-purified with eukaryotic initiation factor-4E when extracts were fractionated using a cap affinity resin, indicating that PHAS-I binding to the initiation factor was increased. Incubating adipocytes with the protein phosphatase inhibitors, calyculin A and okadaic acid, increased PHAS-I phosphorylation and opposed the effects of rapamycin on decreasing PHAS-I phosphorylation. However, neither okadaic acid nor calyculin A abolished the effects of rapamycin on PHAS-I. These results suggest that: the phosphorylation of PHAS-I in response to insulin occurs via the p70(S6K) Signalling pathway. By regulating eukaryotic initiation factor-4E, PHAS-I may have important roles in the control of both protein synthesis and mitogenesis.