Regulation of IGF-1/PI3K/Akt signalling by the phosphoinositide phosphatase pharbin

Pharbin, a 5-phosphatase that induces arborization, is one of the phosphoinositide 5-phosphatases that is highly mutated in patients with Joubert syndrome. Pharbin can hydrolyse PI(4,5)P(2) and PI(3,4,5)P(3) and has the same substrate specificity as SHIP2 and SKIP, which negatively regulate PI3K signalling. Here, we investigated the role of pharbin in IGF-1/PI3K signalling. Ectopic expression of pharbin markedly suppressed the IGF-1-induced activation of Akt without affecting p42/44 MAP kinase phosphorylation. In contrast, pharbin silencing by RNA interference increased the IGF-1-induced phosphorylation of Akt, suggesting that pharbin negatively regulates PI3K/Akt signalling. Pharbin expression also inhibited the phosphorylation of p70 S6 kinase and 4E-BP1 as well as the subsequent protein synthesis in response to IGF-1 treatment. Taken together, these results indicate that pharbin is an important negative regulator of IGF-1/PI3K/Akt signalling and protein synthesis.