124. Regulation of SOCS3 expression by prostaglandin, prolactin and growth hormone: challenging the Jak/STAT signalling dogma

Abstract

SOCS3 is an inhibitor of various cytokine-receptor signalling pathways and is therefore involved in suppression of cellular responsiveness to these critical regulators. SOCS3 expression is thought to be regulated by a STAT responsive element (SRE). However, our research suggests the involvement of other signalling pathways. In T-47D breast cancer cells, we found that PGE2 induces a 3–5 fold increase in SOCS3 mRNA, as determined by real-time PCR. This effect was not due to phosphorylation of STATs, or inhibited by the Jak2 inhibitor, AG490, but was inhibited by the PI3Kinase inhibitor, LY294002, Akt Inhibitor IV and partially inhibited by the PKA inhibitor, H89. It was not affected by inhibitors of MEK, PDK1, mTOR or p38-MAPK. We concurrently examined PRL-induced SOCS3 expression, and found that although STAT1 and 5 phosphorylation was increased, SOCS3 expression was again inhibited by Akt Inhibitor IV and H89 but unaffected by AG-490. To explore this further, we used a model of GH signalling, BaF3 cells stably expressing GH receptor. GH induced a 15–20 fold increase in SOCS3 mRNA, which was accompanied by increased STAT5 phosphorylation. However the SOCS3 response was not inhibited by AG-490 or H89, but was diminished by Akt Inhibitor IV. Analysis of the SOCS3 promoter revealed a FOXO binding site. When we mutated this site in a mouse SOCS3 promoter–luciferase construct, basal and GH-induced promoter activity was significantly increased. These results are consistent with FOXO acting as a repressor, which is inactivated by Akt. We propose that in T-47D cells, SOCS3 expression involves cross-talk between PI3K/Akt and cAMP/PKA, whereas in BaF3 cells, expression is enhanced by Akt phosphorylation and subsequent FOXO inactivation. These findings contrast with the accepted Jak/STAT regulation of SOCS3 expression.

This work is supported by the Australian Research Council.