Regulation of suppressor of cytokine signaling 3 (SOC3) by growth hormone in pro-B cells

Suppressor of cytokine signaling 3 ( SOCS3) is expressed by lymphoid cells and can modulate the sensitivity of these cells to cytokine stimulation through inhibition of Janus kinase ( JAK)/ signal transducers and activators of transcription ( STAT) signaling pathways. This study employed a mouse pro- B cell line expressing the human GH receptor ( BaF/ 3- GHR), to elucidate the signal transduction pathways used by GH to elicit SOCS3 expression. GH treatment of these cells caused a rapid, dosedependent increase in SOCS3 mRNA expression, which was independent of de novo protein synthesis. As expected, GH treatment increased JAK-dependent STAT5 tyrosine phosphorylation, which bound to the proximal STAT response element ( pSRE) on the SOCS3 promoter. This process appeared to involve STAT5b, rather than STAT5a. In addition, GH activation of the SOCS3 promoter required a nearby activator protein ( AP) 1/ cAMP response element ( CRE), which bound cAMP response element binding protein, c- Fos, and c- Jun. Moreover, inhibitors of p38 MAPK and c- Jun N- terminal kinase prevented GH- stimulation of SOCS3 mRNA expression in these cells, suggesting a role for these kinases in SOCS3 transcription. Importantly, GH stimulation increased binding of FOXO3a to the SOCS3 promoter at a site overlapping the AP1/ CRE response element, and overexpression of FOXO3a in these cells augmented SOCS3 promoter activation. In addition, we show a direct interaction between FOXO3a and STAT5 in these cells, which may provide a link between STAT5 and the AP1 transcription factors on the SOCS3 promoter. We conclude that regulation of SOCS3 expression by GH in a pro- B cell involves not only the pSRE, but also a transcriptionally active complex involving cAMP response element binding protein/ c- Fos/ c- Jun and FOXO3a. This study has implications for cytokine regulation of SOCS gene expression in lymphoid cells.