Growth Hormone Regulates Phosphorylation and Function of CCAAT/Enhancer-binding Protein β by Modulating Akt and Glycogen Synthase Kinase-3

Abstract

Growth hormone (GH) regulates transcription factors associated with c-fos, including C/EBPβ. Two forms of C/EBPβ, liver-activating protein (LAP) and liver inhibitory protein (LIP), are dephosphorylated in GH-treated 3T3-F442A fibroblasts. GH-induced dephosphorylation of LAP and LIP is reduced when cells are preincubated with phosphatidylinositol 3′-kinase (PI3K) inhibitors. GH activates Akt and inhibits glycogen synthase kinase-3 (GSK-3). Lithium, a GSK-3 inhibitor, increases GH-dependent dephosphorylation of LAP and LIP. Both are in vitro substrates of GSK-3, suggesting that GSK-3 inactivation contributes to GH-promoted dephosphorylation of C/EBPβ. Alkaline phosphatase increases binding of LAP homodimers and decreases binding of LIP homodimers to c-fos, suggesting that dephosphorylation of C/EBPβ modifies their ability to bind DNA. Both alkaline phosphatase- and GH-mediated dephosphorylation comparably increase binding of endogenous LAP in 3T3-F442A cells. In cells overexpressing LAP and GSK-3, LAP binding decreases, suggesting that GSK-3-mediated phosphorylation interferes with LAP binding. Expression of constitutively active GSK-3 reduced GH-stimulated c-fos promoter activity. These studies indicate that PI3K/Akt/GSK-3 mediates signaling between GH receptor and the nucleus, promoting dephosphorylation of C/EBPβ. Dephosphorylation increases binding of LAP complexes to the c-fos promoter and may contribute to the participation of C/EBPβ in GH-stimulated c-fos expression.