A Novel Transcriptional Repressor-activator Relationship in Growth Hormone-regulated Gene Expression.

Abstract

Growth Hormone (GH), a major regulator of normal growth and metabolism, regulates diverse physiological processes through regulation of specific target genes. A key activator of GH-regulated genes is Signal Transducer and Activator of Transcription (Stat) 5. However, mechanisms by which GH regulates transcriptional repression are poorly understood. A profile of GH-regulated genes in 3T3-F442A adipocytes identified the potent transcriptional repressor Bcl6 (B-cell lymphoma 6) as a novel GH-responsive molecule. Further, the gene for Socs2 (Suppressor Of Cytokine Signaling 2) is strongly inhibited by Bcl6, while Socs2 activation by GH is mediated by Stat5. Chromatin immunoprecipitation (ChIP) showed that endogenous Bcl6 occupies the Socs2 promoter in the absence of GH, and occupancy decreases with GH treatment, while Stat5 occupancy increases reciprocally.

To examine the reciprocal relationship between the repressor Bcl6 and the activator Stat5, we evaluated their genome-wide occupancy by ChIP-Sequencing in GH-responsive 3T3-F442A adipocytes, revealing over 3000 regions of occupancy for Bcl6 and over 900 for Stat5. Among these, gene ontology analysis of potential Bcl6 target genes identified genes involved in growth, differentiation, metabolism, and signaling.

Reciprocal occupancy of Bcl6 and Stat5 was observed not only on the Socs2 promoter, but also on the Cish (Cytokine-inducible SH2-containing protein) and Bcl6 gene promoters. However, Socs2 and Cish are stimulated by GH while Bcl6 is inhibited by GH. Analysis of transcription co-regulators suggests regulatory differences among these genes: p300 and Hdac3 (Histone deacetylase 3) occupied the Bcl6/Stat5 regulatory sequences of Socs2, Cish and Bcl6 constitutively. Promoter activation assays show that p300 co-activates Stat5-mediated induction of Socs2 and Cish expression, while Hdac3 functions as a co-repressor. In contrast, both p300 and Hdac3 may function as co-repressors when regulating Bcl6 expression.

This work provides insight into novel roles for Bcl6 in GH action, and mechanisms of transcriptional regulation involving reciprocal relationships by activating and repressing factors for genes in a major growth regulatory pathway.