Association of the glucocorticoid receptor with STAT3, C/EBPbeta, and the hormone-responsive element within the rat haptoglobin gene promoter during the acute phase response.

Upregulation of haptoglobin (Hp) expression in the rat during the acute phase (AP) response is the result of synergistic effects of IL-6-, IL-1beta-, and corticosterone-activated signaling pathways. IL-6 signaling terminates in cis-trans interactions of the Hp gene hormone-responsive element (HRE) with transcription factors STAT3 and C/EBPbeta. The aim of this study was to examine the unresolved molecular mechanism of glucocorticoid action. A 3-fold rise in serum corticosterone at 2 and 4 h of the AP response induced by turpentine administration preceded a 2.3-fold increase in the rate of Hp gene transcription at 12 h that was accompanied by a 4.8-fold increase in glucocorticoid receptor (GR), the appearance of an 86-kDa STAT3 isoform and 3.9-, 1.9-, and 1.7-fold increased amounts of 91-kDa STAT3, 35- and 42-kDa C/EBPbeta isoforms in the nucleus. These events resulted in 4.6- and 2.5-fold increased Hp levels in the liver and serum at 24 h. HRE affinity chromatography and immunoblot analysis revealed that maximal occupancy of the HRE with GR, STAT3, and C/EBPbeta at 12 h correlated with increased transcriptional activity of the Hp gene. Coimmunoprecipitation experiments showed that activated GR established de novo interaction with STAT3 isoforms while GR-C/EBPbeta interactions observed during basal transcription increased during the AP response. Computer analysis of the HRE disclosed two potential GR-binding sites: one overlapping STAT3, another adjacent to a C/EBPbeta-binding site. This finding and the experimental results suggest that activated GR through direct interactions with STAT3 and C/EBPbeta, participates in Hp gene upregulation as a transcriptional coactivator.