Possible involvement of truncated signal transducer and activator of transcription-5 in the GH pattern-dependent regulation of CYP2C12 gene expression in rat liver

The transcription factors signal transducer and activator of transcription (Stat)5a and Stat5b have been implicated in the GH regulation of CYP2C genes in rodent liver. In addition to full-length Stat5 isoforms, truncated Stat5 proteins (Stat5β), lacking the transactivating domain, have been demonstrated. In this study we found that Stat5β can be formed by proteolytic cleavage in rat liver nuclei and that the activity of the protease is independent of GH. The GH regulation of the female-specific CYP2C12 gene has recently been shown to be conveyed by two adjacent Stat5-binding elements in the 5′-upstream region. We found that binding of Stat5 in liver nuclear extracts to this site involved simultaneous binding of two Stat5 dimers, most likely both Stat5b homodimers and Stat5a/Stat5b heterodimers. We also investigated Stat5 binding to a potential composite Stat5 element in the 3′-untranslated region (UTR) of CYP2C12. Several Stat5 complexes were formed on this element including Stat5β-containing complexes. In transient transfection experiments we could demonstrate that the 3′-UTR element reduced GH activation of a CYP2C12-luciferase reporter construct harboring the 5′-Stat5 elements. We speculate that binding of Stat5β to the 3′-UTR element could be of relevance for the GH-dependent and sex-specific expression of CYP2C12.