12A,B). Concentrations of proliferating cell nuclear antigen, an indicator of cell proliferation, were elevated in liver-specific Stat5-null mice treated with CCl4 (Supporting Fig. 13A,B). To establish GH or TGF-β–dependent apoptosis signaling in vivo, control mice were injected with GH or TGF-β followed by protein and mRNA analyses. Whereas GH treatment of control mice induced caspase-3 activation and expression

of Nox4, Puma, and Bim, no such increase was observed in the absence of GH (Supporting Fig. 14A). TGF-β treatment of control mice, but not experimental mice, induced caspase-3 activation and expression of Nox4, Puma, and Bim mRNA Selleck Gefitinib levels (Supporting Fig. 14B). This finding suggests that caspase-3 activation and expression of Puma and Bim by GH or TGF-β treatment induced apoptosis by STAT5/NOX4. While in many cell types the transcription factor STAT5 provides proliferative and survival cues by activating respective genetic programs, it serves as a bona fide tumor suppressor in liver tissue.3, 25 Loss of STAT5 from liver tissue leads to hepatosteatosis and the development of HCC upon CCl4 treatment.

STAT5′s function as tumor suppressor can be attributed in part to its ability to regulate the cell XL765 datasheet cycle control genes Cdkn2b and Cdkn1a.25 In addition, the presence of STAT5 also suppresses inappropriate cytokine-induced activation of STAT3, an oncoprotein in its own right. We now provide evidence for additional venues used by STAT5 to control cell death and thus suppress the development of HCC. Whereas CCl4 exposure is required to induce HCC in 3-month-old liver-specific see more Stat5-null mice, 17-month-old mice develop HCC in the absence of this chemical insult. Thus, loss of STAT5 by itself is sufficient to fundamentally alter cellular metabolism conducive to disease development. In this study, we have identified and investigated additional STAT5 target genes whose deregulation

likely contribute to the development of HCC in the absence of STAT5. Notably, STAT5 controls ROS production through the activation of the Nox4 gene and it activates the genes encoding the proapoptotic and tumor suppressive proteins PUMA and BIM. We therefore propose that STAT5 protects hepatocytes through several pathways, including the activation of cell death programs executed by NOX4, PUMA, and BIM. Studies on mice from which the genes encoding NOX4, PUMA, and BIM had been deleted, as well as tissue culture cells expressing reduced levels of these proteins, provided sound evidence for these proteins in cell death programs. In hepatocytes, NOX4 is required for TGF-β–induced apoptosis19 and loss of NOX4 from lung epithelium is protective from TGF-β–induced apoptosis.26 In heart tissue, NOX4 protected cells from pressure overload–induced apoptosis.