Abstract 3911: A positive feedback loop between TGF-β and SCF mediates TGF-β1 ligand overexpression in advanced hepatocellular carcinoma
Authors: Zhang Pingyu , Rojas Andres , Wang Ying , Munoz Nina M. , Xiao Lianchun , Wang Jing , Gores Gregory J. , Hung Mien-Chie , Blechacz Boris R. A. 
Published: August 2015

Background: The Tumor Growth Factor-β (TGF-β) pathway is a potent tumor suppressor pathway. Its inactivation has been implicated in hepatocarcinogenesis. However, in patients with advanced stage Hepatocellular Carcinoma (HCC), TGF-β1 serum concentrations are elevated and correlated to tumor size, recurrence and shortened survival. The mechanisms mediating TGF-β1 ligand overexpression in HCC are unknown.

Material and Methods: Immunohistochemical analysis of human HCC samples was performed using Tissue Micro Array. Established human HCC cell lines were used for in vitro studies. Gene expression was assessed by qRT-PCR, ELISA and immunoblot analysis. shRNA was used for gene silencing. Transcription factor DNA binding was evaluated by chromatin immunoprecipitation. For promoter analysis, transcription factor binding site containing DNA fragments were cloned into the enhancer or promoter position of luciferase expression vectors and subsequent transfected.

Results: In up to 68% of advanced stage HCC tumors Stem Cell Factor (SCF) is overexpressed, and SMAD2 and STAT3 are constitutively activated. SCF expression in human HCC tumors is intratumorally and intracellularly correlated to expression of pSer465/467SMAD2 and pTyr705STAT3. In HCC cell lines, SCF stimulation induces TGF-β1 transcription, expression and secretion, all of which are abrogated by genetic silencing of STAT3. TGF-β1 stimulation of human HCC cells induces TGF-β1 transcription, and genetic silencing of SCF prevents TGF-β1 autoregulation. We identified two STAT3 recognition sites within the TGF-β1 gene. In human HCC cells, TGF-β1 stimulation results in STAT3 binding to one of the identified recognition sites. TGF-β1 activates luciferase expression vectors under control of the corresponding STAT3 binding site but not in its absence. TGF-β1-induced STAT3 binding and expression vector activation is abrogated in HCC cells with genetically silenced SCF. TGF-β1 induces SCF expression but SMAD2 knockdown inhibits TGF-β1-induced SCF transcription and expression. We identified 7 putative SMAD2-binding elements (SBE) within the SCF promoter. TGF-β1 stimulation results in SMAD2 binding to the SCF promoter and activation of luciferase expression vectors regulated by the SCF promoter; conversely, genetic silencing of SMAD2 abrogates TGF-β1-induced SCF promoter activation. Importantly, knockdown of SCF restored the antiproliferative functions of TGF-β, and prevented TGF-β-induced tumor cell migration and invasion.

Conclusion: In advanced stage HCC, SCF drives TGF-β1 autoregulation through STAT3-mediated regulation of the TGF-β1 promoter. Thereby, SCF and TGF-β1 form a positive feedback loop that functionally switches the TGF-β pathway from a tumor suppressor to a tumor promoter in advanced stage HCC.

Citation Format: Pingyu Zhang, Andres Rojas, Ying Wang, Nina M. Munoz, Lianchun Xiao, Jing Wang, Gregory J. Gores, Mien-Chie Hung, Boris R. A. Blechacz. A positive feedback loop between TGF-β and SCF mediates TGF-β1 ligand overexpression in advanced hepatocellular carcinoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3911. doi:10.1158/1538-7445.AM2015-3911