Novel tumor suppressor genes SORBS3 and SH2D4A repress IL-6 signaling in hepatocellular carcinoma

Background: Different inflammatory states, e.g. chronic hepatitis B or C viral infection, have been shown to contribute to the development of hepatocellular carcinoma (HCC). In patients with chronic liver disease the serum levels of interleukin 6 (IL-6) are elevated and increase even more when HCC develops. However, there is still not much known about the regulation of IL-6 signaling during hepatocarcinogenesis. Recently, we applied an integrative genomic and transcriptomic approach which revealed that loss of chromosome 8 p is associated with poor prognosis. In addition, we showed that the two chromosome 8 p genes SORBS3 and SH2D4A are functional tumor suppressor genes in vitro and in vivo. The goal of this study is to dissect the molecular mechanisms of SORBS3 and SH2D4A mediated tumor suppression.

Methods: Gene expression analysis was performed to identify chromosome 8 p associated signaling pathways. Furthermore, Tetracycline-inducible SORBS3 or SH2D4A expressing HCC cell lines were generated to identify potential binding partners by mass spectrometry, co-immunoprecipitation and Western blotting. The expression of target genes was measured by quantitative real time PCR. To examine whether SORBS3 and SH2D4A are able to suppress IL-6 induced STAT3 transcriptional activity, STAT3 phosphorylation status was measured and STAT3 luciferase reporter assay were performed.

Results: We found that SORBS3 and SH2D4A function in a convergent manner to inhibit IL-6 signaling. Overexpression of SORBS3 and SH2D4A leads to a decrease of IL-6 target genes Bcl-2, SPINK1 and Bcl-xL and consistently, knockdown to an increase. In addition, loss of SORBS3 mediates a decrease in SH2D4A suggesting an amplifying mechanism. Luciferase assays revealed that both, SH2D4A and SORBS3 inhibit STAT3 transcriptional activity possibly via blocking the phosphorylation of Y705 but not S727. A direct interaction could be observed between STAT3 and SORBS3.

Conclusion: Thus, the chromosome 8 p tumor suppressor genes SORBS3 and SH2D4A are physically and functionally linked and provide a molecular mechanism of inhibition of STAT3-mediated IL-6 signaling in HCC.

Corresponding author: Rössler, Stephanie

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