The Serum Reponse Factor (SRF) is involved in the transcriptional upregulation of the functional p53 inhibitor MDM 4 in hepatocellular carcinoma

MDM 4 is a p53 binding protein that negatively regulates p53-dependent transcription. MDM 4 was found overexpressed in a variety of human cancers and amplifications of the MDM 4 gene locus were reported most of these tumor entities. We have previously demonstrated that MDM 4 is upregulated in about 50% of human hepatocellular carcinomas (HCC), both at messenger RNA (mRNA) and protein level. Although we identified 1q32 gains as a mechanism leading to MDM 4 overexpression in human HCC, many liver tumors display a balanced MDM 4 gene locus. Thus, additional mechanisms are responsible for aberrant MDM 4 expression in the presence of a diploid MDM 4 gene locus. The transcriptional regulation of MDM 4 remains largely elusive. Here, we show that overexpression of SRF is involved in the transcriptional upregulation of MDM 4 in HCC thereby supporting its protumorigenic function.

Using in silico analysis we identified a putative SRF binding site in the MDM 4 promoter region. Overexpression of SRF mRNA compared to normal liver tissue was observed in a collection of human HCCs. In addition, a strong positive correlation between MDM 4 and SRF expression was found in these samples. To test our hypothesis that SRF might play a role in the regulation of MDM 4 expression, we treated HCC cell lines with specific siRNA targeting SRF resulting in a significant decrease of MDM 4 mRNA and protein levels and the re-activation of p53 target genes (e.g. PUMA, BAX and p21) in transfected HCC cell lines. Furthermore functional re-activation of p53 function was indicated by decreased cell viability following SRF knockdown. Finally, upregulation of MDM 4 was observed in SRF-VP16 transgenic mice in vivo.

Taken together, our data demonstrate a crucial role of SRF in the transcriptional activation of MDM 4 expression, thereby supporting the oncogenic MDM 4 activity in HCC in vitro and in vivo.