Upregulation of caveolin-1 in human hepatocellular carcinoma

Abstract

Caveolin-1 (Cav1) is the major structural protein of caveolae and a component at focal adhesions. Cav1 has been reported to be suppressed at the early stages of transformation and carcinogenesis but upregulated in advanced-stage metastasis, suggesting Cav1 as an important player in human cancers. In this study, we aim to determine the clinicopathological significance and functional roles of Cav1 in hepatocellular carcinoma (HCC) and elucidate the mechanism that underlies the upregulation of Cav1. Quantitative PCR and Western blotting analyses were performed on 14 HCC cell lines, both Cav1 mRNA and protein expressions were either undetectable or moderately expressed in non-metastatic cell lines, in contrast to the incredibly high expression in metastatic cell lines. In the physiological context, immunostaining of 14 hepatic resection tissues sets, each consists of non-tumour, tumour and the metastatic counterparts were done. Cav1 expression was not found in non-tumourous liver while progressive increase of Cav1 expression in primary tumor and extrahepatic metastatic tumor of the same patient was observed. To functionally characterize Cav1 in HCC, Cav1 knock-down stable clones were established in non-metastatic cell line SMMC-7721 and metastatic cell lines MHCC97H by lentiviral transduction using Cav1-targeting shRNA. Proliferation rate of stable clones was assessed by BrdU incorporation assay. No significant difference in proliferation rate could be observed among the SMMC-7721 stable clones, while MHCC97H Cav1 knock-down clones showed slower proliferation rate (One-way ANOVA, p=0.0121) when compared to the control. SMMC-7721 stable clones were injected subcutaneously to nude mice, tumour sizes were recorded once a week. Mice were sacrificed at 4-week post-injection, tumours were excised and weighted. Dramatic suppression of tumour formation was resulted in Cav1 knock-down stable clones. Based on our findings that Cav1 was elevated at both the transcriptional and translational levels, Cav1 promoter activity in HCC cells was examined. Reporter assay revealed that Cav1 promoter activity correlates with the Cav1 mRNA level in HCC cells. In addition, Cav1 promoter activity was transactivated by the peroxisome proliferator-activated receptor gamma (PPAR\#947;) transcription factor in a dose-dependent manner. Similar to Cav1, enhanced expression of PPAR\#947; was also observed in metastatic HCC cell lines. Taken together, our findings demonstrated the clinical relevance of Cav1 overexpression with HCC carcinogenesis and metastasis and established its oncogenic role in tumorigenesis. In addition, our results suggested that upregulation of Cav1 in cells was due to the enhanced PPAR\#947;-mediated transcriptional activation of Cav1 promoter.