Molecular and functional characterization of human stanniocalcins

Stanniocalcin-1 (STC1) is a glycoprotein hormone originally discovered in teleost fishes. The hormone is known to regulate plasma Ca2+ homeostasis in fishes. In the last decade, mammalian STC1 and STC2 were cloned and functionally characterized. Different from the fish story, the mammalian hormones are found to be widely expressed in different tissues and only play a minimum role in mineral homeostasis. Recently multiple lines of evidence have highlighted the involvement of human STC1 and STC2 in cancer development. Moreover the transcriptional regulations as well as the biological functions of the hormones are largely not known.;Cancer development usually associates with an aberrant epigenetic regulation, leading to the "loss-of-function" or "gain-of-function" of tumor-suppressor genes or oncogenes. Our data demonstrated that DNA methylation and/or histone modification are involved in the regulation of STC1 and STC2 expression in various human cancer cell-lines. STC1 expression can be stimulated by the treatment with different histone deacetylase inhibitors. The stimulation of STC1 expression was found to be facilitated by both chromatin relaxation as well as the binding of the transcriptional factor NFkappaB to STC1 promoter. Subsequent study also revealed that the removal of the transcriptional repressor Sp1 can potentiate TSA-induced STC1 expression. Conversely CpG methylation was found to be important in silencing STC2 gene expression. Treatment of the human cancer cells with the demethylating agents, (i.e. 5-aza-CdR and hydralazine) can restore the basal and hypoxia-inducible factor-1 (HIF-1) mediated STC2 expression. Among different tumor microenvironments, hypoxia is a critical factor to induce the transformation of malignant cancer cells. Considerable numbers of reports have demonstrated that hypoxia activated the expressions of both STC1 and STC2. Direct evidence on HIF-1 transactivation of STC1 and STC2 genes however were lacking. To address this issue, we investigated the interaction between HIF-1 and the cis-acting elements at the promoter regions of STC1 and STC2. Using deleted promoter constructs, site-directed mutagenesis and ChIP/re-ChIP assays, the direct transactivation role of HIF-1 in the regulation of STC1 and STC2 expressions were elucidated. HIF-1 transactivation of STC1 gene was found to be p300 dependent and was regulated by a cofactor, factor inhibiting HIF-1 (FIH). For STC2 gene activation, HIF-1 transactivation required the interaction with both p300 and HDAC7. The data reveals the unequivocal evidence to prove that both human STC1 and STC2 are new HIF-1 target genes. In addition to the transcriptional regulation, our data revealed the possible biological functions of STC1 and STC2. Using human colon cancer cells HT29, we demonstrated that STC1 was a negative regulator of NFkappaB, to assist apoptosis. Silencing of STC1 significantly reduced the TSA mediated apoptosis in the cells. On the other hand, using ovarian and breast cancer cell models (i.e. SKOV3 and MCF7) STC2 was found to play a role in cell proliferation at hypoxic condition. Our data illustrated that hypoxia-induced cell cycle arrest can be relieved by STC2 overexpression. In contrast, targeting knockdown of STC2 increased cell arrest at G1 or G1-S border. The observations suggest the positive regulatory role of STC2 in tumor progression. Collectively, this study provides a solid scientific basis for our understanding of the transcriptional regulation and the biological functions of STC1 and STC2 in human carcinogenesis.