Recent evidence has accumulated that the dynamic histone methylation mediated by histone methyltransferases and demethylases plays key roles in regulation of chromatin structure and transcription. In the present study, we show that SET and MYND domain-containing protein 3 (SMYD3), a histone methyltransferase implicated in oncogenesis, directly trans-activates the telomerase reverse transcriptase (hTERT) gene that is essential for cellular immortalization and transformation. SMYD3 occupies its binding motifs on the hTERT promoter and is required for maintenance of histone H3-K4 trimethylation, thereby contributing to inducible and constitutive hTERT expression in normal and malignant human cells. Knocking down SMYD3 in tumor cells abolished trimethylation of H3-K4, attenuated the occupancy by the transactivators c-MYC and Sp1, and led to diminished histone H3 acetylation in the hTERT promoter region, which was coupled with down-regulation of hTERT mRNA and telomerase activity. These results suggest that SMYD3-mediated trimethylation of H3-K4 functions as a licensing element for subsequent transcription factor binding to the hTERT promoter. The present findings provide significant insights into regulatory mechanisms of hTERT/telomerase expression; moreover, identification of the hTERT gene as a direct target of SMYD3 contributes to a better understanding of SMYD3-mediated cellular transformation. [Cancer Res 2007;67(6):2626-31]

Hypoxia-inducible factor-1A (HIF-1A) has been implicated in the transcriptional regulation of the telomerase reverse transcriptase (hTERT) gene expression and telomerase activity, essential elements for cellular immortalization and transformation. However, controversial results were obtained in different studies. Moreover, it is totally unclear whether HIF-2A, the paralog of HIF-1A, plays a role in regulating hTERT expression. In the present study, we found that hypoxic treatment enhanced hTERT mRNA expression and telomerase activity in three renal cell carcinoma (RCC) cell lines with different genetic backgrounds. Both HIF-1A and HIF-2A were capable of significantly increasing the hTERT promoter activity in these cells. Moreover, depleting HIF-2A led to a down-regulation of hTERT mRNA level in RCC A498 cells expressing constitutive HIF-2A. It was found that HIF-2A bound to the hTERT proximal promoter and enhanced the recruitment of the histone acetyltransferase p300 and histone H3 acetylation locally in A498 cells treated with hypoxia. Increased levels of hTERT mRNA were observed in two of three hypoxia-treated malignant glioma cell lines. However, HIF-1A stimulated whereas HIF-2A inhibited the hTERT promoter activity in these glioma cell lines. Ectopic expression of HIF-2A resulted in diminished hTERT expression in glioma cells. Collectively, HIF-1A activates hTERT and telomerase expression in both RCC and glioma cells, and HIF-2A enhances hTERT expression in RCC cells, whereas it represses the hTERT transcription in glioma cells.

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