Role of menin in the regulation of telomerase activity in normal and cancer cells

Transcriptional activation of human telomerase reverse transcriptase (hTERT) is critical for telomerase expression, a major step for cellular immortality and carcinogenesis. Although several transcriptional activators have been identified, factors responsible for repressing the hTERT promoter are largely unknown. Gene screening that employed enhanced retroviral mutagenesis has identified potential hTERT repressors. Among these, menin, which is a tumor suppressor and a gene product of MEN1, has been reported to play a critical role. In the present study, we further analyzed menin's role in the transcriptional regulation of hTERT in normal and cancer cells. Luciferase reporter assays that use the hTERT promoter have demonstrated that an overexpression of menin decreases the transcriptional activity of the hTERT gene in a cell-type specific manner. Mutation and deletion analyses of the hTERT promoter demonstrated that there was no specific site on the promoter that was responsible for the menin-mediated transcriptional inhibition. An electrophoretic mobility shift assay using recombinant menin protein generated the binding complexes with the hTERT promoter, which was completely diminished by the addition of poly-dI-dC. This indicates that there is a sequence-independent binding of menin. RT-PCR assays have revealed that overexpression of menin inhibits hTERT mRNA expression in some cell types, although this inhibition does not lead to a significant down-regulation of telomerase activity. In cancer cell lines and in normal cells, the siRNA-based inhibition of MEN1 does not lead to the up-regulation of hTERT mRNA expression. No significant correlation has been found between menin and hTERT mRNA expressions in a variety of cancer cell lines and clinical tissue samples. Thus, while menin appears to have some inhibitory effects on the hTERT promoter, possibly via the sequence-independent binding to the promoter, the present study does not support the hypothesis that menin has a crucial role in the determination of telomerase activity in normal and cancer cells.