Dual regulation of telomerase activity through c-Myc-dependent inhibition and alternative splicing of hTERT.

Telomerase is believed to be induced upon proliferation and inhibited when cells differentiate. Thus, regulation of telomerase activity could be an important mechanism to limit growth of normal and cancer cells. Using transforming growth factor-beta 1 (TGF-beta1), which is known to control proliferation in epithelial cells, we now demonstrate that in the human HaCaT skin keratinocytes, TGF-beta1 downregulated c-Myc, and this blocked proliferation. This also caused a decrease in hTERT expression, which in turn inhibited telomerase activity. Overexpressing hTERT recovered telomerase activity but not proliferation, whereas constitutive expression of c-Myc recovered proliferation and hTERT expression. Nevertheless, telomerase remained inhibited, thus dissociating proliferation and telomerase activity. In addition, we show that TGF-beta1 inhibited telomerase activity despite ongoing hTERT transcription by inducing loss of the full-length hTERT transcript (mediating telomerase activity) and retaining high expression of the inactive beta variant. These changes in the splicing pattern reversed upon TGF-beta1 removal, as did inhibition of telomerase activity, suggesting that alternative splicing may represent a novel mechanism of telomerase regulation by TGF-beta1. In addition, we show that destruction of tissue integrity (in a model for epidermal blistering) resulted in a rapid induction of the inactive beta variant, whereas tissue regeneration (formation of a stratified epithelium) correlated with a shift to the active full-length transcript, which is the dominant form in intact epidermis. Thus alternative splicing may not be restricted to TGF-beta1 but may add a more general mechanism of hTERT regulation in epidermal cells.