Small-molecule-based identification of dynamic assembly of E2F-pocket protein-histone deacetylase complex for telomerase regulation in human cells

Abstract

Activation of telomerase is crucial for cells to gain immortality. Most normal human somatic cells have a limited proliferative life span, and expression of the rate-limiting telomerase catalytic subunit, known as human telomerase reverse transcriptase (hTERT), has been believed to be tightly repressed. This model of hTERT regulation is challenged by the recent identification of the induction of hTERT in normal cycling human fibroblasts during their transit through S phase. Here we show the small-molecule-based identification of the assembly and disassembly of E2F-pocket protein-histone deacetylase (HDAC) complex as a key mechanistic basis for the repression and activation of hTERT in normal human cells. A cell-based chemical screen was used to identify a small molecule, CGK1026, that derepresses hTERT expression. CGK1026 inhibits the recruitment of HDAC into E2F-pocket protein complexes assembled on the hTERT promoter. Chromatin immunoprecipitation analysis reveals dynamic alterations in hTERT promoter occupancy by E2F and pocket proteins according to the cell cycle-dependent regulation of hTERT. Dominant-negative or protein-knockout strategies to disrupt the assembly of E2F-pocket protein-HDAC complex derepress hTERT and telomerase activity. Taken together with the results on the regulatory function of these complexes in cellular senescence and tumorigenesis, our findings suggest that dynamic assembly of E2F-pocket protein-HDAC complex plays a central role in the regulation of hTERT in a variety of proliferative conditions (e.g., normal cycling, senescent, and tumor cells).