Identification of a Klf4-dependent upstream repressor region mediating transcriptional regulation of the myocardin gene in human smooth muscle cells

Phenotypic switching of smooth muscle cells (SMCs) plays a central role in the development of vascular diseases such as atherosclerosis and restenosis. However, the factors regulating expression of the human myocardin (Myocd) gene, the master gene regulator of SMC differentiation, have yet to be identified. In this study, we sought to identify the critical factors regulating Myocd expression in human SMCs. Using deletion/genetic reporter analyses, an upstream repressor region (URR) was localised within the Myocd promoter, herein termed PrmM. Bioinformatic analysis revealed three evolutionary conserved Klf4 sites within the URR and disruption of those elements led to substantial increases in PrmM-directed gene expression. Furthermore, ectopic expression established that Klf4 significantly decreased Myocd mRNA levels and PrmM-directed gene expression while electrophoretic mobility shift assays and chromatin immunoprecipitation (ChIP) assays confirmed specific binding of endogenous Klf4, and not Klf5 or Klf2, to the URR of PrmM. Platelet-derived growth factor BB (PDGF-BB), a potent inhibitor of SMC differentiation, reduced Myocd mRNA levels and PrmM-directed gene expression in SMCs. A PDGF-BB-responsive region (PRR) was also identified within PrmM, overlapping with the previously identified URR, where either siRNA knockdown of Klf4 or the combined disruption of the Klf4 elements completely abolished PDGF-BB-mediated repression of PrmM-directed gene expression in SMCs. Moreover, ChIP analysis established that PDGF-BB-induced repression of Myocd gene expression is most likely regulated by enhanced binding of Klf4 and Klf5 to a lesser extent, to the PRR of PrmM. Taken together, these data provide critical insights into the transcriptional regulation of the Myocd gene in vascular SMCs, including during SMC differentiation.

Feedback regulation of telomerase reverse transcriptase: new insight into the evolving field of telomerase in cancer

Telomerase reverse transcriptase (TERT) is the catalytic component of telomerase, especially the rate-limiting determinant of telomerase activity. So far, TERT has been reported to be over-expressed in more than 90% of cancers, thereby playing a critical role in sustained proliferation and survival potentials of various cancer cells. Over the past decade, a comprehensive network of transcription factors has been shown to be involved in the regulation of TERT. Furthermore, accumulating evidence has suggested that TERT could modulate the expression of numerous genes involved in diverse group of cellular processes, including cell cycle regulation and cellular signaling. Therefore, it indicates that TERT is both an effector and a regulator in carcinoma. However, the mechanisms of the interaction between TERT and its target genes are still not fully understood. Thus, it is necessary to consolidate and summarize recent developments of the cross-talk between TERT and related genes in cancer cells or other cells with cancer cell characteristics, and elucidate these relevant mechanisms. In this review, we focus on various signaling pathways and genes that participate in the feedback regulation of TERT and the underlying feedback loop mechanism of TERT, further providing new insights into non-telomeric functions of telomerase and potentially to be used as a novel therapeutic target for cancer.