Mechanisms And Roles Of BMP4 In Mediating Adriamycin-induced Premature Senececence In Lung Cancer Cells

Cell senescence, an irreversible cell cycle arrest, is a state of permanent growth arrest in which cells are refractory to mitogenic stimuli. There were two distinct cellular senescence programs, i.e., the replicative and the premature senescence. Premature senescence is more rapid and effective to restrain the capacity of uncontrolled cell proliferation than the replicative senescence is. So far, treatment of tumor cells with certain chemotherapeutic agents activates premature senescence to decrease the tumorigenecity. Adriamycin is an important chemotherapy drug, but its mechanism of tumor suppression is not yet entirely understood. In this thesis, our experiments showed that sublethal concentrations of Adriamycin could induce premature senescence in lung cancer cells. Adriamycin treatment resulted in the upregulation of BMP4, which is under-expressed in NSCLC (non small cell lung cancers). Then we used RNAi approach to knock down BMP4 expression, and we found that suppression of endogenous BMP4 expression restrained the Adriamycin-induced senescence, eliciting the critical role of BMP4 in mediating Adriamycin-induced senescence. To study how BMP4 mediates Adriamycin-induced premature senescence, we constructed a stably transfected cell line that can be induced to express BMP4 protein by doxycycline (DOX). Our results suggested that Smad and p38 pathways activated by BMP4 played important roles in modulating Adriamycin-induced premature senescence. Moreover, our results also showed that over-expression of BMP4 was able to induce premature senescence in lung cancer cells and this process required the participation of cyclin/cyclin-dependent kinase (cdk) inhibitors p16^INK4a and p21^WAF1/cip1. We also showed that increases of p16^INK4a and p21^WAF1/cip1 expression in response to BMP4 were mediated by the Smad signaling pathway. Western blotting and chromatin immunoprecipitation (ChIP) experiments demonstrated that BMP4 upregulated p16^INK4a and p21^WAF1/cip1 expression through promoting the binding of the downstream transcription factors Smad1/5/8 to the defined promoter regions. Furthermore, our data revealed that p300 was recruited to p16^INK4a and p21^WAF1/cip1 promoters by Smad1/5/8 to induce the hyperacetylation of histones H3 and H4 at the promoters. The present study provides useful clues to the evaluation of the potentiality of BMP4 as a responsive molecular target for cancer chemotherapy.