Id-1 regulates Bax and Bcl-2 expression through p53 and NF-κB signaling pathways in breast cancer cells

Abstract

Inhibitor of differentiation and DNA binding-1 (Id-1) is an import regulator of cell proliferation and tumorigenesis in many types of human cancers. Increasing evidence supports the function of the Id-1 as a survival factor, but its molecular mechanism has not been elucidated. Therefore, we tested the role of Id-1 in breast cancer by establishing a stable MCF7 cell lines. In cells stably overexpressing Id-1, cell growth tested in serum-free condition was enhanced. The overall cell number was augmented and the expression of several apoptotic regulators was affected by Id-1. Upon quantifying the expression levels of known apoptotic regulators, significantly reduced Bax expression and enhanced Bcl-2 expression, because such reduction and enhancement was reversed upon reduced Id-1 expression by retroviral antisense expression. Upon Id-1 overexpression, the expression levels of several key regulators of Bax and Bcl-2 were changed. Expression of p53 was reduced and such changes were also functional, as p21, a downstream regulator of p53, was reduced. NF-κB expression was augmented and was accompanied by inhibition of IκB and nuclear translocation of p65 that resulted in the induction of NF-κB downstream gene by Id-1. Furthermore, changes in the NF-κB pathway were also functional when tested with an artificial reporter with NF-κB responsive elements or with Il-6 promoter. When tested by real time-PCR, mRNA levels of Bcl-2 were enhanced in Id-1-expressing cells accompanied by enhanced promoter activity. Such enhancement was reversed in a dose-dependent manner with p53 or dominant-negative IκB expression. Suppressed Bax promoter activity by Id-1 was reversed in a dose-dependent manner with p53 expression. However, Id-1 mediated changes in NF-κB pathway were not effective on Bax suppression. Finally, Id-1 played a protective role against taxol-induced apoptosis in breast cancer cells. The reduced expression of p53 by Id-1 was further reduced upon taxol treatment. NF-κB expression was enhanced by taxol in Id-1 overexpressing cells, but the overall amount was low in vehicle-treated control cells. Taken together, we present a molecular mechanism where Id-1 regulates p53 and NF-κB pathways, which in turn regulates Bax and Bcl-2 genes, thus providing a survival advantage under exogenous stress such as serum-free or taxol treatment in breast cancer cells. In this regard, inactivation of Id-1 may provide a potential therapeutic strategy leading to inhibition of breast cancer progression and anti-cancer drug resistance.