Id-1 mediated down-regulation of Bax and up-regulation of Bcl-2 expression are essential for its protective role against taxol-induced apoptosis in breast cancer cells

Abstract

Inhibitor of differentiation and DNA binding-1 (Id-1) inhibits the activity of basic helix-loop-helix transcription factors, and 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 cell line MCF-7 by ectopically overexpressing or inactivating Id-1 by stable transfection or retroviral infection with an antisense construct, respectively. In MCF-7 cells stably overexpressing Id-1, cell proliferation was induced via the activation of ERK signaling pathway. At the protein level, the expression of epidermal growth factor receptor (EGFR), ErbB2 (her-2/neu), early growth response factor-1 (Egr-1) were enhanced by Id-1 as confirmed by western analysis. Upon Id-1 overexpression, the expression levels of several key regulators in cell survival were changed. Nuclear factor-κB (NF-κB) expression was augmented, while those of p53 and IKK were reduced. Furthermore, the expression of several apoptotic regulators was also affected. Bax expression was significantly reduced while Bcl-2 was augmented. However, the expression of Bcl-xL and c-Myc remained unchanged. Finally, Id-1 played a protective role against taxol-induced apoptosis in breast cancer cells. The reduced expression of Bax by Id-1 was further reduced upon taxol treatment. Bcl-2 expression was reduced by taxol in Id-1 overexpressing cells, but the overall amount was equivalent to the level of vehicle-treated control (MCF-7/neo) cells. Such changes in Id-1-expressing cells suggest that the Id-1-induced cellular protection against anticancer-induced apoptosis in breast cancer may be mediated through augmented Bcl-2/Bax ratio. Taken together, we report that in breast cancer cell line MCF-7, Id-1 regulates cell proliferation through activation of ERK signaling pathway, promotes cell survival through activation of p53 and NF-κB signaling pathway, and suppresses cell apoptosis through down-regulation of Bax and up-regulation of Bcl-2. In this regard, inactivation of Id-1 may provide a potential therapeutic strategy leading to inhibition of breast cancer progression.