Evidence of a novel anti-apoptotic factor: Role of Id-1 in anticancer drug-induced apoptosis

Abstract

Id-1 (inhibitor of differentiation or DNA binding), a member of the basic helix-loop-helix (HLH) transcription factor family, is upregulated in many types of human cancer and its expression levels are correlated with poor treatment outcome and shorter survival. To study if Id-1 plays a positive role in protecting cancer cells from programmed cell death, we studied the role of Id-1 on five cancer cell lines derived from nasopharyngeal carcinoma (CNE1), cervical carcinoma (HeLa), breast cancer (MCF7), hepatocarcinoma (Huh7) and prostate cancer (PC3), all of which have been shown overexpression of Id-1 in clinical specimens. Nine commonly used anticancer drugs including two DNA damaging agents (cisplatin, mechlorethamine), one antibiotic (mitomycin C), two topoisomerase II inhibitors (doxorubicin, etoposide), one anti-metabolite (methotrexate), two microtubule disrupting agents (taxol, vincristine), and a green tea extract, epigallocatechin gallate (EGCG), were used to examine the association between Id-1 expression and the anticancer drug-induced apoptosis. After exposured to six types of anticancer agents, all of the cell lines showed a dose dependent downregulation of the Id-1 protein. In addition, the increased PARP cleavage was associated with increased TUNEL positive cells and decreased cell viability. The expression of cleaved PARP and percentage of TUNEL positive cells were suppressed significantly after treated with a caspase 9 inhibitor, Z-LEHD-FMK, though the expression of Id-1 was decreased after drug treatment, which suggested that apoptosis may be the result of Id-1 downregulation by anticancer drug treatment. To investigate if overexpression of Id-1 could protect cancer cells from apoptosis against all nine anticancer drugs, we treated two cell lines with high (CNE1 Id-1) and low (CNE1 pBabe) levels of Id-1 expression and found that overexpression of Id-1 was able to suppress PARP cleavage in response to all anticancer drugs examined. These results were reversed when we suppressed Id-1 expression in CNE1 cells through small RNA interference. We found that downregulation of Id-1 through small RNA technology in CNE1 cells led to increased sensitivity to all six types of chemotherapeutic drugs. In summary, our results demonstrate that Id-1 may be a general negative regulator of anticancer drug-induced apoptosis and suggest a novel therapeutic target in inducing chemosensitization in cancer cells. Our evidence also provides a possible underlying mechanism responsible for the positive role of Id-1 in the progression of human cancer.