The helix-loop-helix transcription factor Id-1 promotes cellular proliferation and survival in esophageal carcinoma cells

Abstract

Id proteins (inhibitor of differentiation or DNA binding) are a group of helix-loop-helix (HLH) transcription factors that can dimerize with basic HLH transcription factors and prevent them from binding to DNA, thus inhibiting the transcription of differentiation associated genes. The ability of Id proteins in stimulating DNAsynthesis and immortalizing mammalian cells either alone or together with additional oncogenes suggests that they may function as potential oncogenes. Although upregulation of Id-1 has been reported for over 20 types of human cancers, including esophageal squamous cell carcinoma (ESCC), its functional role in the molecular pathogenesis of esophageal cancer and the signaling pathways involved are unknown. In this study, we investigated the direct effects of Id-1 on esophageal cancer cell growth, cell cycle distribution and drug-induced apoptosis by transfecting an Id-1 expression vector into an ESCC cell line which showed serum-dependent Id-1 expression to generate stable Id-1 expressing clones. We found that ectopic Id-1 expression increased both cellular proliferation rate and G1-S phase transition in the transfectant clones cultured in serum-free medium. The effects were associated with overexpression of MDM2 (mouse double minute 2) oncoprotein and suppression of p21WAF1 protein, but did not seem to affect CDK4, p16INK4A, pRB and p53 expressions. In addition, Id-1 expression protected ESCC cells from TNF-α-induced apoptosis by up-regulating and activating Bcl-2. Conversely, stable transfection of Id-1 antisense expression vector to inhibit the expression of endogenous Id-1 in another ESCC cell line with serum-independent Id-1 expression reversed the effects on cell growth and expressions of MDM2 and p21WAF1 proteins. In conclusion, our study provides evidence that Id-1 may function as an oncogene in ESCC, and that its effects on esophageal cancer cell proliferation appeared to be mediated through up-regulation of MDM2 and down-regulation of p21WAF1, rather than through inactivation of the p16INK4A/RB pathway as reported in prostate, nasopharyngeal and hepatocellular cancers.