ID4 AS A TUMOR SUPPRESSOR: MECHANISM OF ACTION OF ID4 IN PROSTATE CANCER

Id proteins are members ofbasic helix-loop-helix family. However, Id proteins lack the basic binding domain, which prevents DNA binding, and thereby regulates transcription. There are four members in the Id protein family termed Idl-4. In prostate cancer the expression of Idl and Id3 is high, whereas member Id4 expression is low. Decreased expression of Id4 is due to promoter hypermethylation in prostate cancer as well as many other cancers. This observation led us to hypothesize that Id4 acts as a tumor suppressor in prostate cancer. Furthermore, evidence suggests ectopic Id4 expression in metastatic prostate cancer cell line DU145 induced cell cycle arrest, apoptosis, and senescence. In this study, we expanded on these earlier studies to demonstrate that gain of Id4 attenuates cancer phenotype whereas loss of Id4 promotes cancer phenotype in prostate cancer cell lines DU145 and LNCaP, respectively. Upon over-expression of Id4 in DU145 cells (DU145+W4), there was an increase in apoptosis, due to decreased mitochondrial membrane potential (MMP) and increased expression of 1 pro-apoptotic markers (PUMA, BAX, and p21). Inversely, silencing of Id4 in LNCa