Abstract LB-6: bHLH transcription factor Id4 plays a regulatory role in cell cycle control, apoptosis, and senescence in androgen-insensitive prostate cancer

Abstract

Abstract Id4 belongs to a family of genes that are dominant negative regulators of basic helix-loop-helix (bHLH) transcription factors. Id4 expression shows an inverse correlation to Id1-3 expression in advanced metastatic prostate cancer which act as tumor promoter genes. Recent Id4 promoter hypermethyation data suggests that Id4 may act as a tumor suppressor in prostate cancer, and loss of Id4 expression may correlate with androgen insensitive prostate cancer progression. The inhibitory role of Id4 is characterized by inhibition of cancer progression and up regulation of tumor suppressor genes. In the DU145 prostate cancer cell line, Id4 down regulation is responsible for increased cell survival, metastasis and decreased apoptosis. Id4 absent/androgen-independent prostate cancer cell line DU145 was used to ectopically express Id4 mRNA and protein. This allowed for a novel identification of the role Id4 plays in cell cycle control, apoptosis and proliferation in an advanced prostate cancer model. FACS analysis highlighted the role of Id4 in cell cycle maintenance. Apoptosis assays support the tumor suppressor role of Id4 in DU145 cell lines. Senescence assays were also conducted on in Id4 expressing DU145 cells. RT-PCR and western blot analysis were used to study gene expression of downstream apoptosis and cell cycle control genes E-cadherin, p21, p27, p53, MDM2, E2F1 and, Androgen receptor. Du145 + Id4 prostate cancer cells have significantly decreased proliferation due to an S-phase arrest possibly associated with E2F-1 down regulation. The increased expression of E-cadherin, p27, p21 and p53 strengthens the hypothesis of Id4 as a tumor suppressor by regulating key cell cycle control and apoptosis associated genes. A functional androgen receptor was also induced in this androgen insensitive prostate cancer model. Id4 ectopic expression resulted in a significant decrease in Id1 and Id3, which are known contributors to metastasis and cell survival. This suggests that Id4 may inhibit bHLH transcription factors involved in proliferation, and metastasis of prostate cancer cells. Id4 induces apoptosis whether dependent/independent of the mutated p53 gene in DU145 cells. The presence of senescent cells in Id4 transfected cell lines suggest that Id4 may also play a role in autophagic cell death. We conclude that the mechanism by which Id4 may act as a tumor suppressor is by influencing a hierarchy of cellular processes at multiple levels including, apoptosis, senescence and S-phase mediated cell cycle arrest, which results in changes to cell morphology, and androgen receptor response. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-6.