Abstract 63: Id4 acts as a tumor suppressor by inducing apoptosis and sensitizing prostate cancer cells to chemotherapeutic agent doxorubicin

Abstract

Abstract PURPOSE:Id proteins (Id1, Id2, Id3, and Id4) function as dominant negative regulators of basic helix loop helix transcription factor family. They share the conserved HLH domain but lack the DNA binding basic domain. This domain configuration allows the Id proteins to dimerize with bHLH proteins. The Id-bHLH dimer therefore fails to bind the E box consensus sequence and activate transcription. In general Id1 and Id3 promote proliferation and inhibit differentiation and expression is generally high in many cancers. The function of Id4 appears to be unique as compared to Id1-Id3. Recent studies have demonstrated that Id4 is epigenetically silenced in many cancers including prostate cancer, suggesting that Id4 may in fact act as a tumor suppressor. The broad focus of this study is to investigate how Id4 acts as a tumor suppressor using prostate cancer cell line DU145, in which Id4 is epigenetically silenced. We report that at least two distinct mechanisms by which Id4 acts as a tumor suppressor is by inducing apoptosis and senescence in DU145 cells. Based on these observations, we hypothesize that Id4 will also sensitize prostate cancer cells to chemotherapeutic agents, such as doxorubicin. DESIGN METHODS: Apoptosis was quantitated using Annexin V staining in DU145 cells in which Id4 was ectopically expressed. We also used Mitocasp, which measures Caspase 3/7 activity and mitochondrial membrane potential for cytochrome c release. ηeta-galactosidase staining was used to confirm the senescent cells. Western blotting and other proteomics based approaches were used to determine the levels of apoptotic and senescence markers (Vimentin, Id1, MYC, MDM2, p16, and p21). Lastly, we treated prostate cancer cells with doxorubicin to investigate if Id4 promotes sensitivity to doxorubicin induced apoptosis. RESULTS Apoptosis assay indicated that over-expression of Id4 induced senescence and apoptosis in DU145 cells. At mRNA and protein levels, p21 and vimentin expression was increased. The expression of Id1, c-MYC, and MDM2 were decreased in DU145+Id4 cell line as compared to DU145. Our results also demonstrated that Id4 sensitizes DU145+Id4 cells to doxorubicin induced apoptosis that was Caspase 3/7 and cytochrome c dependent. CONCLUSIONS: Id4 may act as a tumor suppressor by inducing senescence and apoptosis in DU145 cells. Id4 not only induces apoptosis in prostate cancer cell line DU145, but also sensitizes these cells to doxorubicin. Id4 expression can thus be used as a clinical biomarker for prostate cancer progression and determining sensitivity to chemotherapeutic treatments. NIH/NCRR/RCMI grant #G12RR03062, NIH grant #R01CA128914 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 63. doi:1538-7445.AM2012-63