Membrane Androgen Receptor Down-Regulates c-Src-Activity and Beta-Catenin Transcription and Triggers GSK-3beta-Phosphorylation in Colon Tumor Cells

Abstract

Background/Aims: Functional membrane androgen receptors (mARs) have recently been described in colon tumor cells and tissues. Their activation by specific testosterone albumin conjugates (TAC) down-regulates the PI-3K/Akt pro-survival signaling and triggers potent pro-apoptotic responses both, in vitro and in vivo. The present study explored the mAR-induced regulation of gene products implicated in the tumorigenic activity of Caco2 colon cancer cells. Methods: In Caco2 human colon cancer cells transcript levels were determined by RT-PCR, protein abundance and phosphorylation by Western blotting and confocal microscopy, as well as cytoskeletal architecture by confocal microscopy. Results: We report time dependent significant decrease in Tyr-416 phosphorylation of c-Src upon mAR activation. In line with the reported late down-regulation of the PI-3K/Akt pathway in testosterone-treated colon tumors, GSK-3beta was phosphorylated at Tyr-216 after long term stimulation of the cells with TAC, a finding supporting the role of this kinase to promote apoptosis. PCR analysis revealed significant decrease of beta-catenin and cyclin D1 transcript levels following TAC treatment. Moreover, confocal laser scanning microscopic analysis disclosed co-localization of beta-catenin with actin cytoskeleton. It is thus conceivable that beta-catenin may participate in the reported modulation of cytoskeletal dynamics in mAR stimulated Caco2 cells. Conclusions: Our results provide strong evidence that mAR activation regulates late expression and/or activity of the tumorigenic gene products c-Src, GSK-3beta, and beta-catenin thus facilitating the pro-apoptotic response in colon tumor cells.

Keywords

• mARs
• c-Src
• GSK-3beta
• Beta-catenin
• Cyclin D1
• Actin
• Colon tumors