Function and Regulation of Phosphatidylinositol 3,4,5-Triphosphate-Dependent Rac Exchanger 1 (P-Rex1) in Cancer
Up-regulated phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 1 (P-Rex1) promotes tumorigenesis and metastasis of various cancers, and thus could be a potential therapeutic target. The first objective of my study was to determine the molecular mechanism regulating P-Rex1 gene expression. We found that P-Rex1 expression was much higher in metastatic prostate cancer cells than in prostate epithelial cells and non-metastatic prostate cancer cells. Histone deacetylase (HDAC) inhibitors or silence of endogenous HDAC1 and HDAC2 markedly elevated P-Rex1 transcription in non-metastatic prostate cancer cells, whereas overexpression of recombinant HDAC1 in metastatic prostate cancer cells suppressed P-Rex1 expression. HDAC inhibitor trichostatin A (TSA) also significantly increased P-Rex1 promoter activity and caused acetylated histones to accumulate and associate with the P-Rex1 promoter. One Sp1 site, essential for basal promoter activity, was identified as critical for the TSA effect. TSA treatment did not alter the DNA-binding activity of Sp1 toward the P-Rex1 promoter; however, it facilitated the dissociation of the repressive HDAC1 and HDAC2 from the Sp1 binding region. Interestingly, HDAC1 association with Sp1 and with the P-Rex1 promoter was much weaker in metastatic prostate cancer PC-3 cells than in non-metastatic prostate cancer cells, and HDAC inhibitors only had very modest stimulatory effects on P-Rex1 promoter activity and P-Rex1 expression in PC-3 cells. Altogether, our studies demonstrate that HDACs could regulate P-Rex1 gene transcription by interaction with Sp1 and by region-specific changes in histone acetylation within the P-Rex1 promoter. Disassociation of HDACs from Sp1 on the P-Rex1 promoter may contribute to aberrant up-regulation of P-Rex1 in cancer. |HDAC inhibitors are promising anti-cancer drugs because they can restore expression of silenced tumor suppressor genes. However, our data caution that these inhibitors could have severe side effects by derepressing genes, such as P-Rex1, that promote tumor progression. Thus, identifying drugs that target signaling pathways involved in P-Rex1 up-regulation may provide an alternative strategy to treat various cancers. We recently found that phorbol-12-myristate-13-acetate (PMA), a protein kinase C (PKC) activator, down-regulates P-Rex1 expression in breast cancer cells. Thus, the second objective of my study was to investigate the role and mechanism underlying PMA down-regulation of P-Rex1 in breast cancer. We found that PMA treatment caused a time- and concentration-dependent decrease in P-Rex1 protein levels in breast cancer MCF-7 and BT-474 cells with a maximum reduction of 87.2 ± 1.1 and 57.0 ± 8.6%, respectively, at a concentration of 10 ng/ml. Interestingly, either silence of P-Rex1 by siRNA or PMA treatment inhibits breast cancer cell proliferation by over 70%, which can be attenuated by restoring P-Rex1 expression. MCF-7 cells express PMA-sensitive PKCα, δ, ε, and η. The PKC general inhibitor Gö6983, but not the PKCα-selective inhibitor Gö6976, blocked the PMA down-regulation of P-Rex1 expression and breast cancer cell proliferation. Overexpression of a constitutively active PKCε mutant suppressed P-Rex1 expression and breast cancer cell proliferation, which was also blocked by Gö6983 treatment. In contrast, overexpression of wild-type PKCε, wild-type or constitutively active PKCδ and PKCη had no effect. Together, our data suggest that PMA suppresses breast cancer cell proliferation via PKCε-mediated P-Rex1 down-regulation. Since P-Rex1 is highly overexpressed in human breast cancers with estrogen receptor (ER) expression, our study may provide a novel strategy for development of chemotherapeutic agents for patients with ER positive tumors that develop resistance to anti-estrogen therapies.
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