Comprehensive proteomic profiling identifies the androgen receptor axis and other signaling pathways as targets of microRNAs suppressed in metastatic prostate cancer

MicroRNAs are important epigenetic regulators of protein expression by triggering degradation of target mRNAs and/or inhibiting their translation. Dysregulation of microRNA expression has been reported in several cancers, including prostate cancer (PC). We comprehensively characterized the proteomic footprint of a panel of 12 microRNAs that are potently suppressed in metastatic PC (SiM-miRNAs: miR-1, miR-133a, miR-133b, miR-135a, miR-143-3p, miR-145-3p, miR-205, miR-221-3p, miR-221-5p, miR-222-3p, miR-24-1-5p, and miR-31) using reverse-phase proteomic arrays. Re-expression of these SiM-miRNAs in PC cells suppressed cell proliferation and targeted key oncogenic pathways, including cell cycle, apoptosis, Akt/mammalian target of rapamycin signaling, metastasis and the androgen receptor (AR) axis. However, only 12%, at most, of these observed protein expression changes could be explained by predicted direct binding of miRNAs to corresponding mRNAs, suggesting that the majority of these proteomic effects result indirectly. AR and its steroid receptor coactivators (SRCs; SRC-1, -2 and -3) were recurrently affected by these SiM-miRNAs. In agreement, we identified inverse correlations between expression of these SiM-miRNAs and early clinical recurrence, as well as with AR transcriptional activity in human PC tissues. We also identified robust induction of miR-135a by androgen and strong direct binding of AR to the miR-135a locus. As miR-135a potently suppresses AR expression, this results in a negative feedback loop that suppresses AR protein expression in an androgen-dependent manner, while de-repressing AR expression upon androgen deprivation. Our results demonstrate that epigenetic silencing of these SiM-miRNAs can result in increased AR axis activity and cell proliferation, thus contributing to disease progression. We further demonstrate that a negative feedback loop involving miR-135a can restore AR expression under androgen-deprivation conditions, thus contributing to the upregulation of AR protein expression in castration-resistant PC. Finally, our unbiased proteomic profiling demonstrates that the majority of actual protein expression changes induced by SiM-miRNAs cannot be explained based on predicted direct interactions.

Abstract P6-10-03: The PKC inhibitor PKC412 antagonizes breast cancer cell growth and enhances tamoxifen sensitivity

Background: AIB1 (SRC-3, NCoA3), a member of the p160/steroid receptor coactivators family, plays a critical role in cell growth and proliferation. In estrogen receptor-alpha positive (ER+) breast cancer (BC) cells, it coactivates estrogen- and additional transcription factors-dependent gene transcription, reducing the antagonistic activity of tamoxifen and resulting in tamoxifen resistance (TR). We have previously shown that BC patients whose tumors expressed high levels of both AIB1 and HER-2 had worse outcomes with tamoxifen therapy, suggesting that AIB1 may be an important diagnostic and therapeutic target. Our findings that knocking down AIB1 attenuates ER signaling and inhibits breast cancer cell growth further indicate that the manipulation of AIB1 level could be an approach to treating BC and overcoming TR. Recently, it has been shown that protein kinase C (PKC) isoforms phosphorylate AIB1 and prevent its proteasome-mediated degradation. The present study was carried out to test if the multi-targeted kinase and PKC inhibitor PKC412 (midostaurin) is capable of promoting degradation of AIB1, inhibiting BC cell growth, and promoting tamoxifen antagonistic activity.

Methods: The ER+ MCF7, T47D, and ZR75-B BC cells and their tamoxifen-resistant derivatives (TR) were used. The Methylene Blue assay was employed to measure cell viability of BC cell lines after treatment with PKC412 or the combination of PKC412 with tamoxifen. To determine the impact of PKC412 on AIB1 and ER proteins and mRNA levels, we used immunoblotting and RT-qPCR, respectively.

Results: PKC412 successfully inhibited the growth of MCF7, T47D, and ZR75-B cells, and their tamoxifen-resistant derivatives. Treatment with PKC412 depleted AIB1 protein and reduced the level of ER protein without significant alteration in AIB1 mRNA level. Consequently, ER signaling was disrupted, as reflected by decreased expression of ER target genes such as GREB1. PKC412 also enhanced tamoxifen's antagonistic activity in the parental cell lines and sensitized tamoxifen-resistant MCF7 and ZR75-B cells to tamoxifen.

Conclusions: The results of this study suggest that the multi-targeted kinase and PKC inhibitor PKC412 can post-translationally destabilize AIB1 protein and ER, inhibiting BC cell growth and viability. PKC412 enhances tamoxifen's antagonistic activity on BC growth; furthermore, it sensitizes tamoxifen-resistant cells to tamoxifen. Thus, PKC412 is a promising agent to treat BC and, in combination with tamoxifen, to delay or overcome TR.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-10-03.