Abstract 5525: Induced expression of PPM1A in ER-negative breast cancer cells inhibits growth by suppressing CDK phosphorylation
Authors: Mazumdar Abhijit , Hill Jamal , Zhang Yun , Bollu Lakshmi Reddy , Tsimelzon Anna , Chang Jenny , Mills Gordon , Brown Powel 
Published: July 2017

Background: Estrogen receptor (ER) -negative breast cancer is a clinical subtype that is overrepresented among younger women and is associated with a poor prognosis. Current treatments for ER-negative tumors include cytotoxic chemotherapy, or for those overexpress HER2, the anti-HER2 antibody. Targeted therapy for triple-negative breast cancers is urgently needed. In this project, we investigated phosphatases that are differentially expressed in ER-negative as compared to ER-positive breast cancers. We hypothesized that: (1) specific phosphatases govern the growth of ER-negative cancers, (2) Induced expression of specific phosphatases that are under expressed in ER-negative cancers will suppress the growth of ER-negative breast cancers.

Methods: Using 102 human breast tumors (57 ER-positive & 45 ER-negative) from the neo-adjuvant studies from the Baylor Breast Center tumor bank, we isolated RNA and performed Affymetrix microarray studies. Statistical analysis was done with dChip software, and phosphatases over (&gt;1.5 fold; FDR &lt;0.05) or under (&lt;0.66-fold; FDR&lt;0.05) expressed in ER-negative breast cancers as compared to ER-positive cancers were selected for further study. One of the phosphatases under expressed in ER-negative breast cancer was PPM1A. Regulated expression of PPM1A was achieved using a Tet-regulated vector. Cell growth in soft agar and cell cycle analyses were performed using previously published protocols. Mouse xenograft experiments were performed by injecting inducible PPM1A-clones and vector- clones in mammary fat pad of athymic mice and tumor growth was measured over time in randomized groups. To induce PPM1A expression one group was treated with doxycycline when tumor volume reached 50 mm3. Survival analyses were done using Oncomine datasets.

Results: We identified 20 over-expressed and 29 under-expressed phosphatases in ER-negative breast cancers. We selected the under expressed phosphatase PPM1A, for further study. Multivariate cox regression analysis shows PPM1A is an independent predictor for breast cancer survival. Induced expression of PPM1A in ER-negative cells inhibited anchorage-dependent and independent growth but had no effect on ER-positive cell. PPM1A expression also inhibited MDA-MB 231 cells growth in vivo. Induced expression of PPM1A blocked cell cycle progression at G1 phase. PPM1A interacts with CDK6 and inhibits phosphorylation of CDK and MDM2.

Conclusions: We identified a set of over- and under- expressed phosphatases in ER-negative breast cancers as compared to ER-positive cancers. Overexpression of PPM1A in ER-negative breast cancer cells inhibits growth. By identifying the molecules that regulate breast cancer cells growth we are identifying potential new targets for the treatment of these aggressive ER-negative breast cancers. Supported by Komen Promise grant KG081694 & Komen SAB grant

Citation Format: Abhijit Mazumdar, Jamal Hill, Yun Zhang, Lakshmi Reddy Bollu, Anna Tsimelzon, Jenny Chang, Gordon Mills, Powel Brown. Induced expression of PPM1A in ER-negative breast cancer cells inhibits growth by suppressing CDK phosphorylation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5525. doi:10.1158/1538-7445.AM2017-5525