Background: Triple-negative breast cancer (TNBC) has the worst prognosis of all breast cancers, and frequently affects young and in particular African-American Women. Women diagnosed with these tumors currently lack targeted treatment options. To identify novel targets for TNBC treatment, we identified specific phosphatases that are critical for the growth of TNBCs.

Methods: To identify novel targets for TNBC, we investigated the expression of phosphatases in breast tumors, identified those highly expressed in TNBC, and studied whether specific phosphatases are essential for TNBC growth. siRNA were used to knock-down each of the 20 highly expressed phosphatases in 4 TNBC, 4 ER-positive BC cell lines. Cell growth was measured by cell count, and anchorage-independent growth was tested with soft agar assays. Xenograft experiments were performed by injecting TNBC cells expressing inducible shRNA-PTP4A3 into the mammary fat pad of nude mice. Mice with palpable tumors were then randomized to receive either water containing doxycycline (Dox) or vehicle. Tumor size was measured every other day, and the growth rates of tumors were calculated and compared using Student's t-test of the slopes. The prognostic importance of PTP4A3 was evaluated using gene expression and survival data. R statistical software was used to generate Kaplan-Meier curves and determine statistical significance using the log rank (Mantel-Cox) method and perform Cox proportional hazards models analyses. Patients were dichotomized at the mean expression level.

Results: Knockdown of nine phosphatases, including PTP4A3 (PRL-3) significantly reduced growth and anchorage-independent growth of TNBC cell lines. PTP4A3 siRNA treatment induced a G1/S cell cycle block in all breast cancer cell lines. Further analysis of the phosphatase PTP4A3 demonstrated that reduced expression induced apoptosis in four TNBC cell lines, without inducing apoptosis in most ER-positive BC cell lines. Inhibition of PTP4A3 in TNBC xenografts suppressed growth, and a reduction in proliferation and induction of apoptosis was detected by immunohistochemistry. In silico analysis of PTP4A3 expression confirmed elevated expression of PTP4A3 in TNBCs and demonstrated that PTP4A3 is an independent prognostic indicator for breast cancer specific survival.

Conclusion: These studies identified a set of highly expressed phosphatases that represent promising targets for the treatment of TNBCs. We also demonstrated that one of these phosphatases, PTP4A3, is critical for the growth of TNBCs in vitro and in vivo, and PTP4A3 expression is prognostic in women with TNBC. These results provide the rationale for further study of growth promoting phosphatases and suggest that such phosphatases may be targeted for the treatment of this aggressive form of breast cancer.

Research supported by: Susan G. Komen for the Cure (PB, GM), Komen SAB, and the Norman Brinker Award for Research Excellence.

Citation Format: Petra den Hollander, Anna Tismelzon, Jonathan Shepherd, Kathryn R. Rawls, Jamal L. Hill, Abhijit Mazumdar, Susan G. Hilsenbeck, Gordon B. Mills, Powel H. Brown. Phosphatase PTP4A3 is critical for cell growth of triple-negative breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 971. doi:10.1158/1538-7445.AM2014-971