Role of P4HA1 in Breast Cancer Tamoxifen Resistence

Hormone therapy significantly reduces recurrence and mortality in luminal breast cancer patient. However, 40% of the patients with 5-10 years treatment will acquire resistance to tamoxifen. Resistance to tamoxifen and recurrence is one of the leading causes of mortality in breast cancer patients. Understanding how cancer cells develop tamoxifen resistance is critical for the treatment of recurrent luminal breast cancer. We recently found that expression of prolyl 4-hydroxylase subunit alpha-1 (P4HA1), the alpha subunit of prolyl 4-hydroxylase 1 (P4H1), was increased in tamoxifen-resistant luminal breast cancer cells. Knockdown of P4HA1 protein combined with tamoxifen treatment significantly restored tamoxifen sensitivity in luminal breast cancer cells. RNA-seq data showed silencing P4HA1 activates “intrinsic apoptotic signaling pathway and in response to endoplasmic reticulum (ER) stress” signaling pathway in tamoxifen-treated cells. Elevated reactive oxygen species (ROS) levels were detected in P4HA1 knockdown breast cancer cells. Building on these findings, the central hypothesis of this proposal is that P4HA1 plays a crucial role in regulating tamoxifen-resistance by keeping suitable ER stress levels in luminal cancer cells. The objective of this proposal is to characterize the novel tamoxifen resistance mechanism that involves in P4HA1 and determines whether P4HA1 inhibition is a potential strategy overcome endocrine therapy-resistance in luminal breast cancer patients. In Aim 1, we will test the hypothesis that P4HA1 promotes tamoxifen-resistance by regulating ER-stress in luminal breast cancer cells. In Aim 2, we will test the hypothesis that inactivation of P4HA1 combined with tamoxifen treatment reduces tumor growth rate with tamoxifen treatment in vivo. We will examine cell viability, cell apoptosis, cell proliferation and cell cycle in P4HA1-silenced and tamoxifen-treated cells. We also will knockdown proximal ER stress downstream sensors IRE1 or ATF4 and ATF6, and perform rescue experiments in shP4HA1 tamoxifen-resistant cells. In vivo, we will determine whether combination of P4HA1 inactivation with tamoxifen treatment is sufficient to inhibit tamoxifen-resistant breast cancer tumor progression. This investigation will uncover a novel uncanonical role of P4HA1 in the regulation breast cancer progression by regulation ER stress pathway. We may identify P4HA1 as a novel therapeutic target for the hormone therapy-resistant luminal breast cancer.