Protein kinase D1 regulates ERα‐positive breast cancer cell growth response to 17β‐estradiol and contributes to poor prognosis in patients

About 70% of human breast cancers express and are dependent for growth on estrogen receptor α (ERα), and therefore are sensitive to antiestrogen therapies. However, progression to an advanced, more aggressive phenotype is associated with acquisition of resistance to antiestrogens and/or invasive potential. In this study, we highlight the role of the serine/threonine‐protein kinase D1 (PKD1) in ERα‐positive breast cancers. Growth of ERα‐positive MCF‐7 and MDA‐MB‐415 human breast cancer cells was assayed in adherent or anchorage‐independent conditions in cells overexpressing or depleted for PKD1. PKD1 induces cell growth through both an ERα‐dependent manner, by increasing ERα expression and cell sensitivity to 17β‐estradiol, and an ERα‐independent manner, by reducing cell dependence to estrogens and conferring partial resistance to antiestrogen ICI 182,780. PKD1 knockdown in MDA‐MB‐415 cells strongly reduced estrogen‐dependent and independent invasion. Quantification of PKD1 mRNA levels in 38 cancerous and non‐cancerous breast cell lines and in 152 ERα‐positive breast tumours from patients treated with adjuvant tamoxifen showed an association between PKD1 and ERα expression in 76.3% (29/38) of the breast cell lines tested and a strong correlation between PKD1 expression and invasiveness (P < 0.0001). In tamoxifen‐treated patients, tumours with high PKD1 mRNA levels (n = 77, 50.66%) were significantly associated with less metastasis‐free survival than tumours with low PKD1 mRNA expression (n = 75, 49.34%; P = 0.031). Moreover, PKD1 mRNA levels are strongly positively associated with EGFR and vimentin levels (P < 0.0000001). Thus, our study defines PKD1 as a novel attractive prognostic factor and a potential therapeutic target in breast cancer.