Abstract P6-06-27: Expression of ATP-binding cassette transporter C1 (ABCC1) and activated sphingosine kinase 1 in breast cancer are associated with significantly shorter disease free survival
Authors: Yamada A , Nagahashi M , Aoyagi T , Kida K , Milstien S , Spiegel S , Ishikawa T , Endo I , Takabe K 
Published: August 2014

Background and specific objectives: ATP-binding cassette (ABC) transporters are known to be multidrug resistance proteins that efflux various compounds out of cells including chemotherapeutic agents. A number of clinical trails have been conducted targeting ABCB1: however, none showed beneficial effects. The pleiotropic bioactive lipid mediator sphingosine-1-phosphate (S1P), which is generated by sphingosine kinase 1 (SphK1) inside breast cancer cells and exerts its functions by binding to its specific cell surface G-protein coupled receptors (S1PR1-5) after being exported, is now known as a key regulatory molecule in breast cancer progression. We have previously demonstrated that ABCC1 and ABCG2, but not ABCB1 export S1P out of MCF7 human breast cancer cells. We hypothesized that ABCC1 expression in the presence of S1P produced by activated SphK1 in human breast cancer is associated with poor prognosis.

Methods: We constructed a tissue microarray with 281 breast tumors from patients, and analyzed expressions of ABCB1, ABCC1, and ABCG2, activated SphK1 (pSphK1), and S1P receptor-1 (S1PR1) immunohistochemically. Breast cancer subtypes were determined by immunohistochemistry of ER, PR, and HER2. Protein expressions were correlated to clinicopathological characteristics, clinical follow-up, and pathological complete response to neoadjuvant chemotherapy. For in vitro experiments, MCF7 human breast cancer cells were transfected with ABCB1 or ABCC1 and stimulated with estradiol. Cell proliferation was analyzed by WST-8 assay.

Results: The tissue microarray was comprised of 191 luminal A (68.0%), 17 luminal B (6.0%), 27 HER2 (9.6%), and 46 triple-negative (16.4%) tumors. Activated SphK1 was highly expressed in the patients with lymph node metastasis (40.1% vs 27.3%, P = 0.037) and the pSphK1 high expression group had significantly shorter disease free survival (DFS) (P = 0.05). Eighty percent of the patients expressed S1PR1; however, there were no significant differences in prognosis. On the other hand, ABCC1 expression was associated with significantly shorter DFS (P = 0.027). ABCC1 and ABCG2, but not ABCB1, were significantly higher and more frequently expressed in aggressive subtypes. Patients with tumors expressing both pSphK1 and ABCC1 had significantly shorter DFS (P = 0.002), while patients expressing both ABCB1 and pSphK1 did not. Overexpression of ABCC1 in MCF7 cells not only increased S1P secretion, it significantly increased estradiol-dependent proliferation, compared to MCF7 cells transfected with control vector or ABCB1 (P = 0.010 and P = 0.027, respectively).

Conclusions: We have shown that ABCC1 and ABCG2 are highly expressed in aggressive breast cancer subtypes, and that co-expression of pSphK1 and ABCC1 in the tumors is associated with poor prognosis. Our results suggest that inside-out signaling of S1P via ABCC1 may play a significant role in the course of human breast cancer progression.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-06-27.