According to the American Cancer Society, in 2010, 207,090 women will be diagnosed with BrCa and about 39,840 women will die from the disease . Although death rates have been declining since 1990, BrCa mortality is high among African American women . Many of these deaths are due to chemo-resistance, which is a common problem in the treatment of BrCa [12, 13]. Cisplatin and its analogues have been widely used for treatment of human cancers, including advanced BrCa [14–16]. However, resistance to cisplatin represents a major obstacle in the effective management of metastatic BrCa . The balance between survival and apoptotic signals in cancer cells determine the sensitivity to chemotherapy and cancer cells develop resistance to chemotherapies by inactivating apoptotic factors and enhancing survival pathways . However, the factors that promote these remain incompletely understood.
Chemokines direct the migration of leukocytes as well as cancer cells [19, 20] and they play a pivotal role in cell survival . Interactions between CXCR4 and its ligand, CXCL12, promote the survival of breast , pancreatic cancers , and melanoma . CCR9-CCL25 interactions also potentiate anti-apoptotic signaling to immature, or double-positive T cells , and function to retain single positive cells in the thymus until they are fully mature and ready for export .
We have demonstrated that CCR9 is significantly expressed by ovarian and prostate cells and play important role in cell migration and invasion [25, 26]. Here we show that CCR9 also supports BrCa cell growth as well as cell survival or resistance to cisplatin. CCL25 significantly increases BrCa cell proliferation and cisplatin resistance in a CCR9-dependent fashion.
It has been previously shown that CCR9 signaling plays a role in immature T cell survival through PI3K and Gαi protein-dependent activation of Akt/protein kinase B . Downstream PI3K mediators directly phosphorylate and activate Akt [28, 29]. PI3K/Akt signaling pathways are also frequently disturbed in many human cancers and evidence suggests that chemokine receptor signaling activates Akt [28, 30]. Akt modulates the function of numerous substrates involved in the regulation of cell survival, cell cycle progression and cellular growth. The PI3K/Akt pathway is also involved in chemoresistance to cisplatin . Phosphorylated Akt promotes survival by phosphorylating and inactivating pro-apoptotic factors, such as GSK-3β and or FKHR. GSK-3β inhibition by Akt prevents phosphorylation of β-catenin, which impedes its degradation; hence, it is translocated to the nucleus. Once in the nucleus, β-catenin combines with different transcription factors to induce the expression of several genes, such as cyclin D1 . Hence, Akt-mediated phosphorylation of GSK-3β prevents the accumulation of cyclin D1, which is needed to support cell cycle progression. FKHR transactivates the expression of death activating proteins, such as Fas ligand (FasL), Bim, and Bcl-6. Phosphorylating FKHR1 at its threonine and serine residues prevents its translocation to the nucleus and any associated gene transcription .
Our studies strongly support the hypothesis that CCR9-CCL25 signaling enhances BrCa cell growth and survival. Specifically, we show that CCL25 induces the activation of the PI3K/Akt pathway and phosphorylation of its downstream mediators, e.g., GSK-3β and FKHR. PI3K inhibition completely abrogated CCL25-mediated and CCR9-dependent cisplatin resistance, Akt, GSK-3β, and FKHR phosphorylation. However, it was also plausible that other Akt-activation pathways supported BrCa cell survival following cisplatin (and CCL25) treatment. In this regard, chemokine-chemokine receptor interactions also support integrin clustering to potentially activate FAK, which is a protein tyrosine kinase involved in cell proliferation, migration and survival . Activated FAK also interacts with PI3K through integrin clustering . However, our findings show that FAK is not involved in CCL25-mediated and CCR9-dependent Akt activation or subsequent GSK-3β and FKHR phosphorylation.
In support of our findings, a recent study has shown that the PI3K/Akt signaling pathway is an important event downstream of amphiregulin for the development of cisplatin resistance in BrCa cells . Taken together, these results suggest that CCL25 treatment induces BrCa cell survival and cisplatin resistance. We also show that CCR9-dependent anti-apoptotic signaling involves the PI3K/Akt pathway and phosphorylation of its downstream mediators, GSK-3β and FKHR - through CCR9 and PI3K/Akt, but independent of FAK, supporting our hypothesis that CCR9-CCL25 interaction promotes BrCa cell survival and resistance to cisplatin.