A major cause of the high mortality rates due to OvCa is chemotherapy resistance. Cisplatin is often the first drug of choice for OvCa treatment. Unfortunately, cisplatin resistance is a major obstacle that impedes successful chemotherapy and a major cause of treatment failure in human OvCa. The balance between survival and apoptotic signals determine a cell's sensitivity to chemotherapy. Indeed, cancer cells develop resistance to chemotherapy by means of inactivating apoptotic factors and enhancing survival pathways that antagonize apoptosis signals [2
]. However, the precise mechanisms of OvCa cell cisplatin-sensitivity or survival are not known.
Chemokines function to direct leukocyte and cancer cell migration, and play pivotal roles in cell survival [9
]. Studies have demonstrated that CXCR4-CXCL12 interactions promote the survival of tumor cells, allowing growth under less favourable conditions. In particular, CXCR4 mediates survival in glioma cells [10
]. Recent studies have also suggested that CCR9-CCL25 interactions potentiate anti-apoptotic signalling to immature T cells [11
]. We have demonstrated that OvCa cells and tissues express CCR9 and play important role in cell migration, invasion under the chemotactic gradient of CCL25 (unpublished observations). Here we show that CCR9 also supports OvCa cell survival and cisplatin resistance. For the first time, we show that CCL25 significantly increases the proliferation of the OvCa cells in a CCR9-dependent fashion. In the presence of cisplatin, CCL25 also supported OvCa cell survival. Even though higher doses of cisplatin abrogated this CCL25-mediated resistance, our findings demonstrate that CCL25 confers significant cisplatin resistance.
Studies have shown that CCR9 signalling plays a role in immature T cell survival through PI3K and Gαi
protein-dependent activation of Akt/protein kinase B [11
]. By phosphorylation of its downstream effectors, Akt propagates cell survival signalling that promotes cell proliferation, maintains cell growth and inhibits apoptosis. The PI3K/Akt pathway has also been shown to be involved in cisplatin resistance. Recent studies show that Akt inactivation, through a PI3K inhibition, sensitizes OvCa cells to cisplatin-induced cell death [6
]. Phosphorylated Akt promotes survival by phosphorylating and inactivating pro-apoptotic factors, such as FKHR and GSK-3β [7
]. FKHR is a transcription factor that transactivates the expression of death-activating proteins, such as Fas ligand (FasL) and Bim [12
]. Phosphorylation of FKHRL1 at Thr32, Ser253 and Ser315 prevents translocation of this protein to the nucleus and loss of FKHR-mediated gene transcription [13
]. Recently, it was shown that activation of chemokine receptors lead to phosphorylation of GSK-3β and FKHR in a PI3K/Akt-dependent manner [14
Taken together, our studies strongly support that CCR9-CCL25 signalling enhances OvCa survival and cisplatin resistance. Specifically, we show that CCL25 induces robust activation predominately through the PI3K/Akt pathway and its downstream mediators, FKHR and GSK-3β. Moreover, PI3K inhibition completely abrogated CCL25-mediated and CCR9-dependent cisplatin resistance, Akt, GSK-3β, and FKHR phosphorylation.
Chemokines-chemokine receptor interactions also support integrin clustering, which also increase FAK activation. FAK is a cytoplasmic protein tyrosine kinase involved in the regulation of cell proliferation, migration, and survival. FAK is constitutively associated with β-integrins. Activated FAK has also been shown to support PI3Kp85 phosphorylation following integrin clustering, but the mechanism(s) is not fully understood [15
]. FAK inhibition did not effect CCL25-mediated PI3K, Akt, FKHR, or GSK-3β phosphorylation in OvCa cells, which suggest CCR9 signalling and survival mechanisms are independent of FAK activity.
Conflicting studies demonstrated cisplatin activates Akt in several cancer cell lines, which leads to cisplatin resistance [16
]. Moreover, it has been shown that cisplatin can transiently induce Akt-mediated phosphorylation of FKHRL1 in the cisplatin-resistant cell line, CAOV-3, with subsequent cytoplasmic retention of FKHRL1 and cell survival [17
]. However, cisplatin-treatment alone did not lead to significant increases in phosphorylation of PI3K, Akt, GSK-3β, or FKHR. In fact, cisplatin treatment led to a slight down regulation of Akt activation. However in the presence of CCL25 along with cisplatin, phosphorylation of Akt, GSK-3β and FKHR elevated to significant levels. Taken together, these results suggest that CCL25 treatment contributes to OvCa survival and cisplatin resistance. Moreover, we show that CCR9-dependent anti-apoptotic signalling in OvCa cells involves the PI3K/Akt cascade and phosphorylation of its downstream mediators, GSK-3β and FKHR (Figure ).
Figure 6 Inhibition of apoptotic signal in cisplatin resistant tumor cells. More than one mechanism is usually observed in resistant cells, and this contributes to the multi-factorial nature of cisplatin resistance. CCR9-dependent anti-apoptotic signalling in (more ...)