Role of proton-coupled folate transporter in pemetrexed-resistance of mesothelioma: clinical evidence and new pharmacological tools

Abstract

Background: Thymidylate synthase (TS) has a predictive role in pemetrexed-treatment of mesothelioma, however additional chemoresistance mechanisms are poorly understood. Here we explored the role of the reduced-folate carrier (RFC/SLC19A1) and proton-coupled folate-transporter (PCFT/SLC46A1) in antifolate-resistance in mesothelioma.Patients and methods:PCFT, RFC and TS RNA and PCFT protein levels were determined by quantitative-RT-PCR of frozen tissues and immunohistochemistry of tissue-microarrays, respectively, in two cohorts of pemetrexed-treated patients. Data were analyzed by t-test, Fisher/log-rank test and Cox-proportional models. The contribution of PCFT expression and PCFT-promoter methylation on pemetrexed activity were evaluated in mesothelioma cells and spheroids, through 5-aza-2′-deoxycytidine-mediated demethylation and siRNA-knockdown.Results: Pemetrexed-treated patients with low-PCFT had significantly lower rates of disease control, and shorter overall-survival (OS), in both the test (N = 73, 11.3 vs. 20.1 months, P = 0.01) and validation (N = 51, 12.6 vs. 30.3 months, P = 0.02) cohorts. Multivariate analysis confirmed PCFT independent prognostic role. Low-PCFT protein levels were also associated with shorter OS. Patients with both low-PCFT and high-TS levels had the worst prognosis (OS, 5.5 months), whereas associations were neither found for RFC nor in pemetrexed-untreated patients. PCFT silencing reduced pemetrexed sensitivity, while 5-aza-2′-deoxycytidine overcame resistance.Conclusions: These findings identify for the first time PCFT as a novel mesothelioma prognostic biomarker, prompting prospective trials for its validation. Moreover, preclinical data suggest that targeting PCFT-promoter methylation might eradicate pemetrexed-resistant cells characterized by low-PCFT expression.

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