A dual role of Proline Dehydrogenase in Lung Cancer

Lung cancer is still a global health problem. Present therapeutic approaches, among which Tyrosine Kinase Inhibitors (TKIs), have improved outcomes of patients with lung adenocarcinoma (LUAD). However, resistance mechanisms occur. Proline dehydrogenase (PRODH) could be a novel target and/or biomarker of therapy response in EGFR mutant LUADs. In mitochondria, PRODH oxidizes proline to pyrroline 5-carboxylate. The electrons produced during the reaction can be used to generate ATP or ROS species, thus affecting several cellular processes, such as survival, senescence, and apoptosis, playing a critical role in cancer. Using different cellular models, we have shown a negative correlation between EGFR and PRODH expression, which appears to be mediated by STAT3. We hypothesized that STAT3 decreases PRODH expression, by recognizing specific response elements (REs) in the PRODH gene (Aim 1).Concurrently, we observed an increase in PRODH expression in EGFR mutant LUAD cell lines with in vitro acquired resistance to EGFR TKIs, compared to their parental cell lines, and asked if PRODH could play a role in acquired resistance (Aim 2). Luciferase assays were carried out in different cell lines using constructs carrying PRODH intronic regions containing putative STAT3 binding sites identified bioinformatically. To investigate PRODH role in acquired resistance to TKIs, we carried out PRODH silencing or inhibition in HCC827 cells, which carry EGFR exon 19 p.E746-A750del, and in its derivative HCC827-GR5 (2nd gen. TKI, gefitinib-resistant) cells, grown in 2D or 3D, and evaluated viability, and apoptosis. Results 1. We observed luciferase activity upon STAT3 inhibition in DU145 cells compared to those treated with vehicle. Luciferase activity instead decreased when we ectopically expressed STAT3 in NCI-H1299 cells. Our data suggest that STAT3 may directly bind its specific REs in the PRODH gene to repress its expression and favor cell growth. Results 2. We observed that PRODH inhibition strongly impaired 2D and 3D growth of EGFR mutant, resistant LUAD cells but instead favored the growth of the parental cell line, in line with previous data. Interestingly, in TKI-resistant cells, PRODH inhibition determined an increased expression of cell cycle arrest (p21) and apoptotic (PUMA, caspase 8) markers. From the data shown here, PRODH may exert different roles in early- and late-stage lung cancer and upon acquired drug resistance, based on the activated signaling pathways.