Peroxisome proliferator-activated receptors as novel targets of small cell lung cancer circulating tumor cells

Small cell lung cancer (SCLC) has a dismal prognosis with a low 2-year survival rate. Chemotherapy for recurrent SCLC fails invariably, and novel tumor targets are needed. Here, the effects of agents targeting the peroxisome proliferator-activated receptors (PPARs) in SCLC are investigated. Initial screening of 96 PPAR-directed agents was performed in two SCLC CTC-derived lines (BHGc10, BHGc16). Compounds showing high cytotoxicity were subsequently tested in two pleural effusion-derived lines (S457, S1392) and the SCLC line NCI-H69. Several PPARs emerged as actionable targets: eight PPARγ ligands and nine ligands for PPARα, PPARα/δ, or PPARβ/δ. For the six most effective compounds, treatment-induced protein changes were further profiled in BHGc16 using protein arrays. Cytotoxicity varied by compound, while the PPARγ agonist pioglitazone and the PPARα agonist fenofibrate were preferentially active in CTC lines, DG172 hydrochloride was selective for pleural effusion-derived lines, while rosiglitazone maleate, cloxiquine, and agrimol B showed no selectivity. Mechanistically, in the CTC-derived cell line BHGc16, these six PPAR-directed agents increased pro-apoptotic proteins (Bax, Bad, caspase-3/9), decreased anti-apoptotic and invasion proteins (Bcl-2, Bcl-XL, c-FLIP-L, ICAM-1, CXCR4), and suppressed Akt/mTOR, MEK/ERK, p38 MAPK, and JAK2/STAT3 signaling. These findings support PPARs as clinically relevant targets in SCLC, with PPAR-directed agents showing cytotoxic effects comparable to those reported in other malignancies. Such agents may aid SCLC treatment and help delineate biological differences between CTCs and resident tumor cells.