PAD4 inhibitors: potential sensitizers of tumour cells to TRAIL-induced apoptosis

Abstract

Targeting tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a mediator of apoptosis (cell death) and death ligand belonging to the TNF superfamily, represents a promising approach in anti-cancer therapy due to its selectivity to target cancer cell populations yet not normal cells. However, resistance to TRAIL is a common occurrence in tumours, limiting the effects of TRAIL to a minority of TRAIL-sensitive tumours. Peptidylarginine deiminase 4 (PAD4) is a Ca2+-dependent enzyme catalysing the post-translational conversion of arginine residues to citrulline within histones (citrullination), implicated in the epigenetic modulation of gene expression, with a potential role in tumourigenesis. This study aimed to determine whether the PAD4 inhibitor F-amidine may sensitize tumour cells to TRAIL-induced apoptosis. Expression of PAD4 mRNA was assessed in a panel of 18 cancer cell lines via real-time qRT-PCR to determine PAD4 sensitive cell lines. Prostate (LNCaP), breast (MCF-7), malignant glioma (U87-MG) and acute myeloid leukaemic (HL-60) cell lines were challenged with F-amidine in the presence or absence of death receptor 5 (DR5) agonists and nuclear morphology assessed via Hoechst 33342 staining. PAD4 mRNA expression was detected in 11 cancer cell lines from breast, prostate, leukaemic, glioma, neuroblastoma, myeloma, hepatocellular carcinoma and pluripotent embryonal carcinoma origin. F-amidine synergistically and significantly enhanced TRAIL responses within the PAD4 expressing TRAIL-resistant U87-MG cell line. PAD4 expressing TRAIL-sensitive HL-60 and resistant MCF-7 cell lines were not significantly sensitized to TRAIL-induced apoptosis. Significant synergistic TRAIL responses were observed in the TRAIL-resistant and PAD4 mRNA-negative LNCaP cell line, revealing that F-amidine may also potentiate TRAIL responses independent of PAD4 inhibition. Overall, this study presents some of the first evidence that PAD inhibitors may possess roles as novel TRAIL sensitizers in epigenetic anti-cancer therapy, potentially independent of PAD4 inhibition.