Prodigiosin is a bacterial tripyrrole pigment with potent cytotoxicity against diverse human
cancer cell lines. Endoplasmic reticulum (ER) stress is initiated by accumulation of unfolded or misfolded
proteins in the ER lumen and may induce cell death when irremediable. In this study, the role of ER stress in
prodigiosin-induced cytotoxicity was elucidated for the first time. Comparable to the ER stress inducer
thapsigargin,
prodigiosin up-regulated signature ER stress markers
GRP78 and CHOP in addition to activating the IRE1, PERK and ATF6 branches of the unfolded protein response (UPR) in multiple human
breast carcinoma cell lines, confirming
prodigiosin as an ER stress inducer.
Prodigiosin transcriptionally up-regulated CHOP, as evidenced by its promoting effect on the CHOP promoter activity. Of note, knockdown of CHOP effectively lowered
prodigiosin's capacity to evoke PARP cleavage, reduce cell viability and suppress colony formation, highlighting an essential role of CHOP in
prodigiosin-induced cytotoxic ER stress response. In addition,
prodigiosin down-regulated BCL2 in a CHOP-dependent manner. Importantly, restoration of BCL2 expression blocked
prodigiosin-induced PARP cleavage and greatly enhanced the survival of
prodigiosin-treated cells, suggesting that CHOP-dependent BCL2 suppression mediates
prodigiosin-elicited cell death. Moreover, pharmacological inhibition of JNK by
SP600125 or dominant-negative blockade of PERK-mediated eIF2α phosphorylation impaired
prodigiosin-induced CHOP up-regulation and PARP cleavage. Collectively, these results identified ER stress-mediated cell death as a mode-of-action of
prodigiosin's tumoricidal effect. Mechanistically,
prodigiosin engages the IRE1-JNK and PERK-eIF2α branches of the UPR signaling to up-regulate CHOP, which in turn mediates BCL2 suppression to induce cell death.