Pro-apoptotic effect of haem oxygenase-1 in human colorectal carcinoma cells via endoplasmic reticular stress

Several biological effects of haem oxygenase (HO)-1, including anti-inflammatory, antiapoptotic and antioxidative properties were reported; however, the role of HO-1 in apoptosis is still unclear. In the presence of stimulation by cobalt protoporphyrin (CoPP), an HO-1 inducer, apoptotic characteristics were observed, including DNA laddering, hypodiploid cells, and cleavages of caspase (Casp)-3 and poly(ADP) ribose polymerase (PARP) proteins in human colon carcinoma COLO205, HCT-15, LOVO and HT-29 cells in serum-free (SF) conditions with increased HO-1, but not heat shock protein 70 (HSP70) or HSP90. The addition of 10% foetal bovine serum (FBS) or 1% bovine serum albumin accordingly inhibited CoPP-induced apoptosis and HO-1 protein expression in human colon cancer cells. CoPP-induced apoptosis of colon cancer cells was prevented by the addition of the pan-caspase inhibitor, Z-VAD-FMK (VAD), and the Casp-3 inhibitor, Z-DEVD-FMK (DEVD). N-Acetyl cysteine inhibited reactive oxygen species-generated H₂O₂-induced cell death with reduced intracellular peroxide production, but did not affect CoPP-induced apoptosis in human colorectal carcinoma (CRC) cells. Two CoPP analogs, ferric protoporphyrin and tin protoporphyrin, did not affect the viability of human CRC cells or HO-1 expression by those cells, and knockdown of HO-1 protein expression by HO-1 small interfering (si)RNA reversed the cytotoxic effect elicited by CoPP. Furthermore, the carbon monoxide (CO) donor, CORM, but not FeSO₄ or biliverdin, induced DNA ladders, and cleavage of Casp-3 and PARP proteins in human CRC cells. Increased phosphorylated levels of the endoplasmic reticular (ER) stress proteins, protein kinase R-like ER kinase (PERK), and eukaryotic initiation factor 2α (eIF2α) by CORM and CoPP were identified, and the addition of the PERK inhibitor, GSK2606414, inhibited CORM- and CoPP-induced apoptosis. Increased GRP78 level and formation of the HO-1/GRP78 complex were detected in CORM- and CoPP-treated human CRC cells. A pro-apoptotic role of HO-1 against the viability of human CRC cells via induction of CO and ER stress was firstly demonstrated herein.