Differential Effects of Procaspase-3 Activating Compounds in the Induction of Cancer Cell Death

Abstract

The evasion of apoptosis is a key characteristic of cancer, and thus strategies to selectively induce apoptosis in cancer cells hold considerable promise in personalized anticancer therapy. Structurally similar procaspase activating compounds PAC-1 and S-PAC-1 restore procaspase-3 activity through the chelation of inhibitory zinc ions in vitro, induce apoptotic death of cancer cells in culture, and reduce tumor burden in vivo. IP or IV administrations of high doses of PAC-1 are transiently neurotoxic in vivo, while S-PAC-1 is safe even at very high doses and has been evaluated in a Phase I clinical trial of pet dogs with spontaneously occurring lymphoma. Here we show that PAC-1 and S-PAC-1 have similar mechanisms of cell death induction at low concentrations (less than 50 µM), but at high concentrations PAC-1 displays unique cell death induction features. Cells treated with a high concentration of PAC-1 have a distinctive gene expression profile, unusual cellular and mitochondrial morphology, and an altered intracellular Ca2+ concentration, indicative of endoplasmic reticulum (ER) stress-induced apoptosis. These studies suggest strategies for anticancer clinical development, specifically bolus dosing for PAC-1 and continuous rate infusion for S-PAC-1.

Antitumor activity of paclitaxel is significantly enhanced by a novel proapoptotic agent in non–small cell lung cancer

“…In addition, we have also shown that PAC-1 significantly enhances the antitumor activity of paclitaxel, both in vitro and in NSCLC xenograft model. Our data on NSCLC complement previous studies on proapoptotic effects of PAC-1 in different cancers [20]. The proapoptotic effects of PAC-1 are proportional to the cellular levels of PC-3.…”

Removal of Metabolic Liabilities Enables Development of Derivatives of Procaspase-Activating Compound 1 (PAC-1) with Improved Pharmacokinetics

“…65 The mechanism of action of PAC-1 most likely involves the chelation of labile zinc from procaspase-3, relieving the zinc-mediated inhibition and allowing procaspase-3 to process itself to the active form. 36, 37, 41 Using genetically-encoded zinc sensors, PAC-1 has been shown to mobilize the labile zinc pool in cancer cells. 41 Providing further support to this direct procaspase-3 activation mechanism, cells treated with PAC-1 or a derivative show cleaved procaspase-3 and poly-ADP ribose polymerase-1 prior to release of cytochrome c from the mitochondria or cleavage of initiator procaspases-8 and -9, 8, 42, 43, 66 and PAC-1 -mediated apoptosis occurs regardless of the status of Bcl-2 family proteins.…”

“…36, 37, 41 Using genetically-encoded zinc sensors, PAC-1 has been shown to mobilize the labile zinc pool in cancer cells. 41 Providing further support to this direct procaspase-3 activation mechanism, cells treated with PAC-1 or a derivative show cleaved procaspase-3 and poly-ADP ribose polymerase-1 prior to release of cytochrome c from the mitochondria or cleavage of initiator procaspases-8 and -9, 8, 42, 43, 66 and PAC-1 -mediated apoptosis occurs regardless of the status of Bcl-2 family proteins. 67, 68 Because of this unique mechanism, PAC-1 is increasingly being used as a tool to directly activate procaspase-3 in a variety of biological settings.…”

“…38 It is likely that the improved safety profile is due in large part to the decreased ability of S-PAC-1 to cross the blood-brain barrier, as compared to PAC-1 . 41 In addition to the ability of S-PAC-1 to induce cell death to a variety of cancer cell lines in culture, 38 recent efforts have demonstrated the potential for S-PAC-1 to sensitize cancer cells in culture to ionizing radiation. 74 Encouragingly, S-PAC-1 was effective in reducing or stabilizing tumor growth in four out of six canine patients with spontaneously occurring lymphoma, and the compound was well tolerated in all six dogs.…”

Turning ON Caspases with Genetics and Small Molecules

“…These findings are consistent with previous studies showing that zinc inhibits caspase-3 with an IC 50 of 0.1 μ M (Perry et al, 1997). They also reported that the zinc-chelation mechanism was responsible for the cell death observed in cell culture leading to altered intracellular Ca 2+ concentration, indicative of endoplasmic reticulum stress-induced apoptosis (West et al, 2012). In summary, PAC-1 may not be a direct activator of procaspase, but led to interesting discoveries about the role of metal chelation in caspase activation and initiation of cell death.…”