Gambogic Acid Induces Pyroptosis of Colorectal Cancer Cells through the GSDME-Dependent Pathway and Elicits an Antitumor Immune Response

SIMPLE SUMMARY: The treatment of colorectal cancer (CRC) is still a major challenge, and the manipulation of pyroptosis has become a novel therapeutic strategy for cancers. Gambogic acid (GA), a common traditional Chinese medicine, has been reported to be a potential agent for cancer treatment. However, the anticancer effects of GA in CRC and the molecular mechanism remain unclear. We conducted this study to solve this problem. The results demonstrated that GA inhibits the viability of CRC cells through GSDME-dependent pyroptosis by regulating the activation of caspase-3. Encouragingly, the process of GA-induced pyroptosis elicits an anti-tumor immune response. These results reveal for the first time that GSDME-dependent pyroptosis is a previously unrecognized mechanism by which GA inhibits CRC, and these findings have important implications for GA use in the treatment of cancers. ABSTRACT: Pyroptosis is a recently identified form of programmed cell death (PCD) that exerts a vital influence on the antitumor immune response. GA, a xanthone structure isolated from gamboge resin, is a naturally occurring bioactive ingredient with several anticancer activities, such as activities that affect cell cycle arrest, apoptosis, and autophagy. Here, we found that GA decreased the viability of the CRC cell lines, HCT116 and CT26, in a dose- and time-dependent manner, and multiple pores and large bubbles in the membranes, which are morphological characteristics of pyroptosis, were observed by light microscopy and transmission electron microscopy (TEM). Furthermore, the cleavage of gasdermin E (GSDME) was observed after exposure to GA, along with concomitant activation of caspase-3. Additionally, GSDME-dependent pyroptosis triggered by GA could be attenuated by siRNA-mediated knockdown of GSDME and treatment with the caspase-3 inhibitor. Moreover, we found that GA induced pyroptosis and significantly inhibited tumor growth in CT26 tumor-bearing mice. Strikingly, significantly increased proportions of CD3(+) T cells, cytotoxic T lymphocytes (CTLs), and dendritic cells (DCs) were observed in the tumor microenvironment in the GA-treated groups. Moreover, significantly increased proportions of CTLs and effector memory T cells (TEM) (CD8(+) CD44(+) CD62L(−)) were also detected in the spleens of the GA-treated groups, suggesting that the pyroptosis-induced immune response generated a robust memory response that mediated protective immunity. In this study, we revealed a previously unrecognized mechanism by which GA induces GSDME-dependent pyroptosis and enhances the anticancer immune response. Based on this mechanism, GA is a promising antitumor drug for CRC treatment that induces caspase-3-GSDME-dependent pyroptosis. This study provides novel insight into cancer chemoimmunotherapy.