Glaucocalyxin A impairs tumor growth via amplification of the ATF4/CHOP/CHAC1 cascade in human oral squamous cell carcinoma

Ethnopharmacological relevance: The natural extract glaucocalyxin A (GLA), purified from the aboveground sections of the Chinese traditional medicinal herb Rabdosia japonica (Burm. f.) Hara var. glaucocalyx (Maxim.) Hara, has various pharmacological benefits, such as anti-bacterial, anti-coagulative, anti-neoplastic, and anti-inflammatory activities. Although GLA has shown anti-tumor activity against various cancers, the therapeutic potential and biological mechanisms of GLA remain to be further explored in oral squamous cell carcinoma (OSCC).

Aim of the study: This study aimed to elucidate the therapeutic potential and regulatory mechanisms of GLA in OSCC.

Materials and methods: The cell proliferation and Apoptosis effects of GLA were analyzed by CCK-8, clone formation, Annexin V/PI staining, and apoptotic protein expression in vitro. An OSCC xenograft model was applied to confirm the anti-neoplastic effect in vivo. Furthermore, the changes of Reactive Oxygen Species (ROS) were determined by DCFH-DA probe and GSH/GSSG assay, and inhibited by the pan-caspase inhibitor Z-VAD(OMe)-FMK and the ROS scavenger N-acetylcysteine (NAC). The modulation of GLA on mitochondria and ER-dependent Apoptosis pathways was analyzed by JC-1 probe, quantitative Real-Time PCR, and Western blot. Finally, public databases, clinical samples, and transfection cells were analyzed to explore the importance of GLA's indirect targeting molecule CHAC1 in OSCC.

Results: GLA significantly inhibited cell proliferation and induced Apoptosis in vitro and in vivo. GLA perturbed the redox homeostasis, and cell Apoptosis was totally rescued by Z-VAD(OMe)-FMK and NAC. Furthermore, GLA activated the mitochondrial Apoptosis pathway. Simultaneously, the overexpression and knockdown of CHAC1 dramatically affected GLA-mediated Apoptosis. The endoplasmic reticulum stress-associated ATF4/CHOP signal was identified to participate in GLA-upregulated CHAC1 expression. Finally, we found that CHAC1 expression was lower in OSCC compared with normal tissues and positively correlated with 4-Hydroxynonenal (4-HNE) level. High CHAC1 expression also indicated better overall survival. Moreover, CHAC1 selectively regulated the viability of oral Cancer cells.

Conclusion: GLA is a promising therapeutic agent that activates the ROS-mediated ATF4/CHOP/CHAC1 axis in OSCC patients.