Betulinic acid, a major therapeutic triterpene of Celastrus orbiculatus Thunb., acts as a chemosensitizer of gemcitabine by promoting Chk1 degradation.

Abstract

Celastrus orbiculatus Thunb., also called as oriental bittersweet vine or climbing spindle berry, a traditional Chinese herbal medicine has been used to treat a spectrum of painful and inflammatory diseases for centuries. Explored for their unique medicinal properties, C.orbiculatus offers additional therapeutic effects on cancerous diseases. The effect of single-agent gemcitabine on survival has not long been encouraging, combination therapies provide patients multiple chances of benefit for improved clinical response. This study aims at expounding the chemopotentiating effects and underlying mechanisms of betulinic acid, a primary therapeutic triterpene of C. orbiculatus in combination with gemcitabine chemotherapy. The preparation of betulinic acid was optimized using ultrasonic-assisted extraction method. Gemcitabine-resistant cell model was established by induction of the cytidine deaminase. MTT, colony formation, EdU incorporation and Annexin V/PI staining assays were used to evaluate cytotoxicity, cell proliferation and apoptosis in BxPC-3 pancreatic cancer cell line and H1299 non-small cell lung carcinoma cell line. Comet assay, metaphase chromosome spread and γH2AX immunostaining were applied for DNA damage assessment. Western blot and co-immunoprecipitation was used to detect the phosphorylation and ubiquitination of Chk1. Mode of action of gemcitabine in combination with betulinic acid was further captured in BxPC-3-derived mouse xenograft model. We noticed that the extraction method had an impact on the thermal stability of C. orbiculatus. Ultrasound-assisted extraction at room temperature in shorter processing time could maximize the overall yields and biological activities of C. orbiculatus. The major constituent was identified as betulinic acid, and the pentacyclic triterpene represented the prominent anticancer activity of C. orbiculatus. Forced expression of cytidine deaminase conferred acquired resistance to gemcitabine, while betulinic acid displayed equivalent cytotoxicity toward gemcitabine-resistant and sensitive cells. A combination therapy of gemcitabine with betulinic acid produced synergistic pharmacologic interaction on cell viability, apoptosis and DNA double-strand breaks. Moreover, betulinic acid abrogated gemcitabine-triggered Chk1 activation by destabilizing Chk1 loading via proteasomal degradation. The combination of gemcitabine and betulinic acid significantly retarded BxPC-3 tumor growth in vivo compared to single-agent gemcitabine treatment alone, accompanied with reduced Chk1 expression. These data provide evidence that betulinic acid is a potential candidate for chemosensitization as a naturally occurring Chk1 inhibitor and warrants further preclinical evaluation.