Abstract:
Lung cancer is a major cause of cancer-related death. Tumor growth is a fundamental process of cancer progression. Vital characteristics that potentiate tumor growth are cancer stem cell (CSC) and enhance cell proliferation. Previous studies reported that targeting on CSC-promoting pathways or proliferative signals was a promising strategy in cancer treatment. Gigantol, a bibenzyl compound from Dendrobium draconis, was reported to have a suppressive activity on CSC phenotypes, in vitro. However, the effects of gigantol on tumorigenicity in vivo, cancer cell proliferation and related CSC-promoting and proliferation regulatory mechanisms have not been explored. In this study, ectopic tumor xenograft in nude mice was utilized to gain understanding on how gigantol attenuated CSCs and tumorigenicity capacity, and proliferation assay and colony formation assay were used to evaluate an anti-proliferative effect of gigantol. Proteomics analysis was used to identify molecular mechanisms of gigantol on the CSC-phenotype and proliferation regulation. The results of tumor xenografts showed that the tumor raised from gigantol-treating cells had lower tumor weight and density than its own paired untreated control within each mouse. Morevover, the tumors raised from gigantol-pretreated cells showed significantly lower proliferative tumor cells, indicating by Ki-67 labeling. For proliferation, gigantol reduced growth rates and colony forming units of lung cancer cells. The proteomics analysis revealed that gigantol suppressed functional proteins manipulating the CSC properties, which were signaling proteins in PI3K/Akt/mTOR and JAK/STAT pathways. Also, c-Myc, a proliferation- and stemness-promoting transcription factor, was down-regulated by gigantol. It was known that Akt controls c-Myc by inhibiting GSK3β. Western blot analysis and immunoprecipitation were used to confirm, and the results showed that gigantol facilitated GSK3β function, which enhanced c-Myc degradation by increasing c-Myc-ubiquitin complex. In conclusion, this study reveals novel insights into the anti-tumor mechanisms of gigantol focused on CSC targeting, destabilizing tumor integrity, and suppressing tumor growth via suppression of the PI3K/Akt/mTOR and JAK/STAT pathways, as well as, induction of GSK3β-mediated c-Myc ubiquitin-proteasome degradation. This data supports the potential of gigantol to be developed as a drug for lung cancer.
