Highly Potent Antiausterity Agents from Callistemon citrinus and Their Mechanism of Action against the PANC-1 Human Pancreatic Cancer Cell Line

Abstract

Human pancreatic cancer cells display remarkable tolerance to nutrition starvation that help them to survive in a hypovascular tumor microenvironment, a phenomenon known as "austerity". The elucidation of agents countering this tolerance is an established antiausterity strategy in anticancer drug discovery. In this study, a Callistemon citrinus leaf extract inhibited the viability of PANC-1 human pancreatic cancer cells preferentially under nutrient-deprived medium (NDM) with a PC 50 value of 7.4 μg/mL. Workup of this extract resulted in the isolation of three new meroterpenoids, callistrilones L−N (1−3), together with 14 known compounds (4−17). The structure elucidation of the new compounds was achieved by HRFABMS and by NMR and ECD spectroscopic analysis. The new compounds showed highly potent preferential cytotoxicity against PANC-1 cells with PC 50 values ranging from 10 to 65 nM in NDM. Of these, callistrilone L (1) inhibited PANC-1 cell migration and colony formation in a normal nutrient-rich condition. Callistrilone L (1) also strongly suppressed the migration of PANC-1 cells in real time. Mechanistically, 1 was found to inhibit the Akt/mTOR and autophagy activation pathway. Callistrilone L (1) and related meroterpenoids are promising leads for anticancer drug development based on the antiausterity strategy used in this work.

GDP Induces PANC‐1 Human Pancreatic Cancer Cell Death Preferentially under Nutrient Starvation by Inhibiting PI3K/Akt/mTOR/Autophagy Signaling Pathway

“…At the highest concentration of GDP (100 μM), colony formation was reduced by over 80 % compared to the control cells. These results prove the inhibitory effect of GDP on pancreatic cancer colony formation, similar to many previously discovered antiausterity agents, such as ancistrolikokine E3, callistrilone L, and ursenolide [11,20,23] …”

“…Quantitive real‐time cancer cell migration inhibition studies have rarely been reported. Callistrilone L and ursenolide, two potent anti‐austerity agents recently reported by our group, exerted remarkable cell migration inhibitory activities against PANC‐1 cells in real‐time [11,20] . In continuation, we herein also provide quantitative real‐time live‐evidence of the PANC‐1 cancer cell migration inhibition by GDP [21] .…”

“…For the control cells, the wound area was decreased to 26 % of the initial area after 24 h. Interestingly, in the case of the treatment with GDP, the wound area drastically remained at around 50 % (25 μM) and 70 % (50 and 100 μM) of the initial area. Therefore, GDP showed strong inhibition of PANC‐1 tumor cells’ migration, similar to fragranone A and callistrilone L, two previously identified anti‐austerity agents [11,21] . Thus, GDP is not only cytotoxic to PANC‐1 cells under nutrient deprivation conditions, but also could inhibit cancer cell migration in nutrient‐rich conditions, suggesting its possible therapeutic effect against pancreatic cancer metastasis from the hypo‐vascular microenvironment to hyper‐vascular organs like lungs, and liver where cancer cells can thrive to form large colonies.…”

“…This phenomenon is known as ‘colonization’, [22] Finding agents that can prevent the colonization process would be beneficial for preventing the development of cancer metastasis. Several antiausterity agents could exert potent colony formation inhibitory effects under nutrient‐rich conditions [11,17] . Therefore, GDP was tested for its colony formation inhibitory effects on PANC‐1 cells in DMEM.…”

“…Since the discovery of this phenomenon, a screening strategy has been developed for identifying agents that can eliminate cancer cells′ ability to tolerate nutrition starvation. Utilizing this strategy has resulted in the discovery of several potent anti‐austerity agents, including arctigenin, grandifloracin, and callistrilone L, and angelmarin [7–11] . Arctigenin, one of the most potent anti‐austerity agents, has completed an early Phase II human clinical trial, proving a positive survival outcome for pancreatic cancer patients, without any toxic side effects [7] .…”