Apoptotic toxicity of destruxin B in human non-Hodgkin lymphoma cells

Anticancer fungal natural products: Mechanisms of action and biosynthesis

Many fungal metabolites show promising anticancer properties both in vitro and in animal models, and some synthetic analogs of those metabolites have progressed into clinical trials. However, currently, there are still no fungi-derived agents approved as anticancer drugs. Two potential reasons could be envisioned: 1) lacking a clear understanding of their anticancer mechanism of action, 2) unable to supply enough materials to support the preclinical and clinic developments. In this review, we will summarize recent efforts on elucidating the anticancer mechanisms and biosynthetic pathways of several promising anticancer fungal natural products.

Altered membrane rigidity via enhanced endogenous cholesterol synthesis drives cancer cell resistance to destruxins

Destruxins, secondary metabolites of entomopathogenic fungi, exert a wide variety of interesting characteristics ranging from antiviral to anticancer effects. Although their mode of action was evaluated previously, the molecular mechanisms of resistance development are unknown. Hence, we have established destruxin-resistant sublines of HCT116 colon carcinoma cells by selection with the most prevalent derivatives, destruxin (dtx)A, dtxB and dtxE. Various cell biological and molecular techniques were applied to elucidate the regulatory mechanisms underlying these acquired and highly stable destruxin resistance phenotypes. Interestingly, well-known chemoresistance-mediating ABC efflux transporters were not the major players. Instead, in dtxA- and dtxB-resistant cells a hyper-activated mevalonate pathway was uncovered resulting in increased de-novo cholesterol synthesis rates and elevated levels of lanosterol, cholesterol as well as several oxysterol metabolites. Accordingly, inhibition of the mevalonate pathway at two different steps, using either statins or zoledronic acid, significantly reduced acquired but also intrinsic destruxin resistance. Vice versa, cholesterol supplementation protected destruxin-sensitive cells against their cytotoxic activity. Additionally, an increased cell membrane adhesiveness of dtxA-resistant as compared to parental cells was detected by atomic force microscopy. This was paralleled by a dramatically reduced ionophoric capacity of dtxA in resistant cells when cultured in absence but not in presence of statins. Summarizing, our results suggest a reduced ionophoric activity of destruxins due to cholesterol-mediated plasma membrane re-organization as molecular mechanism underlying acquired destruxin resistance in human colon cancer cells. Whether this mechanism might be valid also in other cell types and organisms exposed to destruxins e.g. as bio-insecticides needs to be evaluated.

Selective apoptotic cell death effects of oral cancer cells treated with destruxin B

BACKGROUND

Recent studies have revealed that destruxins (Dtx) have potent cytotoxic activities on individual cancer cells, however, data on oral cancer cells especial human are absent.

METHODS

Destruxin B (DB) was isolated and used to evaluate the selective cytotoxicity with human oral cancer cell lines, GNM (Neck metastasis of gingival carcinoma) and TSCCa (Tongue squamous cell carcinoma) cells, and normal gingival fibroblasts (GF) were also included as controls. Cells were tested with different concentrations of DB for 24, 48, and 72 h by MTT assay. Moreover, the mechanism of cytotoxicity was investigated using caspase-3 Immunofluorescence, annexin V/PI staining, and the expression of caspase-3, Bax, and Bcl-2 by western blotting after treated with different concentrations of DB for 72 h as parameters for apoptosis analyses.

RESULTS

The results show that DB exhibited significant (p < 0.01) and selective time- and dose-dependent inhibitory effects on GNM and TSCCa cells viability but not on GF cells. The data suggested that DB is capable to induce tumor specific growth inhibition in oral GNM and TSCCa cancer cells via Bax/Bcl-2-mediated intrinsic mitochondrial apoptotic pathway in time- and dose-dependent manners.

CONCLUSIONS

This is the first report on the anti-proliferation effect of DB in oral cancer cells. The results reported here may offer further evidences to the development of DB as a potential complementary chemotherapeutic target for oral cancer complications.

Destruxin B inhibits hepatocellular carcinoma cell growth through modulation of the Wnt/β-catenin signaling pathway and epithelial-mesenchymal transition

The aberrant activation of Wnt/β-catenin signaling plays an important role in the carcinogenesis and progression of hepatocellular carcinoma (HCC). Therefore, the Wnt/β-catenin signaling molecules are attractive candidates for the development of targeted therapies for this disease. The present study showed that destruxin B (DB) inhibits the proliferation and induces the apoptosis of HCC cells by decreasing the protein expression of anti-apoptotic Bcl-2 and Bcl-xL and increasing the expression of the proapoptotic protein Bax. More importantly, DB also attenuates Wnt-signaling in HCC cells by downregulating β-catenin, Tcf4, and β-catenin/Tcf4 transcriptional activity, which results in the decreased expression of β-catenin target genes, such as cyclin D1, c-myc, and survivin. Furthermore, DB affects the migratory and invasive abilities of Sk-Hep1 cells through the suppression of markers of the epithelial-mesenchymal transition (EMT). A synergistic anti-proliferative and migratory effect was achieved using the combination of DB and sorafenib in Sk-Hep1 cells. In conclusion, DB acts as a novel Wnt/β-catenin inhibitor and reduces the aggressiveness and invasive potential of HCC by altering the cells' EMT status and mobility. DB in combination with sorafenib may be considered for future clinical use for the management of metastatic HCC.