Xanthone-rich extract from Gentiana dinarica transformed roots and its active component norswertianin induce autophagy and ROS-dependent differentiation of human glioblastoma cell line

Gordana Tovilovic-Kovacevic, Dijana Krstic-Milosevic, Branka Vinterhalter, Mina Toljic, Vladimir Perovic, Vladimir Trajkovic, Ljubica Harhaji-Trajkovic, Nevena Zogovic

Glioblastoma multiforme (GMB) is the most malignant of all brain tumors with poor prognosis. Anticancer potential of xanthones, bioactive compounds found in Gentiana dinarica, is well-documented. Transformation of G. dinarica roots with Agrobacterium rhizogenes provides higher xanthones accumulation, which enables better exploitation of these anticancer compounds. The aim of this study was to investigate antiglioma effect of three different G. dinarica extracts: E1-derived from untransformed roots, E2-derived from roots transformed using A. rhizogenes strain A4M70GUS, and E3-derived from roots transformed using A. rhizogenes strain 15834/PI. Further, mechanisms involved in anticancer potential of the most potent extract were examined in detail, and its active component was determined. The cell viability was assessed using MTT and crystal violet test. Cell cycle analysis, the expression of differentiation markers, the levels of autophagy, and oxidative stress were analyzed by flow cytometry. Autophagy and related signaling pathways were assessed by immunoblotting. E3, in contrast to E1 and E2, strongly reduced growth of U251 human glioblastoma cells, triggered cell cycle arrest in G2/M phase, changed cellular morphology, and increased expression of markers of differentiated astrocytes (glial fibrillary acidic protein) and neurons (β-tubulin). E3 stimulated autophagy, as demonstrated by enhanced intracellular acidification, increased microtubule-associated light chain 3B (LC3-I) conversion to autophagosome associated LC3-II, and decreased level of selective autophagy target p62. Induction of autophagy was associated with Akt-dependent inhibition of main autophagy suppressor mammalian target of rapamycin (mTOR). Both genetic and pharmacological inhibition of autophagy suppressed the expression of differentiation markers, but had no effect on cell cycle arrest in E3-treated cells. E3 stimulated oxidative stress, and antioxidants vitamin E and N-acetyl cysteine inhibited autophagy and differentiation of E3-treated U251 cells. The most prevalent compound of E3, xanthone aglycone norswertianin, also arrested glioblastoma cell proliferation in G2/M phase and induced glioblastoma cell differentiation through induction of autophagy and oxidative stress. These results indicate that E3 and its main active component norswertianin may serve as a potential candidate for differentiation therapy of glioblastoma.