Hydroalcoholic extract of Buxus sempervirens shows antiproliferative effect on melanoma, colorectal carcinoma and prostate cancer cells by affecting the autophagic flow

Buxus sempervirens (European Box, Buxaceae, boxwood) has been used in folk medicine to treat rheumatism, arthritis, fever, malaria and skin ulceration while, in recent years, interest has grown on possible employment of boxwood extracts in cancer therapy. We studied the effect of hydroalcoholic extract from dried leaves of Buxus sempervirens (BSHE) on four human cell lines (BMel melanoma cells, HCT116 colorectal carcinoma cells, PC3 prostate cancer cells, and HS27 skin fibroblasts) to ascertain its possible antineoplastic activity. This extract inhibited proliferation of all cell lines in different degree as shown, after 48 h-exposure and MTS assay, by the values of GR(50) (normalized growth rate inhibition(50)) that were 72, 48, 38, and 32 μg/mL for HS27, HCT116, PC3 and BMel cells, respectively. At the above GR(50) concentrations, 99% of all studied cells remained vital showing accumulation of acidic vesicles in the cytoplasm, mainly around nuclei, whereas a higher extract concentration (125 μg/mL) was cytotoxic causing, after 48 h-exposure, death of all BMel and HCT116 cells. Immunofluorescence showed microtubule-associated light chain three protein (LC3, a marker for autophagy) to be localized on the above acidic vesicles when cells were treated for 48 h with BSHE (GR(50) concentrations). Western blot analysis revealed, in all treated cells, a significant increase (2.2–3.3 times at 24 h) of LC3II, i.e., the phosphatidylethanolamine conjugate of the cytoplasmic form LC3I that is recruited in autophagosome membranes during autophagy. Such increase was accompanied, in all cell lines treated for 24 h or 48 h with BSHE, by a significant increment (2.5–3.4 times at 24 h) of p62, an autophagic cargo protein undergoing degradation during the autophagic process. Therefore, BSHE appeared to promote autophagic flow with its following blockade and consequent accumulation of autophagosome or autolysosomes. The antiproliferative effects of BSHE also involved cell cycle regulators such as p21 (HS27, BMel and HCT116 cells) and cyclin B1 (HCT116, BMel and PC3 cells) whereas, among apoptosis markers, BSHE only decreased (30%–40% at 48 h) the expression of the antiapoptotic protein survivin. It was concluded that BSHE impairs autophagic flow with arrest of proliferation and death in both fibroblasts and cancer cells, being the latter much more sensitive to these effects.