Carvacrol and rosemary oil at higher concentrations induce apoptosis in human hepatoma HepG2 cells

Natural essential oils are volatile herbal complex compounds which manifest cytotoxic effects on living cells depending on their type and concentration but usually they are not genotoxic. Our previous studies showed that carvacrol (CA) and rosemary essential oil (RO) induced growth inhibition of both human cell lines HepG2 and BHNF-1, with hepatoma HepG2 cells being more sensitive to either compound tested. Cytotoxic concentrations of CA and RO induced the formation of DNA strand breaks. Further ex vivo studies showed that extracts prepared from hepatocytes of CA- and RO-supplemented rats did not increase incision repair activity compared to extracts from liver cells of control animals. Therefore, the aim of this work was to determine the effect of cytotoxic concentrations of CA and RO on the cell cycle and the ability of both natural volatiles to induce DNA fragmentation and apoptotic death of human hepatoma HepG2 cells. These effects were measured after 24 h incubation of HepG2 cells with CA and RO using three independent methods - flow cytometry, internucleosomal DNA fragmentation (electrophoresis) and micronucleus assay. Evaluation of morphological changes and formation of micronuclei in HepG2 cells showed no increase in the number of micronuclei in cells treated by CA and RO compared to control cells. On the other hand, CA and RO induced morphological changes typical for apoptosis in concentration-dependent manner. The presence of necrosis was negligible. Both natural compounds caused shrinking of cytoplasmic membrane and formation of apoptotic bodies. In addition, the highest concentrations of CA and RO induced internucleosomal DNA fragmentation (formation of DNA ladder) in HepG2 cells. Cell cycle analysis revealed the accumulation of cells in the G1 phase, which was accompanied by a reduction in the number of cells in the S phase after 24 h exposure to the substances tested. The cell division was thus slowed down or stopped and this process resulted in cell death.