## 4 Discussion
hortensis_ L. extract corresponds to the reduction in the level of lipid peroxidation product, which is MDA. Extracts from the vegetative phases F1-3 characterized by a lower level of all phenolic compounds also had a higher level of MDA compared to control cells. In the case of treating cells with extracts from the V1 and V2 phases with low concentrations, a significantly higher level of phenolic compounds was accompanied by preventing the increase in MDA levels even after UV radiation. Physical properties and functionality of cell membranes are directly affected by formation of lipid peroxidation products. The lipid asymmetry is interrupted reducing the hydrophobicity of the inner layer of lipid membrane causing depolarization [50]. As a result, membrane integrity may be lost [51]. Regardless of the above, lipid peroxidation products are chemically reactive molecules due to their carbon-carbon double bond structure and electrophilic characteristics, which reacts with the cell's nucleophilic components, including DNA, lipids, peptides, and proteins, leading to disturbances in cell metabolism. During these reactions, free reactive lipid peroxidation products decrease, and at the same time adducts with proteins increase that promote disruption of cell signaling and affect metabolic modifications resulting in cell dysfunction and apoptosis [52]. Regardless of the antioxidant activity of the polyphenols contained in _S. hortensis_ L. extracts, other components of the extract can completely modify the lipid peroxidation process. Earlier it was pointed out that carvacrol present in very high concentration in _S. hortensis_ L. extract may increase the permeability of bacterial cell membranes, which leads to their death [53]. It can be suggested that the results of this study confirm the opposite effect of carvacrol on lipid peroxidation than polyphenols. The consequence of this is an increase in lipid peroxidation for extracts from V2 phase, full flowering and end of flowering, and at higher concentrations of extracts, and therefore even higher carvacrol concentration. Moreover, the higher lipid peroxidation after using extracts V1 and V2 may also be caused by lowest level of rutin and rosmarinic acid in these extracts. Both these polyphenols are powerful antioxidants, and their higher level in F1-F3 extracts guarantees protection of membrane phospholipids. Rutin that is known for its antioxidant activity, in the case of other skin cells, has been shown as a molecule that not only directly reacts with free radicals but also inactivates enzymes responsible for their generation [28]. Furthermore, rutin protective effect on the skin relates to its involvement in intracellular signaling leading to cytoprotective proteins expression, antioxidant enzymes activation, as well as apoptosis prevention [54]. Moreover, rutin antioxidant action significantly prevents lipid oxidative metabolism, which might be observed as lower lipid peroxidation products level, but also decreased level of other lipid metabolism markers such as isoprostanes or endocannabinoids, which are known as pro-inflammatory mediators [55]. On the other hand, rutin also shows cytotoxic action against various types cancer cells [56; 57; 58], which can be connected with a higher level of lipid peroxidation products in cancer cell compared to non-transformed cells what was also observed in this study. Simultaneously, also rosmarinic acid that level is the highest in studied extracts, additionally increases skin cells antioxidant capacity. It has also been shown that rosmarinic acid can stimulate the activity of antioxidant enzymes such as glutathione peroxidase, catalase, and superoxide dysmutase [59; 60], as well as prevent lipid peroxidation [61]. The cytoprotective action is visible mainly in skin melanocytes, but not in the case of melanoma, which is partially confirmed by data for other cancer types [62].