## 4 Discussion
This may be one of the mechanisms of pesticide action promoting cancer cell proliferation. Namely, they stimulate an increase in GSH levels relative to GSSG, thereby promoting cancer growth. This applies to both tested lines, because in none of the tested cases was a decrease in GSH/GSSG noted relative to the control. In both cell lines, catalase activity after exposure to the fungicide is either increased (at the lower tested concentration) or maintained at the control level. This correlates with the increase in GSH levels and may mean that cancer cells are predisposed to reduce the level of free radicals by increasing the level of catalase due to the action of the fungicide. According to [PERSON] et al., inhibition of catalase activity results in an increase in the level of free radicals [28]. Therefore, it is most likely that cancer cells increase their protection against excessive oxidative stress by stimulating this enzyme. Also, the activity of glutathione peroxidase in cells of both tested lines exposed to cyprodinil increases, especially at the lower analyzed concentration. The increase in GPx activity is associated with the elimination of free radicals and with maintaining the state of oxidative balance, which in turn promotes the proliferation of cancer cells. Our results are consistent with the literature data indicating that exposure to pesticides causes an increase in GPx activity [29]. In the presented study, apoptotic cells were analyzed using acridine orange and ethidium bromide staining. Microscopic examination was used to supplement the results obtained from the cytometer and from the caspase 3/7 activity test. Compared to the control group, no apoptotic changes were observed in the cells of both lines exposed to cyprodinil. Similarly to the Caco-2 cells exposed to mesotriene in our previous experiment, no increase in the number of apoptotic cells was observed here [15]. This is a visible correlation of the stimulation of cell proliferation of both tested lines under the influence of the analyzed compound with the inhibition of the apoptosis process. Cyprodinil did not stop, but increased cell proliferation, without stimulating apoptosis. This was confirmed by the results obtained from the analysis carried out using the LC-ESI-MS/MS method. Cyprodinil penetrated both A-375 and DLD-1 cells to a high degree, especially at the lower analyzed concentrations. In summary, significant stimulation of cancer cells of both tested cell lines was observed as a result of their exposure to the tested fungicide, which is the result of effective penetration of this compound through cell membranes and stimulation of oxidative stress to a level supporting cancer cell proliferation, while reducing the level of apoptosis. Analysis of the zeta potential as a function of electrolyte pH for A375 and DLD-1 cells incubated with cyprodinil indicates that the tested fungicide does not alter the electrical properties of the cell membranes when applied at low concentrations (0.05 uM for A-375 cells and 0.025 uM for DLD-1 cells). This may suggest efficient intracellular penetration of cyprodinil into cancer cells. However, treatment with higher concentrations results in adsorption of the compound onto the cell surface, accompanied by a shift in zeta potential toward higher absolute values, thereby enhancing the electrostatic stability of the cells. Data obtained as a result of LC-ESI-MS/MS analysis confirm the ease of penetration of cyprodinil into cancer cells. Moreover, the obtained results are consistent with our previously published data indicating a very high permeability of cyprodinil into MCF-7 cancer cells [10]. Similarly to the cited publication, and in the current situation, cyprodinil migrated into the interior of the cells in greater amounts when it was applied at a lower concentration. This applies to both tested cell lines. The EFSA (European Food Safety Authority) presented data on cyprodinil taking into account the proposed MRLs (maximum residue levels).