## 4 Discussion  
Because of the non-specific symptoms of the disease and diagnosis at a late stage, lung cancer results in a poor prognosis and limited therapeutic options. Members of the ABC transporter superfamily, including P-gp, are involved in the development of resistance to a wide range of drugs that are structurally unrelated and have different mechanisms of action. By increasing the efflux of chemotherapeutic agents, cancer cells can reduce intracellular drug levels and prevent them from reaching therapeutic concentrations at the target site [29]. In studies using cell lines and animals, it has been shown that many substances act as effective P-gp inhibitors. However, the results of clinical trials indicate the low potency and specificity of these substances compared to most clinically used drugs [30]. On the other hand, an increased amount of P-gp was found in many types of cancer cells, including tumors that had not been treated with chemotherapeutic agents. This suggests that the activity of the transporter may be related to the pathomechanism of tumor development. It is probable that the amount of P-gp in a cell is correlated with its exposure to toxins and xenobiotics, which are directly related to DNA damage, leading to the formation of the cancer cell phenotype. Whether the most extensively studied _ABCB1_ gene polymorphisms affect treatment responses or the side effects of chemotherapeutics remains a matter of speculation. This issue is complex, and it is known that many factors can modulate the process of transcription and protein formation. Therefore, in the presented study, the prognostic significance of the _ABCB1_ mRNA level in lung cancer was assessed using data collected in publicly available bioinformatics databases. Figure 12: The correlations between _ABCB1_ expression, selected clinical parameters, and DNA methylation probes using MEXPRESS in LUSC (* \(p<0.05\); ** \(p<0.01\); *** \(p<0.001\)). The obtained results confirmed a large diversity in mRNA levels for \(ABCB1\) depending on the tissue type. High expression of this gene and P-gp in adrenal, kidney, liver, and intestine tissues was confirmed in many previous studies [31; 32]. The exceptions were the colon and stomach, where no significant differences in \(ABCB1\) expression were shown between cancer tissues and normal tissues. Additionally, for gastric cells, a negligible level of the transcript was observed. In a study using cell lines, significantly lower \(ABCB1\) mRNA levels in gastric cancers compared to colon cancer were shown [33], reflecting the physiological role of this transporter in the absorption in the small/large intestines. In the presented study, non-cancerous lung tissue indicated higher levels of \(ABCB1\) mRNA compared to NSCLC cells. Our findings were consistent with the results of research related to breast [34] and colorectal cancer [35]. This suggests a close link between the reduction in efflux pump function and the transformation of, i.a., normal lung cells to malignant cells. Furthermore, cell-line-based research indicates that metastatic cells develop a secondary multidrug resistance involving P-gp under the influence of chemotherapy. Several research teams found significant increases in \(ABCB1\) expression after repeated courses of treatment with chemotherapeutic drugs [36; 37; 38]. These observations are also clinically significant due to the strong positive correlation between the amounts of mRNA and P-gp protein, which was confirmed in the presented study and other clinical lung cancer samples [37]. Tobacco smoking is an important risk factor for lung cancer development. Specifically, this includes SCLC and the squamous histological type of NSCLC cancer. It is thought that chemicals found in tobacco smoke alter the expression or the activity of drug-metabolizing enzymes and xenobiotic transporters and could directly suppress P-gp activity [39]. This may be an explanation for the results obtained in the presented study among LUSC cancer patients.