Introduction: Cyclooxygenase-2 (COX-2) is an inducible COX isoenzyme, whose expression is very low in most tissues, but increases in states of acute and chronic inflammation, under the influence of numerous physical, chemical and biological factors, including carcinogens. Increased expression of the enzyme COX-2 and its most important product prostaglandin E2 (PGE2) has been detected in malignant tumors. DNA methylation, an epigenetic event mediated by DNA methyltransferases (DNMT) enzymes, is thought to play an important role in the transcriptional “silencing” of tumor-suppressor genes associated with COX-2 signaling pathways. Objective: In vitro determination of the cytotoxic activity of celecoxib, a selective COX-2 inhibitor, as well as 5-aza-2′-deoxycytidine (decitabine), a demethylating agent, on human cell lines of lung adenocarcinoma (A549), colon adenocarcinoma (HT-29) and fetal lung fibroblasts (MRC-5). The aim is also to examine the expression of genes involved in the process of carcinogenesis in A549, HT-29 and MRC-5 cells treated with celecoxib and decitabine, as well as with their combinations with natural substances – silibinin and bile acids (chenodeoxycholic (CDCA) and ursodeoxycholic acid (UDCA)). Methods: The cytotoxicity of the tested substances on A549, HT-29 and MRC-5 cells was evaluated by the MTT test. Quantification of gene expression was performed using the qRT-PCR method, and was analyzed using the comparative ΔΔCt method, with ACTB as a reference gene. Statistical processing of the results was done by one-factor analysis of variance (ANOVA) with Taki’s post-hoc test. Results: Celecoxib and decitabine exhibit concentration-dependent cytotoxicity, i.e. inhibition of viability and proliferation of malignant and normal cells. In A549 and HT-29 cells, compared to the results obtained in the untreated, control group of cells, it was observed that celecoxib decreased the expression of the genes involved in the synthesis of COX-2 and PGE2 (PTGS2 and PTGES), and increased the expression of the gene responsible for the catabolism of PGE2 (HPGD), which encodes the synthesis of the enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH). Silibinin showed a synergistic anti-inflammatory effect with celecoxib in terms of increasing HPGD gene expression in A549 and HT-29, but also decreasing PTGS2 gene expression in HT-29 cells. UDCA exhibited a fully synergistic anti-inflammatory effect with celecoxib in A549, while in HT-29 cells this synergism was observed in terms of decreasing PTGS2 expression and increasing HPGD gene expression. In both malignant cell lines, celecoxib reduced the mRNA level for the pro-inflammatory transcription factor NFκB, with observed the additive effect of co-treatment with silibinin. The tested bile acids exhibited this synergistic effect only in HT-29 cells. Examining the expression of tumor-suppressor genes showed that celecoxib increased the expression of TP53 in A549 cells, and PTEN expression in A549 and HT-29. In both cell lines addition of silibinin and UDCA to celecoxib increased the amounts of TP53 and PTEN. In A549 cells CDCA also acted synergistically with celecoxib to increase the levels of these tumor-suppressors. Celecoxib caused an increase in the amount of mRNA for the proapoptotic BAX gene, and decreased the amount of mRNA for the antiapoptotic BCL2 gene in A549 and HT-29 cells. Silibinin and bile acids acted differently on the genes involved in the regulation of the apoptosis, but the addition of these natural substances to celecoxib inevitably directed the malignant cells towards apoptosis, observing the BAX/BCL2 mRNA ratio. The results show that the treatment of malignant cells with celecoxib increased the expression of the CDH1 gene, which encodes the synthesis of the tumor-suppressor epithelial protein E-cadherin, and decreased the level of the genes responsible for the synthesis of important matrix metalloproteinases (MMP2 and MMP9) that play a role in tumor invasion and metastasis. Silibinin and UDCA showed beneficial effects on reducing the metastatic potential of A549 and HT-29 cells, considering the synergistic effect with celecoxib regarding the modulation of the expression of the mentioned genes. Decitabine negatively affected mRNA levels for all three DNMT enzymes (DNMT1, DNMT3A, and DNMT3B) in A549 and HT-29 cells. The results show that the treatment of A549 cells with decitabine, by reducing the expression of the transcriptional repressor ZEB1, led to the transcriptional activation of CDH1 and HPGD tumor-suppressor genes. A complete synergistic effect with decitabine in A549 was exhibited by silibinin and UDCA. Conclusion: Taking into account the obtained IC50 values for celecoxib, it is concluded that malignant cells are more sensitive to selective COX-2 inhibition, compared to normal cells, which indicates an important role of the COX-2 enzyme in the survival and proliferation of lung and colon adenocarcinoma cells. It is also concluded that there is a possibility of modulating the expression of genes involved in carcinogenesis using the selective COX-2 inhibitor. It was shown that the demethylating agent has a role in the reactivation of epigenetically “silenced” tumor-suppressor genes that encode the synthesis of E-cadherin and 15-PGDH in lung adenocarcinoma cells. Also, the synergistic antineoplastic effects of silibinin and bile acids with celecoxib and decitabine were confirmed, which opens the possibility of applying new combined pharmacological strategies in the therapy of lung and colon adenocarcinoma.