VDR Decrease Enhances the Efficacy of 1,25-Dihydroxyvitamin D3 Inhibiting Gefitinib Resistance by Regulating...

Background: Gefitinib is a first-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) targeting EGFR-mutated non-small cell lung cancer (NSCLC) and is a current first-line treatment for NSCLC. However, acquired resistance leads to the failure of treatment and remains a challenge. Therefore, identifying novel therapeutic approaches to combat EGFR-TKI resistance is crucial. Methods: The Cancer Genome Atlas (TCGA) database analysis and gefitinib-resistant cell lines were used to analyze VDR expression in NSCLC. Cell proliferation and apoptosis were assessed via MTT assay, colony formation assay, and flow cytometry. Immunofluorescence, qPCR, and Western blotting were used to measure mRNA and protein expression levels of VDR and other related molecules. Xenograft tumors in BALB/c nude mice were employed to investigate the effects of VDR and 1,25-dihydroxyvitamin D3 (1,25(OH)₂D₃) on gefitinib-resistant tumors in vivo. Results: We found that VDR was significantly upregulated in EGFR-TKI-resistant NSCLC cells. Patients with high VDR expression exhibited poor prognosis. VDR knockdown significantly inhibited cell proliferation, tumor growth, and reduced gefitinib resistance, whereas VDR overexpression enhanced resistance. VDR knockdown downregulated EGFR and FASN expression. Silencing either EGFR or FASN confirmed the existence of a positive feedback regulatory loop involving VDR, EGFR, and FASN. Treatment with 1,25(OH)₂D₃ increased VDR levels but decreased EGFR and FASN expression. VDR knockdown significantly enhanced the inhibitory effect of 1,25(OH)₂D₃ on gefitinib resistance. The combination of VDR knockdown and 1,25(OH)₂D₃ treatment was more effective than either treatment alone in suppressing EGFR and FASN expression. Conclusions: VDR promotes NSCLC resistance to EGFR-TKIs by regulating EGFR and FASN expression through a positive feedback loop. Knocking down VDR effectively enhances the ability of 1,25(OH)₂D₃ to overcome gefitinib resistance, mediated by the synergistic downregulation of EGFR and FASN expression. Targeting VDR represents a potential strategy to enhance the efficacy of 1,25(OH)₂D₃ in overcoming EGFR-TKI resistance.