Transcriptional Regulation of Receptor-Mediated Mitophagy in Sunitinib-Resistant Renal Cancer Cells: Response to Succinic Acid

Background/Objectives: Drug resistance is a major challenge in cancer therapy, and mitochondria contribute to this process by controlling both metabolic adaptability and cell survival signaling. Mitophagy, the selective lysosomal removal of dysfunctional mitochondria, has been implicated in therapy resistance, yet its role in sunitinib-resistant renal cancer remains poorly defined. Methods: In this study, acquired sunitinib resistance was established in ACHN renal cancer cells through eight months of stepwise dose escalation. Initial selection conditions were determined using CCK-8 viability and crystal violet colony assays in parental ACHN cells, whereas sustained proliferation under continuous sunitinib exposure was used as the operational criterion for the resistant phenotype. Resistant and parental sensitive cells were treated with 25 µM and 50 µM succinic acid, alone or in combination with sunitinib. Gene expression of BNIP3, NIX, FUNDC1, LC3, PINK1, Parkin, PGAM5, SRC, LONP1, and ATP5F1A was measured by RT-qPCR, and BNIP3 and NIX protein levels were assessed by ELISA. Results: Resistant cells showed significant upregulation of receptor-mediated mitophagy components BNIP3, NIX and FUNDC1 (p < 0.05), with no significant change in LC3, alongside suppression of PINK1, Parkin, and mitochondrial homeostasis-associated genes LONP1, PGAM5, and ATP5F1A (p < 0.05). Succinic acid predominantly reduced BNIP3 and NIX protein levels in both cell lines and suppressed BNIP3, NIX, and LC3 mRNA expression in resistant cells. In contrast, the sunitinib + 50 µM succinic acid combination selectively increased PARKIN, PGAM5, LONP1, and ATP5F1A expression in resistant cells (2.49- to 5.98-fold; p < 0.005), a pattern not observed in parental cells. Conclusions: These findings indicate that sunitinib resistance in ACHN cells is associated with upregulated transcription of receptor-mediated mitophagy components and downregulated transcription of PINK1/Parkin pathway genes, and that exogenous succinic acid selectively upregulates PARKIN and other mitochondrial homeostasis-related gene expression in resistant, but not parental, cells.