Vitamin C enhances cisplatin sensitivity in bladder cancer via 5hmC-mediated epigenetic modulation of ATF4

Cisplatin resistance remains a major challenge in the clinical treatment of bladder cancer (BC), and the epigenetic regulation of this resistance, particularly involving 5-hydroxymethylcytosine (5hmC), has not been fully elucidated. Here, we investigated the role of 5hmC and vitamin C (VC) in modulating cisplatin sensitivity in BC. Clinical analyses of 36 BC patients receiving cisplatin-based neoadjuvant chemotherapy showed that reduced 5hmC levels in pre-chemotherapy tumor tissues were significantly associated with cisplatin resistance (CR-BC) and poor prognosis, with low 5hmC correlating with shorter progression-free survival (PFS). In vitro, we established two cisplatin-resistant cell lines (T24-CR, UMUC-3-CR) that exhibited reduced 5hmC compared to parental cells. Treatment with 100 μM VC significantly restored 5hmC levels in CR-BC cells by activating TET enzymes, inhibited cell proliferation, and enhanced cisplatin sensitivity; these effects were abrogated by the TET inhibitor Bobcat339, confirming VC acts in a TET-dependent manner. Mechanistically, genome-wide 850 K methylation array and RNA-seq analyses revealed that VC upregulated methylation specifically at the promoter of ATF4, a downstream effector of the MAPK pathway, thereby downregulating ATF4 expression. ATF4 knockdown in CR-BC cells increasing cisplatin sensitivity, while Bobcat339 reversed VC-induced ATF4 downregulation. In vivo, VC combined with cisplatin significantly inhibited tumor growth in T24-CR xenografts, and co-treatment with ATF4 knockdown further enhanced this effect, accompanied by elevated 5hmC and reduced Ki67 in tumors. Collectively, our findings identify reduced 5hmC as a hallmark of cisplatin-resistant BC and reveal a novel mechanism by which VC enhances cisplatin sensitivity. VC activates TET enzymes to increase 5mC at the ATF4 promoter, downregulating ATF4 and modulating the MAPK pathway. This highlights VC as a potential epigenetic adjuvant to overcome cisplatin resistance in BC.