High-effect synergy from dual HDAC6 and YAP-TEAD inhibition in ovarian clear cell carcinoma.

BACKGROUND: Ovarian clear cell carcinoma (OCCC) is an aggressive epithelial subtype characterized by intrinsic chemoresistance and limited targeted therapies. Transcriptional programs driven by Yes-associated protein (YAP) and TEA domain (TEAD) factors support tumor stress adaptation, suggesting that this axis represents therapeutic vulnerability. Histone deacetylase 6 (HDAC6) has been linked to Hippo/YAP regulation. We hypothesized that dual inhibition of HDAC6 and YAP-TEAD would converge on the YAP axis and yield synergistic cytotoxicity in OCCC. METHODS: We first analyzed a tissue microarray (TMA) of 171 OCCC cases stained for HDAC6, YAP, and connective tissue growth factor (CTGF) to assess correlations and survival associations. Guided by these findings, we treated HAC2, OVTOKO, and RMG-1 cells with fixed-ratio combinations of the HDAC6 inhibitor citarinostat and the YAP-TEAD disruptor verteporfin (VP). Drug interactions were quantified using Loewe isobolograms and the Chou-Talalay combination index (CI). Immunofluorescence was used to measure total YAP (t-YAP) and the nuclear-cytoplasmic (N/C) ratio, while RT-qPCR assessed YAP, CTGF, and CYR61 transcripts. In HAC2, western blotting analysis was performed for the expression of CTGF protein. The selected assays were reproduced with the distinct YAP-TEAD inhibitor CA-3. RESULTS: In patient tissues, cytoplasmic HDAC6 correlated positively with cytoplasmic YAP and CTGF, and inversely with nuclear HDAC6, but none of the markers predicted survival. Across all three cell lines, dual treatment produced robust, high-effect synergy (minimum CI ≈ 0.08–0.28). In HAC2, the combination increased t-YAP but reduced the N/C ratio, indicating restricted nuclear access. VP alone decreased t-YAP, whereas synergy was confirmed with CA-3, suggesting that TEAD disruption is the relevant mechanism. RT-qPCR revealed divergent cell line-specific transcriptional responses, with CTGF and CYR61 often upregulated by citarinostat and suppressed by VP or combination. The CTGF-HDAC6 correlation in the TMA supports the biological relevance of this axis, whereas early transcript changes likely reflect compensatory feedback. CONCLUSIONS: An investigation from the clinic to bench identified an HDAC6-YAP-CTGF axis in OCCC and demonstrated that dual HDAC6 and YAP-TEAD inhibition yields strong high-effect synergy by restricting YAP nuclear access and limiting TEAD engagement. These findings may provide a rationale for biomarker-guided validation of citarinostat-VP combinations in organoids, in vivo models, and early-phase clinical studies. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12935-026-04204-7.