<p>Combination BET Family Protein and HDAC Inhibition Synergistically Elicits Chondrosarcoma Cell Apoptosis Through RAD51-Related DNA Damage Repair</p>

Abstract

Background: Chondrosarcoma is the second-most common type of bone tumor and has inherent resistance to conventional chemotherapy. Present study aimed to explore the therapeutic effect and specific mechanism(s) of combination BET family protein and HDAC inhibition in chondrosarcoma. Methods: Two chondrosarcoma cells were treated with BET family protein inhibitor (JQ1) and histone deacetylase inhibitors (HDACIs) (vorinostat/SAHA or panobinostat/PANO) separately or in combination; then, the cell viability was determined by Cell Counting Kit-8 (CCK-8) assay, and the combination index (CI) was calculated by the Chou method; cell proliferation was evaluated by 5-ethynyl-2′-deoxyuridine (EdU) incorporation and colony formation assay; cell apoptosis and reactive oxygen species (ROS) level were determined by flow cytometry; protein expressions of caspase-3, Bcl-XL, Bcl-2, γ-H2AX, and RAD51 were examined by Immunoblotting; DNA damage was determined by comet assay; RAD51 and γ-H2AX foci were observed by immunofluorescence. Results: Combined treatment with JQ1 and SAHA or PANO synergistically suppressed the growth and colony formation ability of the chondrosarcoma cells. Combined BET and HDAC inhibition also significantly elevated the ROS level, followed by the activation of cleaved-caspase-3, and the downregulation of Bcl-2 and Bcl-XL. Mechanistically, combination treatment with JQ1 and SAHA caused numerous DNA double-strand breaks (DSBs), as evidenced by the comet assay. The increase in γ-H2AX expression and foci formation also consistently indicated the accumulation of DNA damage upon cotreatment with JQ1 and SAHA. Furthermore, RAD51, a key protein of homologous recombination (HR) DNA repair, was found to be profoundly suppressed. In contrast, ectopic expression of RAD51 partially rescued SW 1353 cell apoptosis by inhibiting the expression of cleaved-caspase-3. Conclusion: Taken together, our results disclose that BET and HDAC inhibition synergistically inhibit cell growth and induce cell apoptosis through a mechanism that involves the suppression of RAD51-related HR DNA repair in chondrosarcoma cells.

Pharmaco-transcriptomic correlation analysis reveals novel responsive signatures to HDAC inhibitors and identifies Dasatinib as a synergistic interactor in small-cell lung cancer

“…therapeutic success rate through inhibition of the tumour resistance mechanism. The rationales of therapeutic combinations with HDAC inhibitors have been discussed, mainly including combined DNAdamaging chemotherapeutic agents, radiotherapy, hormonal therapies, DNA methyltransferase inhibitors, and immunotherapies, as well as various other small-molecule inhibitors [39,40,[55][56][57][58].…”

Epigenetic Approaches to Overcome Fluoropyrimidines Resistance in Solid Tumors

“…Recent data have shown that BET and HDAC inhibitors exert a synergistic effect on cellular processes in cancer cells; thus, dual BET/HDAC inhibitors have been designed, and preclinical studies are ongoing [150,151]. Unfortunately, the association of BET/HDAC inhibitors or dual inhibitors with fluoropyrimidines has not been tested in clinical studies.…”

BET Proteins as Attractive Targets for Cancer Therapeutics

“…The suppression of chromatin interaction with BET would explain the similar cellular effects evoked by HDACis and BETis and the synergistic effect of their combination. A mechanistic study revealed that the cytotoxic effect induced by the cotreatment with JQ1 and HDACi was attributed to the induction of DNA damage and impaired DNA repair through the suppression of RAD51, a key homologous recombination (HR) protein [93]. The ectopic expression of RAD51 partially compromised the cytotoxic effect elicited by the cotreatment with JQ1 and HDACi, indicating that RAD51 downregulation could be significant for clinical benefit.…”

“…Apart from repressing BRCA1, RAD51 and CtIP expression, BETis downregulated the expression of HR proteins such as BRCA2, BRIP1, FANCD2, CHK1, CHK2, MRE11, RAD50 TIP60, WEE1 and EZH2 [89,93,99,100]. BRCA2 is the primary mediator of RAD51 nucleofilament formation and strand exchange in HR.…”

“…Additionally, BETis directly influenced DDR by increasing the formation of γH2AX, a marker of DSBs (Figure 4) [25,93,100,168]. Specific depletion of BRDT, BRD2, BRD3 and BRD4 indicates that BRD2 and BRD4 knockdown increased DSBs levels, suggesting that cells deficient in these BETs are susceptible to spontaneous DSB formation [168].…”