Inhibition of non-homologous end joining mitigates paclitaxel resistance resulting from mitotic slippage in non-small cell lung cancer.

Mitotic slippage, which enables cancer cells to bypass cell death by transitioning from mitosis to the G1 phase without undergoing normal cytokinesis, is one likely mechanism of paclitaxel (PTX) resistance. DNA double-strand breaks (DSBs) in the G1 phase are mainly repaired through non-homologous end joining (NHEJ). Therefore, inhibiting NHEJ could augment the PTX-induced cytotoxicity by impeding the repair of PTX-induced DSBs during the G1 phase following mitotic slippage. We aimed to evaluate the effects of NHEJ inhibition on mitotic slippage after PTX treatment in non-small cell lung cancer (NSCLC). H1299, A549, H1975, and H520 NSCLC cell lines were employed. In addition, A-196 and JQ1 were used as NHEJ inhibitors. H1299 cells were PTX-resistant and exhibited an increased frequency of mitotic slippage upon PTX treatment. NHEJ inhibitors significantly augmented the PTX-induced cytotoxicity, DSBs, and apoptosis in H1299 cells. The newly generated PTX-resistant cells were even more prone to mitotic slippage following PTX treatment and susceptible to the combined therapy. Docetaxel further demonstrated synergistic effects with the NHEJ inhibitor in PTX-resistant cells. NHEJ inhibition may overcome intrinsic or acquired PTX resistance resulting from mitotic slippage by synergistically increasing the cytotoxic effects of antimitotic drugs in NSCLC.

Evaluating the immunoproteasome as a potential therapeutic target in cisplatin-resistant small cell and non-small cell lung cancer.

PURPOSE: We evaluated the expression of proteasome subunits to assess whether the proteasome could be a therapeutic target in cisplatin-resistant lung cancer cells. METHODS: Cisplatin-resistant (CR) variants were established from three non-small cell lung cancer (NSCLC) cell lines (A549, H1299, and H1975) and two small cell lung cancer (SCLC) cell lines (SBC3 and SBC5). The expression of proteasome subunits, the sensitivity to immunoproteasome inhibitors, and 20S proteasomal proteolytic activity were examined in the CR variants of the lung cancer cell lines. RESULTS: All five CR cell lines highly expressed one or both of the immunoproteasome subunit genes, PSMB8 and PSMB9, while no clear trend was observed in the expression of constitutive proteasome subunits. The CR cells expressed significantly higher levels of PSMB8 and PSMB9 proteins, as well. The CR variants of the H1299 and SBC3 cell lines were more sensitive to immunoproteasome inhibitors, and had significantly more proteasomal proteolytic activity than their parental counterparts. CONCLUSIONS: The immunoproteasome may be an effective therapeutic target in a subset of CR lung cancers. Proteasomal proteolytic activity may be a predictive marker for the efficacy of immunoproteasome inhibitors in cisplatin-resistant SCLC and NSCLC.

Bromodomain and extraterminal domain inhibition synergizes with WEE1-inhibitor AZD1775 effect by impairing nonhomologous end joining and enhancing DNA damage in nonsmall cell lung cancer.

Bromodomain and extraterminal domain (BET) inhibitors are broadly active against distinct types of cancer, including nonsmall cell lung cancer (NSCLC). Previous studies have addressed the effect of BET-inhibiting drugs on the expression of oncogenes such as c-Myc, but DNA damage repair pathways have also been reported to be involved in the efficacy of these drugs. AZD1775, an inhibitor of the G2-M cell cycle checkpoint kinase WEE1, induces DNA damage by promoting premature mitotic entry. Thus, we hypothesized that BET inhibition would increase AZD1775-induced cytotoxicity by impairing DNA damage repair. Here, we demonstrate that combined inhibition of BET and WEE1 synergistically suppresses NSCLC growth both in vitro and in vivo. Two BET inhibitors, JQ1 and AZD5153, increased and prolonged AZD1775-induced DNA double-strand breaks (DSBs) and concomitantly repressed genes related to nonhomologous end joining (NHEJ), including XRCC4 and SHLD1. Furthermore, pharmaceutical inhibition of BET or knockdown of the BET protein BRD4 markedly diminished NHEJ activity, and the BET-inhibitor treatment also repressed myelin transcription factor 1 (MYT1) expression and promoted mitotic entry with subsequent mitotic catastrophe when combined with WEE1 inhibition. Our findings reveal that BET proteins, predominantly BRD4, play an essential role in DSB repair through the NHEJ pathway, and further suggest that combined inhibition of BET and WEE1 could serve as a novel therapeutic strategy for NSCLC.