Antagonistic effects of nitric oxide in a glioblastoma photodynamic therapy model:mitigation by BET bromodomain inhibitor JQ1

Endogenous nitric oxide (NO) generated by inducible NO synthase (iNOS) promotes glioblastoma cell proliferation and invasion, and also plays a key role in glioblastoma resistance to chemotherapy and radiotherapy. Non-ionizing photodynamic therapy (PDT) has anti-tumor advantages over conventional glioblastoma therapies. Our previous studies revealed that glioblastoma U87 cells upregulate iNOS after a photodynamic challenge and that resulting NO not only increased resistance to apoptosis, but rendered surviving cells more proliferative and invasive. These findings were largely based on the effects of inhibiting iNOS activity and scavenging NO. Demonstrating now that iNOS expression in photostressed U87 cells is mediated by NF-κB, we hypothesized that (i) recognition of acetylated lysine (acK) on NF-κB p65/Rel A by bromodomain and extra-terminal (BET) protein Brd4 is crucial, and (ii) by suppressing iNOS expression, a BET inhibitor (JQ1) would attenuate the negative effects of photostress. The following evidence was obtained: (i) Like iNOS, Brd4 protein and p65-acK levels increased several fold in photostressed cells; (ii) JQ1 at minimally toxic concentrations had no effect on Brd4 or p65-acK upregulation after PDT, but strongly suppressed iNOS, survivin, and Bcl-xL upregulation, along with the growth and invasion spurt of PDTsurviving cells; (iii) JQ1 inhibition of NO production in photostressed cells closely paralleled that of growth/invasion inhibition; (iv) At 1% the concentration of iNOS inhibitor 1400W, JQ1 reduced post-PDT cell aggressiveness to a far greater extent. This is the first evidence for BET inhibitor targeting of iNOS expression in cancer cells and how such targeting can markedly improve therapeutic efficacy.