p300 bromodomain inhibitors impair cell proliferation by inducing senescence in HPV positive head and neck squamous cell carcinoma

The incidence of human papillomavirus (HPV)-associated head and neck squamous cell carcinoma (HNSCC) has risen sharply in recent decades and now accounts for most new oropharyngeal cancers diagnosed in the United States. Standard of care therapies are effective but cause significant toxicities and long-term morbidities, highlighting a critical need for alternative treatment strategies. HPV-associated cancers, including oropharyngeal tumors, present a unique opportunity for targeted therapeutic intervention since their pathogenesis depends on the inactivation of major tumor suppressor pathways by the viral oncoproteins E6 and E7. One key oncogenic mechanism is the ability of HPVE6 to bind the transcriptional co-activator p300/CBP, thereby blocking p53 acetylation and activation. Therefore, we hypothesized that p300/CBP bromodomain inhibitors (BDIs) may disrupt the HPVE6-p300/CBP interaction and restore the p53 tumor suppressor program. We evaluated the effect of two p300/CBP BDIs, CCS1477 and GNE-781, on the HPV16+ HNSCC cell line, UM-SCC-47. Cell proliferation was assessed by Incucyte® live cell imaging, clonogenic survival by colony formation assays, and cell-cycle distribution by flow cytometry with propidium iodide. Senescence was measured by beta-galactosidase staining, and protein levels of key p53 targets were analyzed by immunoblot. Both p300/CBP BDIs markedly reduced cell proliferation and colony formation. Proliferation decreased in a dose-dependent manner, with an IC50 of 23.66 nM for CCS1477 and 9.06 nM for GNE-781. At 100 nM, CCS1477 and GNE-781 reduced colony formation by 94% (p<0.0001) and 97.9% (p<0.0001), respectively. p300/CBP BDIs induced G1 arrest, reflected by a 15% (p<0.0001) increase in the proportion of cells in G1 and a 30% (p<0.0001) reduction in S phase. Both inhibitors triggered a >8-fold (p<0.0001) increase in the number of senescent cells. Immunoblot analysis showed increased levels of p53, acetylated p53 (K382Ac), and p21 with no change in caspase 3, supporting senescence, not apoptosis, as the main mechanism responsible for the anti-proliferative effects. In summary, the p300/CBP BDIs, CCS1477 and GNE-781, show anti-tumor activity in HPV16+ UM-SCC-47 cells, reactivating the p53 program and inducing senescence. These findings support further evaluation of p300/CBP BDIs in other HPV+ cancer model systems.