Abstract 4773: Knockdown of the p300 histone acetylase stimulates the rapid and selective decay of ERBB2 mRNA as induced by class 1 histone deacetylase inhibitors

We have recently shown that translation-dependent and 39UTR-targeted decay of ERBB2 and other oncogenic transcripts can be rapidly induced by either protein knockdown or selective enzymatic inhibition of class-1 histone deacetylases (e.g. HDAC1/2) using an approved therapeutic like Romidepsin (FK228). Nanomolar doses of FK228 sufficient to acetylate endogenous histones but not tubulin in ERBB2 amplified and overexpressing SKBr3 cells, produces significant cell growth inhibition and apoptosis detectable by 72 h and preceded by the rapid (within 4 h) decay of ERBB2 transcripts, similar to that produced by a pan-HDAC inhibitor like trichostatin-A (TSA) but not by any class-2 selective HDAC inhibitor. As polysome profiling indicated that TSA or FK228 induced decay of ERBB2 mRNA was associated with 80S ribosome loss coupled with the appearance of pre-40S acetylated proteins, mass spectrometry (MS) was used to identify these acetylated proteins. Prominent among the MS identified acetylated proteins was the histone acetyl transferase, p300, where 12 acetylated lysines localized largely to p3009s HAT domain were detected following 2h of FK228 treatment. Of note, p300 was recently identified as a key component of a HDAC1/2 regulated complex broadly regulating mRNA stability via poly(A) length (Sharma et al., Cell, 2016). Although considered to be a chromatin regulator that is predominantly nuclear in its subcellular distribution, in recent immunohistochemical studies of human breast cancers p300 was found to be overexpressed and predominantly localized in the cytoplasm (+/- nucleus) as compared to its nuclear localization in normal breast tissue samples (Fermento et al., Exp Mol Pathol, 2014). In our SKBr3 cell line model, siRNA knockdown of p300 induced a rapid decay in ERBB2 transcript levels (ERBB2/GAPDH = 0.3 at 48 h) comparable to that observed with FK228 treatment and exceeding that produced by partial knockdown of HDAC1/2 proteins (ERBB2/GAPDH = 0.7 at 48 h). Ongoing studies with specific inhibitors of p300 bromo and HAT domains are in progress along with intracellular assays of p300 catalytic activity. As well, our MS search is now focused on potential p300-associated substrates that may also be mechanistically involved with FK228 induced ERBB2 transcript destabilization. Altogether, these recent findings shed new light on the relatively unrecognized subcellular mechanism by which HDACs regulate translational control and the stability of cell growth and cancer-promoting transcripts like ERBB2. As new therapeutic strategies are increasingly needed to counter clinical resistance to currently available ERBB2 receptor targeted agents, the repurposing of selective HDAC1/2 inhibitors as well as the emergence of new p300 inhibitors may find appropriate clinical utility against treatment refractory ERBB2-positive breast cancers. Citation Format: Gary K. Scott, Lei Wei, Mariah Alejo, Jacob Rose, Birgit Schilling, Christopher Benz. Knockdown of the p300 histone acetylase stimulates the rapid and selective decay of ERBB2 mRNA as induced by class 1 histone deacetylase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4773.