Effect of PROTAC-mediated BRD4 degradation on bone marrow-derived mesenchymal stromal cells [BM-MSCs]

BRD4, a member of the BET family of histone readers, binds to acetylated lysine of histone H3 and promotes assembly of super-enhancer complexes that drive expression of key oncogenes in acute myeloid leukemia (AML) and other cancers. ARV-825 is a proteolysis-targeting chimera (PROTAC) that targets BRD4 for CRBN-mediated ubiquitination and degradation. BM-MSCs are an important element of the bone marrow microenvironment of AML. To understand how targeting BRD4 in BM-MSCs may contribute to the overall effect on AML of targeting BRD4, we treated BM-MSCs from two normal donors with ARV-825 in vitro. Treatment of BM-MSC monocultures with ARV-825 for 24 hr caused extensive changes in gene expression, highly uniform between triplicates. Although the cultures from the two normal donors showed different profiles, their changes with ARV-825 were highly similar. These changes implicated effects on oxidative stress, osteogenic differentiation, retinoid metabolism, F-actin polymerization, CXCL12, and proliferation. BM-MSCs were prepared from BM aspirates of normal donors as previously described (PMID: 24599548). BM-MSC from each donor were incubated for 24 hr in triplicate with either DMSO control or 25 nM ARV-825, a concentration similar to the 50 nM used to treated AML cells in monocultures. ARV-825 treatment did not affect the number or viability of BM-MSCs at 24 hr, although there was a slight relative decrease in cell number at 72 hr. Western blotting showed marked reductions in BRD4 and MYC protein levels at 24 hr.