Gain-of-function p53 mutants such as p53-R175H form stable aggregates that accumulate in cells and play important roles in cancer progression. Selective degradation of gain-of-function p53 mutants has emerged as a highly attractive therapeutic strategy to target cancer cells harboring specific p53 mutations. We identified a small molecule called MCB-613 to cause rapid ubiquitination, nuclear export, and degradation of p53-R175H through a lysosome-mediated pathway, leading to catastrophic cancer cell death. In contrast to its effect on the p53-R175H mutant, MCB-613 causes slight stabilization of p53-WT and has weaker effects on other p53 gain-of-function mutants. Using state-of-the-art genetic and chemical approaches, we identified the deubiquitinase USP15 as the mediator of MCB-613’s effect on p53-R175H, and established USP15 as a selective upstream regulator of p53-R175H in ovarian cancer cells. These results confirm that distinct pathways regulate the turnover of p53-WT and the different p53 mutants and open new opportunities to selectively target them.

In eukaryotic cells, the gene expression status is strictly controlled by epigenetic modifications on chromatin. The repressive status of chromatin largely contributes to HIV latency. Studies have shown that modification of histone H3K27 acts as a key molecular switch for activation or suppression of many cellular genes. In this study, we found that K27-acetylated histone H3 specifically recruited Super Elongation Complex (SEC), the transcriptional elongation complex essential for HIV-1 long terminal repeat (LTR)-mediated and general cellular transcription. Interestingly, H3K27 acetylation further stimulates H3R26 methylation, which subsequently abrogates the recruitment of SEC, forming a negative feedback regulatory loop. Importantly, by inhibiting methyltransferase activity of CARM1, the enzyme responsible for H3R26 methylation, HIV-1 transcription is reactivated in several HIV latency cell models, including a primary resting CD4+ T cell model. When combined with other latency disrupting compounds such as JQ1 or vorinostat/SAHA, the CARM1 inhibitor achieved synergistic effects on HIV-1 activation. This study suggests that coordinated and dynamic modifications at histone H3K27 and H3R26 orchestrate HIV-1 LTR-mediated transcription, and potentially opens a new avenue to disrupt latent HIV-1 infection by targeting specific epigenetic enzymes.

Approximately 20% of early-stage breast cancers display amplification or overexpression of the ErbB2/HER2 oncogene, conferring poor prognosis and resistance to endocrine therapy. Targeting HER2⁺ tumors with trastuzumab or the receptor tyrosine kinase (RTK) inhibitor lapatinib significantly improves survival, yet tumor resistance and progression of metastatic disease still develop over time. Although the mechanisms of cytosolic HER2 signaling are well studied, nuclear signaling components and gene regulatory networks that bestow therapeutic resistance and limitless proliferative potential are incompletely understood. Here, we use biochemical and bioinformatic approaches to identify effectors and targets of HER2 transcriptional signaling in human breast cancer. Phosphorylation and activity of the Steroid Receptor Coactivator-3 (SRC-3) is reduced upon HER2 inhibition, and recruitment of SRC-3 to regulatory elements of endogenous genes is impaired. Transcripts regulated by HER2 signaling are highly enriched with E2F1 binding sites and define a gene signature associated with proliferative breast tumor subtypes, cell-cycle progression, and DNA replication. We show that HER2 signaling promotes breast cancer cell proliferation through regulation of E2F1-driven DNA metabolism and replication genes together with phosphorylation and activity of the transcriptional coactivator SRC-3. Furthermore, our analyses identified a cyclin-dependent kinase (CDK) signaling node that, when targeted using the CDK4/6 inhibitor palbociclib, defines overlap and divergence of adjuvant pharmacologic targeting. Importantly, lapatinib and palbociclib strictly block de novo synthesis of DNA, mostly through disruption of E2F1 and its target genes. These results have implications for rational discovery of pharmacologic combinations in preclinical models of adjuvant treatment and therapeutic resistance. Cancer Res; 76(6); 1463–75. ©2016 AACR.

Here we demonstrate that reprogramming steroid receptor coactivator-3 (SRC-3) function by changing its posttranslational modification (PTM) code drastically influences systems biology. These findings support the physiological importance of PTMs in directing in vivo functions of a master coregulator. We previously reported that the transactivation potential of SRC-3 is controlled in part by PTMs, although this data emanated from in vitro studies. To test the physiological implications of PTMs on SRC-3, we developed a knock-in mouse model containing mutations at four conserved phosphorylation sites. These mice displayed a systems biology phenotype with increased body weight and adiposity, coupled with reduced peripheral insulin sensitivity. Collectively, these phenotypes result from increased IGF1 signaling, due to elevated IGFBP3 levels. We provide convincing evidence that these mutations in SRC-3 promoted enhanced transcription of the IGFBP3 gene and globally influenced growth and metabolism. Consequently, these mice displayed increased liver tumorigenesis, which likely results from elevated IGF1 signaling.