NOP16 is a histone mimetic that regulates Histone H3K27 methylation and gene repression.

Post-translational modifications of histone tails alter chromatin accessibility to regulate gene expression. Some viruses exploit the importance of histone modifications by expressing histone mimetic proteins that contain histone-like sequences to sequester complexes that recognize modified histones. Here we identify an evolutionarily conserved and ubiquitously expressed, endogenous mammalian protein Nucleolar protein 16 (NOP16) that functions as a H3K27 mimic. NOP16 binds to EED in the H3K27 trimethylation PRC2 complex and to the H3K27 demethylase JMJD3. NOP16 knockout selectively globally increases H3K27me3, a heterochromatin mark, without altering methylation of H3K4, H3K9, or H3K36 or acetylation of H3K27. NOP16 is overexpressed and linked to poor prognosis in breast cancer. Depletion of NOP16 in breast cancer cell lines causes cell cycle arrest, decreases cell proliferation and selectively decreases expression of E2F target genes and of genes involved in cell cycle, growth and apoptosis. Conversely, ectopic NOP16 expression in triple negative breast cancer cell lines increases cell proliferation, cell migration and invasivity in vitro and tumor growth in vivo , while NOP16 knockout or knockdown has the opposite effect. Thus, NOP16 is a histone mimic that competes with Histone H3 for H3K27 methylation and demethylation. When it is overexpressed in cancer, it derepresses genes that promote cell cycle progression to augment breast cancer growth.

Alternative splicing of GSDMB modulates killer lymphocyte-triggered pyroptosis.

Granzyme A from killer lymphocytes cleaves gasdermin B (GSDMB) and triggers pyroptosis in targeted human tumor cells, eliciting antitumor immunity. However, GSDMB has a controversial role in pyroptosis and has been linked to both anti- and protumor functions. Here, we found that GSDMB splicing variants are functionally distinct. Cleaved N-terminal (NT) fragments of GSDMB isoforms 3 and 4 caused pyroptosis, but isoforms 1, 2, and 5 did not. The nonfunctional isoforms have a deleted or modified exon 6 and therefore lack a stable belt motif. The belt likely contributes to the insertion of oligomeric GSDMB-NTs into the membrane. Consistently, noncytotoxic GSDMB-NTs blocked pyroptosis caused by cytotoxic GSDMB-NTs in a dominant-negative manner. Upon natural killer (NK) cell attack, GSDMB3-expressing cells died by pyroptosis, whereas GSDMB4-expressing cells died by mixed pyroptosis and apoptosis, and GSDMB1/2-expressing cells died only by apoptosis. GSDMB4 partially resisted NK cell-triggered cleavage, suggesting that only GSDMB3 is fully functional. GSDMB1-3 were the most abundant isoforms in the tested tumor cell lines and were similarly induced by interferon-γ and the chemotherapy drug methotrexate. Expression of cytotoxic GSDMB3/4 isoforms, but not GSDMB1/2 isoforms that are frequently up-regulated in tumors, was associated with better outcomes in bladder and cervical cancers, suggesting that GSDMB3/4-mediated pyroptosis was protective in those tumors. Our study indicates that tumors may block and evade killer cell-triggered pyroptosis by generating noncytotoxic GSDMB isoforms. Therefore, therapeutics that favor the production of cytotoxic GSDMB isoforms by alternative splicing may improve antitumor immunity.

De novo induction of lineage plasticity from human prostate luminal epithelial cells by activated AKT1 and c-Myc.

Neuroendocrine prostate cancer (NEPC) is an aggressive variant of prostate cancer that either develops de novo or arises from prostate adenocarcinoma as a result of treatment resistance. Although the prostate basal cells have been shown to directly generate tumor cells with neuroendocrine features when transduced with oncogenic signaling, the identity of the cell-of-origin for de novo NEPC remains unclear. We show that the TACSTD2high human prostate luminal epithelia cells highly express SOX2 and are relatively enriched in the transition zone prostate. Both TACSTD2high and TACSTD2low luminal cells transduced by constitutively activated AKT1 (caAKT1), and c-Myc can form organoids containing versatile clinically relevant tumor cell lineages with regard to the expression of AR and the neuroendocrine cell markers Synaptophysin and Chromogranin A. Tumor organoid cells derived from the TACSTD2high luminal cells are more predisposed to neuroendocrine differentiation along passaging and are relatively more castration-resistant. Knocking down TACSTD2 and SOX2 both attenuate neuroendocrine differentiation of tumor organoid cells. This study demonstrates de novo neuroendocrine differentiation of the human prostate luminal epithelial cells induced by caAKT1 and c-Myc and reveals an impact of cellular status on initiation of lineage plasticity.

The Sca-1+ and Sca-1- mouse prostatic luminal cell lineages are independently sustained.

The phenotypic and functional heterogeneity of the mouse prostate epithelial cell lineages remains incompletely characterized. We show that the Sca-1+ luminal cells at the mouse proximal prostate express Sox2. These cells are replicative quiescent, castration resistant, and do not possess secretory function. We use the Probasin-CreERT2 and Sox2-CreERT2 models in concert with a fluorescent reporter line to label the Sca-1- and Sca-1+ luminal cells, respectively. By a lineage tracing approach, we show that the two luminal cell populations are independently sustained. Sox2 is dispensable for the maintenance of the Sca-1+ luminal cells but is essential for their facultative bipotent differentiation capacity. The Sca-1+ luminal cells share molecular features with the human TACSTD2+ luminal cells. This study corroborates the heterogeneity of the mouse prostate luminal cell lineage and shows that the adult mouse prostate luminal cell lineage is maintained by distinct cellular entities rather than a single progenitor population.