Retinoblastoma (RB) protein inactivation during tumor progression is often associated with acquisition of immature phenotypes and resistance to therapy. Determination of an RB inactivation signature in a context of gaining undifferentiated phenotype in a p53-null sarcoma system revealed a critical role for interleukin (IL)-6. Using a Gene Set Enrichment Analysis (GSEA), we discovered that poorly differentiated breast cancers are enriched for this RB inactivation signature. Accelerated IL-6 secretion following RB inactivation in an RB-intact luminal-type breast cancer cell line MCF-7 promoted a positive feed forward loop between IL-6 and STAT3 driving tumor growth and endocrine therapy resistance. In addition, some of RB-intact basal-like type breast cancer cell lines exhibited a similar phenotype following RB depletion. The mechanism whereby RB inactivation increases IL-6 production in MCF-7 cells appeared to involve fatty acid oxidation (FAO)-dependent mitochondrial metabolism and c-Jun NH(2)-terminal kinase (JNK). In addition, IL-6, via STAT3-mediated feedback to mitochondria, autonomously adjusts mitochondrial superoxide to levels suitable to maintain stem cell-like activity. The gene expression profile of luminal-type breast cancer patients with low RB expression revealed high enrichment of genes involved in mitochondrial respiration and downstream targets of IL-6. These findings unveiled an unexpected strategy whereby RB suppresses malignant features of cancer cells through metabolic reprogramming and cell-autonomous inflammation.

Epigenetic regulators are attractive anticancer targets, but the promise of therapeutic strategies inhibiting some of these factors has not been proven in vivo or taken into account tumor cell heterogeneity. Here we show that the histone methyltransferase G9a, reported to be a therapeutic target in many cancers, is a suppressor of aggressive lung tumor-propagating cells (TPCs). Inhibition of G9a drives lung adenocarcinoma cells towards the TPC phenotype by de-repressing genes which regulate the extracellular matrix. Depletion of G9a during tumorigenesis enriches tumors in TPCs and accelerates disease progression metastasis. Depleting histone demethylases represses G9a-regulated genes and TPC phenotypes. Demethylase inhibition impairs lung adenocarcinoma progression in vivo. Therefore, inhibition of G9a is dangerous in certain cancer contexts, and targeting the histone demethylases is a more suitable approach for lung cancer treatment. Understanding cellular context and specific tumor populations is critical when targeting epigenetic regulators in cancer for future therapeutic development.