Person:

Yeh, Jennifer

Interleukin-6 (IL-6)–mediated activation of signal transducer and activator of transcription 3 (STAT3) is a mechanism by which chronic inflammation can contribute to cancer and is a common oncogenic event. We discovered a pathway, the loss of which is associated with persistent STAT3 activation in human cancer. We found that the gene encoding the tumor suppressor microRNA miR-146b is a direct STAT3 target gene, and its expression was increased in normal breast epithelial cells but decreased in tumor cells. Methylation of the miR-146b promoter, which inhibited STAT3-mediated induction of expression, was increased in primary breast cancers. Moreover, we found that miR-146b inhibited nuclear factor κB (NF-κB)–dependent production of IL-6, subsequent STAT3 activation, and IL-6/STAT3–driven migration and invasion in breast cancer cells, thereby establishing a negative feedback loop. In addition, higher expression of miR-146b was positively correlated with patient survival in breast cancer subtypes with increased IL6 expression and STAT3 phosphorylation. Our results identify an epigenetic mechanism of crosstalk between STAT3 and NF-κB relevant to constitutive STAT3 activation in malignancy and the role of inflammation in oncogenesis.

Inappropriate activation of the transcription factors STAT3 and STAT5 has been shown to drive cancer pathogenesis through dysregulation of genes involved in cell survival, growth, and differentiation. Although STAT3 and STAT5 are structurally related, they can have opposite effects on key genes, including BCL6. BCL6, a transcriptional repressor, has been shown to be oncogenic in diffuse large B cell lymphoma. BCL6 also plays an important role in breast cancer pathogenesis, a disease in which STAT3 and STAT5 can be activated individually or concomitantly. To determine the mechanism by which these oncogenic transcription factors regulate BCL6 transcription, we analyzed their effects at the levels of chromatin and gene expression. We found that STAT3 increases expression of BCL6 and enhances recruitment of RNA polymerase II phosphorylated at a site associated with transcriptional initiation. STAT5, in contrast, represses BCL6 expression below basal levels and decreases the association of RNA polymerase II at the gene. Furthermore, the repression mediated by STAT5 is dominant over STAT3-mediated induction. STAT5 exerts this effect by displacing STAT3 from one of the two regulatory regions to which it binds. These findings may underlie the divergent biology of breast cancers containing activated STAT3 alone or in conjunction with activated STAT5.

A recent article published by Britschgi et al. in Cancer Cell, “JAK2/STAT5 Inhibition Circumvents Resistance to PI3K/mTOR Blockade: A Rationale for Cotargeting These Pathways in Metastatic Breast Cancer,” describes a positive feedback loop of JAK2/STAT5 activation that drives resistance to PI3K/mTOR inhibition in breast cancer. The authors found that genetic or pharmacological inhibition of JAK2 circumvents resistance to PI3K/mTOR inhibition and go on to show the efficacy of combined PI3K/mTOR and JAK2 inhibition on reducing cancer cell number, tumor growth, and metastasis as well as increasing in vivo survival. These results provide strong support for combination therapy with JAK2/STAT5 and PI3K/mTOR inhibitors in breast cancer. Here we discuss how the article by Britschgi et al. proposes a novel mechanism to explain how breast cancer cells overcome inhibition of a key signaling pathway driving cell proliferation. We also discuss the interplay between activation of the transcription factors STAT5 and STAT3 in breast cancer.

One cause of morbidity and mortality in chronic lymphocytic leukemia (CLL) is infection, which results from defects in a number of components of the immune system. In particular, dendritic cells (DCs) are functionally defective in patients with CLL. To understand the molecular mechanism for this abnormality, we focused on signal transduction pathways that regulate the function of monocyte-derived dendritic cells (Mo-DCs). Monocytes from CLL patients exhibit high IL-4Rα expression due to the enhanced activation of STAT3. However, IL-4R signaling is decoupled from activation of its downstream mediator STAT6 by enhanced levels of the negative regulator SOCS5. This impairs differentiation of functionally mature DCs leading to decreased expression of HLA-DR and costimulatory molecules, and reduced secretion of pro-inflammatory cytokines in LPS-activated DCs. Moreover, Mo-DCs from CLL patients display a decreased ability to induce pro-inflammatory T-cell responses. IL-10-treatment of monocytes from healthy donors mimics the alteration in signaling observed in CLL patients, through enhanced STAT3-dependent expression of SOCS5. The higher level of SOCS5 inhibits STAT6 activation and leads to defective DC differentiation. These findings indicate that SOCS5 mediates the impaired function of DCs in CLL patients, and has the potential to be a new therapeutic target for reversing cancer-associated immune suppression.

Since the neoplastic phenotype of a cell is largely driven by aberrant gene expression patterns, increasing attention has been focused on transcription factors that regulate critical mediators of tumorigenesis such as signal transducer and activator of transcription 3 (STAT3). Here we investigate how the inappropriate activation of STAT3 contributes to cancer pathogenesis and how it can be targeted therapeutically. As proteins that interact with STAT3 may be key in addressing these questions, we took three complementary approaches: a proteomics approach to identify novel STAT3-interacting proteins, a chemical biology approach to identify STAT3-interacting proteins critical for oncogenesis, and molecular analysis of a STAT3 structural domain known to mediate protein-protein interactions. First, we performed mass spectrometry on STAT3-containing complexes from breast cancer cells that have constitutively active STAT3 and are dependent on STAT3 function. We identified granulin as a novel STAT3-interacting protein that is critical to STAT3 transcriptional activity and STAT3-mediated tumorigenic phenotypes. Furthermore, granulin expression positively correlated with STAT3 gene expression signatures in breast cancer patients. We then applied this mass spectrometry approach to investigate the mechanism of two small molecules – ST3-01 and Pyrimethamine (Pyr) – identified by transcription-based reporter screens to inhibit STAT3 activity without altering its activation or nuclear localization. ST3-01 and Pyr reduced STAT3 interaction with the chromatin remodeler, BRG1, which we found to be necessary for STAT3 function. Next, we studied the role of the STAT3 N-terminal domain (NTD), which mediates interactions between two STAT3 dimers for cooperative DNA binding. We identified STAT3 target genes dependent on the NTD for transcriptional regulation. We then showed that NTD mutations which inhibit cooperative DNA binding reduced the induction of a subset of STAT3 target genes by decreasing STAT3 binding to their regulatory regions. These studies demonstrate that a proteomics approach can reveal critical modulators of transcription factor function. Moreover, our characterization of the impact of the STAT3 NTD on STAT3-dependent activity provides a deeper mechanistic understanding of STAT3 signaling as well as a structural template for drug design. Collectively, these insights into how STAT3 protein-protein interactions modulate its transcriptional function may guide future therapies that target this oncogenic signaling pathway.