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A novel TGFβ modulator that uncouples R-Smad/I-Smad-mediated negative feedback from R-Smad/ligand-driven positive feedback.

ABSTRACT: As some of the most widely utilised intercellular signalling molecules, transforming growth factor β (TGFβ) superfamily members play critical roles in normal development and become disrupted in human disease. Establishing appropriate levels of TGFβ signalling involves positive and negative feedback, which are coupled and driven by the same signal transduction components (R-Smad transcription factor complexes), but whether and how the regulation of the two can be distinguished are unknown. Genome-wide comparison of published ChIP-seq datasets suggests that LIM domain binding proteins (Ldbs) co-localise with R-Smads at a substantial subset of R-Smad target genes including the locus of inhibitory Smad7 (I-Smad7), which mediates negative feedback for TGFβ signalling. We present evidence suggesting that zebrafish Ldb2a binds and directly activates the I-Smad7 gene, whereas it binds and represses the ligand gene, Squint (Sqt), which drives positive feedback. Thus, the fine tuning of TGFβ signalling derives from positive and negative control by Ldb2a. Expression of ldb2a is itself activated by TGFβ signals, suggesting potential feed-forward loops that might delay the negative input of Ldb2a to the positive feedback, as well as the positive input of Ldb2a to the negative feedback. In this way, precise gene expression control by Ldb2a enables an initial build-up of signalling via a fully active positive feedback in the absence of buffering by the negative feedback. In Ldb2a-deficient zebrafish embryos, homeostasis of TGFβ signalling is perturbed and signalling is stably enhanced, giving rise to excess mesoderm and endoderm, an effect that can be rescued by reducing signalling by the TGFβ family members, Nodal and BMP. Thus, Ldb2a is critical to the homeostatic control of TGFβ signalling and thereby embryonic patterning.

SUBMITTER: Gu W

PROVIDER: S-EPMC4321984 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Triple-negative breast cancer (TNBC) is an aggressive subtype with few treatment options for chemo-resistant disease. In both preclinical models and patient circulating tumor cells, androgen receptor (AR) expression is increased in anchorage independent TNBC. The AR inhibitor enzalutamide (Enza) leads to reduced TNBC growth in soft agar, invasion, mammosphere formation in vitro, and reduced tumorigenicity and recurrence when combined with chemotherapy in vivo pre-clinical models. Transforming growth factor β (TGFβ) pathway gene signatures are also increased during TNBC anchorage independent survival both in vitro and in vivo in pre-clinical models and CTC from patients during relapse while on chemotherapy. We hypothesized that a positive loop between AR and TGFβ signaling facilitates TNBC anchorage independent survival (anoikis resistance). We previously published that AR protein levels and transcriptional activity increased during anchorage independent conditions and we now find that that multiple components of the TGFβ pathway, including TGFβ1 and 3, as well as pathway activity, as measured by nuclear localization and transcriptional activity of pSmad3, are enhanced in anchorage independent conditions. Indeed, exogenous TGFβ increased AR protein and TGFβ inhibition decreased AR and TNBC viability, particularly under anchorage independent culture conditions. ChIP-Seq experiments revealed AR binding to genomic regions near the TGFB1 and SMAD3. TGFB3 and AR expression were positively correlated in clinical datasets and high levels of co-expression correspond to significantly worse recurrence-free and overall survival in both ER- and basal-like breast cancer. Finally, combining Enza with a TGFβ inhibitor decreased cell survival more than either drug alone, particularly under anchorage independent conditions, where the effect was more than additive. These findings warrant further investigations into whether combined inhibition of AR and TGFβ pathways might decrease metastatic recurrence rates and mortality from TNBC.

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A novel TGFβ modulator that uncouples R-Smad/I-Smad-mediated negative feedback from R-Smad/ligand-driven positive feedback.

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