Abstract 2035: ELF3 is required for IL-1alpha activation of mesenchymal stem cells in the perivascular niche to immune reactive fibroblasts
Authors: Tran Linda L. , Rowley David R. 
Published: August 2019

Abstract
Tissue resident mesenchymal stem cells (MSC) have been identified in many adult tissues and are a potential origin for key components of the reactive stroma adjacent to cancer foci. Attempts to target cancer associated fibroblasts as a form of cancer treatment have had mixed results. This may be due in part to our lack of understanding in the heterogeneity of stromal populations and mechanisms that regulate their co-evolution in tumors. Previous work in our lab indicates that tissue-resident MSCs may occupy a perivascular stem cell niche that becomes activated in the presence of IL-1α. We developed a Cspg4 lineage tracing mouse to address the cell ontogeny of reactive stroma. We have shown that Cspg4+ cells occupy the perivascular niche in multiple tissues and that the cells isolated from the microvasculature are more responsive to IL-1α than prostate fibroblasts. Employing a syngeneic tumor model, we’ve shown Cspg4+ cells can be recruited from the perivascular niche and expand into the tumor.
To elucidate IL-1α regulated mechanisms, co-cultures using normal human prostate, CSPG4+ MSCs (HPS-19I) and epithelial cells (PNT1A) engineered to overexpress IL-1α were performed. A genome-wide gene expression microarray was utilized to determine IL-1α regulated gene expression in the stromal compartment. Among the ~1200 genes upregulated, gene ontology analysis presented enrichment for the immune and inflammatory responses. E74-Like Factor 3 (ELF3) was identified as a candidate transcription factor that may mediate the MSC response to inflammatory signals and subsequent differentiation. Transient knockdown of ELF3 using small interfering RNA in HPS-19Is showed a greatly reduced response to IL-1α and inhibited cellular proliferation. Of interest, exposure to IL-1α induced MSC differentiation to a pro-inflammatory immune fibroblast cell type that was restricted from further differentiation to prototypical myofibroblasts by TGFβ. While responsive to TGFβ, these cells continue to express inflammatory genes, including IL-1β, IL6, IL8, and CSF2, although they were refractory to induced smooth muscle α-actin and collagen expression, hallmarks of myofibroblast differentiation. Moreover, co-activation by TGFβ and IL-1α was associated with activation of the Hippo pathway and inhibition of YAP resulted in restricted myofibroblast differentiation of HPS-19Is. Together these data suggest that activation and differentiation of tissue-resident MSCs by reactive stroma regulators IL-1α and TGFβ results from complex interplay between ELF3 and Hippo pathway components.
MSCs represent one source of the stromal cells that compose reactive stroma in tumors. By more fully understanding the ontogeny and specific regulatory pathways of MSC expansion and differentiation to stromal cell types in the tumor microenvironment, we can develop better therapeutics that target the stromal compartment.
Citation Format: Linda L. Tran, David R. Rowley. ELF3 is required for IL-1alpha activation of mesenchymal stem cells in the perivascular niche to immune reactive fibroblasts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2035.