Abstract
Branching morphogenesis is a mechanism used by many species for organogenesis and tissue maintenance. Receptor tyrosine kinases (RTKs), including epidermal growth factor receptor (EGFR) and the sprouty protein family are believed to be critical regulators of branching morphogenesis. The aim of this study was to analyze the expression of Sprouty-2 (SPRY2) in the mammary gland and study its role in branching morphogenesis. Human breast epithelial cells, breast tissue and mouse mammary glands were used for expression studies using immunoblotting, real rime PCR and immunohistochemistry. Knockdown of SPRY2 in the breast epithelial stem cell line D492 was done by lentiviral transduction of shRNA constructs targeting SPRY2. Three dimensional culture of D492 with or without endothelial cells was done in reconstituted basement membrane matrix. We show that in the human breast, SPRY2 is predominantly expressed in the luminal epithelial cells of both ducts and lobuli. In the mouse mammary gland, SPRY2 expression is low or absent in the virgin state, while in the pregnant mammary gland SPRY2 is expressed at branching epithelial buds with increased expression during lactation. This expression pattern is closely associated with the activation of the EGFR pathway. Using D492 which generates branching structures in three-dimensional (3D) culture, we show that SPRY2 expression is low during initiation of branching with subsequent increase throughout the branching process. Immunostaining locates expression of phosphorylated SPRY2 and EGFR at the tip of lobular-like, branching ends. SPRY2 knockdown (KD) resulted in increased migration, increased pERK and larger and more complex branching structures indicating a loss of negative feedback control during branching morphogenesis. In D492 co-cultures with endothelial cells, D492 SPRY2 KD generates spindle-like colonies that bear hallmarks of epithelial to mesenchymal transition. These data indicate that SPRY2 is an important regulator of branching morphogenesis and epithelial to mesenchymal transition in the mammary gland.
Highlights
Branching morphogenesis is a highly conserved developmental process, where epithelial-based organs are able to increase their surface area and form the correct functional histoarchitecture [1,2]
Sprouty proteins have been identified as regulators of fibroblast growth factor receptors (FGFRs), c-Met and epidermal growth factor receptor (EGFR) signaling in lung, kidney and vasculogenesis but their role in the human breast gland morphogenesis has not been systematically analyzed [22]
Using D492, we show that SPRY2 is expressed at the branching tips and suppression of SPRY2 through shRNA gene knockdown increases branching morphogenesis and promotes epithelial to mesenchymal transition when cultured with endothelial cells
Summary
Branching morphogenesis is a highly conserved developmental process, where epithelial-based organs are able to increase their surface area and form the correct functional histoarchitecture [1,2] This process gives rise to the airways of the lungs [3], the urine collecting ducts [4], the prostate [5], salivary glands [6] and the mammary glands [7,8]. Receptor tyrosine kinases (RTKs), such as fibroblast growth factor receptors (FGFRs) and epidermal growth factor receptors (EGFRs) are key mediators of signals that regulate proliferation, differentiation and branching morphogenesis in the mammary gland [1,10] Extracellular cues such as FGFs and EGFs act via their respective receptors to activate intracellular pathways, such as the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI-3 kinase) pathways which are critical for proper development of branched organs [1]. SPRY2 sequester c-Cbl molecules from activated EGFR and disregulate EGFR ubiquitination and downregulation, thereby potentiating the amplitude and longevity of intracellular signals [23,24]
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