SLIT/ROBO1 Signaling Suppresses Mammary Branching Morphogenesis by Limiting Basal Cell Number

Abstract

Summary In the field of breast biology, there is a growing appreciation for the “gatekeeping function” of basal cells during development and disease processes; yet, mechanisms regulating the generation of these cells are poorly understood. Here, we report that the proliferation of basal cells is controlled by SLIT/ROBO1 signaling and that production of these cells regulates outgrowth of mammary branches. We identify the negative regulator TGF-β1 upstream of ROBO1 and show that it induces Robo1 expression specifically in the basal layer, functioning together with SLIT2 to restrict branch formation. Loss of SLIT/ROBO1 signaling in this layer, alone, results in precocious branching due to a surplus of basal cells. SLIT2 limits basal cell proliferation by inhibiting canonical WNT signaling, increasing the cytoplasmic and membrane pools of β-catenin at the expense of its nuclear pool. Together, our studies provide mechanistic insight into how specification of basal cell number influences branching morphogenesis.

CD151 represses mammary gland development by maintaining the niches of progenitor cells

“…7B). In contrast, expression of Slit-2 and Robo-1, both shown to enhance branching morphogenesis in mice upon deletion as seen in CD151-targeted mice, 45 exhibited a minimal change (Fig. 7A).…”

“…6), even though both of these molecules have recently been shown to repress the pubertal outgrowth of mammary glands. 45 In this regard, the skewed distribution of basal or myoepithelial cells detected in CD151-targeted mice may be partially attributed to the altered function of Ron, rather than Slit-2/Robo-1 receptor complexes (Fig. 7C).…”

The Proto-Oncogene Myc Is Essential for Mammary Stem Cell Function

“…Even though Myc was deleted from the BCs only, luminal cells displayed decreased proliferation rates, and luminal progenitor population was diminished in mutant epithelium. A recent study provided evidence that BCs contribute to the control of mammary branching morphogenesis probably due to production of soluble growth factors stimulating proliferation of luminal cells [33]. We, therefore, suggest that a lack of paracrine or direct intercellular signaling between basal and luminal layers due to low BC number or/and impaired gene expression in Myc-deficient BCs could account for the diminished proliferation and low progenitor cell content in the luminal compartment and decreased branching complexity observed in K5cre;myc F/F mammary epithelium.…”

The mammary myoepithelial cell

“…Furthermore, simultaneous loss-of-function mutations of slit2 and ntn1 genes resulted in an enhanced phenotype with separated luminal and basal cell layers in the mammary ducts suggestive of synergy between Slit2 and Netrin 1 during ductal morphogenesis. A recent report from the same laboratory implicated Slit/Robo1 signaling in the control of mammary branching morphogenesis (Macias et al, 2011). The authors suggested that basal myoepithelial cells control the formation of new ductal branches via the production of mitogens for the luminal cells.…”

“…The authors suggested that basal myoepithelial cells control the formation of new ductal branches via the production of mitogens for the luminal cells. Macias and coworkers revealed that the TGF-b1-induced expression of Robo1 in basal myoepithelial cells and the interaction of Slit2 with Robo1 inhibited b-catenin signaling, limiting basal cell proliferation and preventing the formation of new branches (Macias et al, 2011).…”