PTHrP regulates mammary cell fate by specifying the mammary mesenchyme during embryonic mammary development. Loss of PTHrP or its receptor allows mammary bud cells to adopt an epidermal fate, whereas, overexpression of PTHrP in the epidermis induces inappropriate differentiation of the ventral epidermis into nipple-like skin. Previous studies have suggested that the Wnt pathway is one of the downstream pathways activated by PTHrP in specifying the mammary mesenchyme. We confirmed that PTHrP modulates Wnt signaling in the mammary mesenchyme using a reporter of Wnt signaling, TOPGAL-C. Reporter expression is completely abolished by loss of PTHrP signaling and ectopic mesenchymal Wnt signaling is induced by overexpression of PTHrP. To further investigate the requirement for Wnt signaling, we examined mutant mice overexpressing PTHrP but lacking Lef1, an essential component and target of the Wnt pathway. Our observations suggest that loss of Lef1 partially reverses the nipple-like differentiation of the ventral skin. Therefore, we concluded that PTHrP-driven mammary mesenchyme specification is partially mediated by Wnt signaling. Surprisingly, ectopic TOPGAL-C expression is maintained in K14-PTHrP;Lef1-null mice. These data suggest that PTHrP induces a Wnt signal that activates Lef1 expression in the mesenchyme. To determine if Wnt signaling regulates mesenchyme specification and Lef1 expression, we deleted beta-catenin, the primary mediator of the canonical Wnt pathway, in the mammary mesenchyme. Reduced mesenchymal Wnt signaling correlates with abnormal bud morphology and reduced expression of mammary mesenchyme markers. To identify other Wnt pathway components that are regulated by PTHrP during mammary mesenchyme specification, we performed microarray analyses of RNA from ventral skins of WT and K14-PTHrP mice and from RNA extracted from mammary buds of PTHrP-null and WT mice. We identified genes that are upregulated in PTHrP-overexpressing ventral skin, compared to WT, and genes that are downregulated in PTHrP-null mammary buds, compared to WT, as candidate genes that are regulated by PTHrP. Of these, we are currently investigating RSPO1 and WNT11 as Wnt pathway regulators that potentially act downstream of PTHrP during mesenchyme specification.
Available at: http://works.bepress.com/minoti_hiremath/8/