Mammary epithelial-restricted expression of activated c-src rescues the block to mammary gland morphogenesis due to the deletion of the C-terminus of Patched-1

Mesenchymal dysplasia (mes) mice expressing a C-terminally truncated version of the Hedgehog (Hh)-ligand receptor, Patched-1 (Ptch1), exhibit a limited spectrum of developmental defects including blocked ductal morphogenesis of the mammary gland during puberty. Given that the Hh-ligands can stimulate signalling cascades distinct from the canonical pathway involving Smo and the Gli-family proteins and that Ptch1 binds to factors harbouring SH3-domains, we determined whether the mes mammary gland defect could be rescued by activating non-canonical signalling pathways downstream of Ptch1. We demonstrate here that expression of constitutively active c-src (c-src(Act)) in mammary epithelial cells overcomes the block to mammary epithelial morphogenesis in mes mice. Specifically, MMTV-directed expression of c-src(Act) rescued blocked ductal morphogenesis in mes mice, albeit only after animals were more than 15 weeks of age. The overall morphology resembled wild type mice expressing c-src(Act) although 40% of mes/MMTV-c-src(Act) mice exhibited terminal end buds at 24 weeks of age. C-src(Act) restored the proliferative capacity of mes epithelial cells, self-renewal capacity of mammary progenitor cells and increased the expression of Esr1, Ptch1 and Gli1. These data reveal the cooperative interactions between signalling cascades involving c-src and Ptch1 and suggest that Hh-signalling may be permissive for c-src/Esr1-dependent mammary gland morphogenesis.