The plasminogen receptor, Plg-RKT, is essential for mammary lobuloalveolar development and lactation

Background: Lactational competence requires plasminogen, the zymogen of the serine protease, plasmin. Plg-R-KT is a unique transmembrane plasminogen receptor that promotes plasminogen activation to plasmin on cell surfaces. Plg-R-KT(-/-) mice are viable, but no offspring of Plg-R-KT(-/-) female mice survive to weaning. Objectives: We investigated potential lactational failure in Plg-R-KT(-/-) mice and addressed causal mechanisms. Methods: Fibrin accumulation, macrophage infiltration, processing of extracellular matrix components, effects of genetic deletion of fibrinogen, expression of fibrosis genes, and proliferation and apoptosis of epithelial cells were examined in lactating mammary glands of Plg-R-KT(-/-) and Plg-R-KT(+/+) mice. Results: Milk was not present in the stomachs of offspring of Plg-R-KT(-/-) female mice and the pups were rescued by foster mothers. Although the mammary ductal tree developed normally in Plg-R-KT(-/-) glands, lobuloalveolar development was blocked by a hypertrophic fibrotic stroma and infiltrating macrophages were present. A massive accumulation of fibrin was also present in Plg-R-KT(-/-) alveoli and ducts. Although this accumulation was decreased when Plg-R-KT(-/-) mice were made genetically heterozygous for fibrinogen, defects in lobuloalveolar development were not rescued by fibrinogen heterozygosity. Transcriptional profiling revealed that EGF was downregulated 12-fold in Plg-R-KT(-/-) glands. Furthermore, proliferation of epithelial cells was not detectable. In addition, the pro-survival protein, Mcl-1, was markedly downregulated and apoptosis was observed in Plg-R-KT(-/-) but not Plg-R-KT(+/+) glands. Conclusions: Plg-R-KT is essential for lactogenesis and functions to maintain the appropriate stromal extracellular matrix environment, regulate epithelial cell proliferation and apoptosis, and, by regulating fibrinolysis, preserve alveolar and ductal patency.