Pleiotrophin (PTN) growth factor function in epithelial cells during mouse mammary gland development

Mammary gland development is a complex process requiring numerous systemic hormones as well as local regulatory factors. Expression of the heparin-binding growth factor, pleiotrophin (PTN) in the mouse mammary gland and in normal human breast has been reported but its function during mammary gland development is not known. Our lab has found that PTN expression is hormonally regulated in a tissue-specific manner in the mouse mammary gland during pregnancy suggesting that PTN might play a role during mammary gland development. We observed that both PTN and its receptor ALK are expressed in mouse mammary epithelial cells and their expression is regulated in parallel during pregnancy. During mid-pregnancy, a 30-fold down-regulation of PTN mRNA expression was measured, which coincides with beginning of lobular-alveolar differentiation of mammary epithelial cells suggesting that PTN expression in mammary epithelial cells regulates the differentiation state of the mouse mammary gland. We also found a significant decrease of PTN expression in mammary glands of multiparous mice and enrichment in differentiated mammary epithelial cells after repeated pregnancy has been previously suggested. We thus employed treatments of mice with an anti-PTN mouse monoclonal blocking antibody to investigate PTN function during mammary gland development. The maturation of mammary glands was inhibited by PTN activity as seen by the decrease in ductal elongation and branching via the inhibition of phospho ERK1/2 signaling. To further investigate PTN function in mammary epithelial cells (MECs), we employed treatment with the PTN blocking antibody of cultured MECs. Treatment of MECs in 3D culture with the anti-PTN blocking antibody suggests that PTN maintains the progenitor phenotype of MECs by impairing their organization and the expression of mammary progenitor cell markers. The findings from MECs cultured on a plastic surface (2D) suggest that PTN activity in MECs is dependent on the proper growth conditions provided in vivo by the surrounding stroma. We propose that PTN paracrine activity has inhibitory functions towards the differentiation of epithelial cells during mouse mammary gland development and maintains their progenitor phenotype.