PRL-stimulated ubiquitination of ZnT2 mediates a transient increase in Zn secretion followed by ZnT2 degradation in mammary epithelial cells [Cell Biology]
July 11th, 2014 by Seo, Y. A., Lee, S., Hennigar, S. R., Kelleher, S. L.
The zinc (Zn) transporter ZnT2 imports Zn into secretory vesicles to export Zn from the mammary epithelial cell. Mutations in ZnT2 substantially impair Zn secretion into milk and result in neonatal Zn deficiency. Basal levels of the lactogenic hormone prolactin (PRL) are high during lactation and transiently increase in response to suckling. We found previously that PRL chronically maintains ZnT2 expression through activation of the Jak2/STAT5 signaling pathway. How PRL acutely affects ZnT2-mediated Zn secretion has not been explored. Herein, we report that PRL transiently augments Zn secretion from mammary epithelial cells which was abrogated by 4 h. Moreover, acute Zn secretion was abrogated by stimulating the ubiquitination and internalization of ZnT2, which targeted ZnT2 for proteasome-dependent degradation. Mutagenesis of two N-terminal lysine residues (K4R and K6R) inhibited ubiquitination and degradation, which confined ZnT2 to the cell membrane and maintained Zn efflux from the cell. These findings indicate that PRL regulates Zn secretion in a multifactorial manner; first, by enhancing Zn secretion in an acute, ZnT2-dependent manner, then by subsequently activating ubiquitin-dependent ZnT2 degradation to abrogate Zn secretion. This provides insight into novel mechanisms through which Zn transport is tightly regulated in mammary epithelial cells.