XBP1 Is Required for Expansion of the Mammary Epithelial Compartment in Early Lactation

Presentation Number: MON-0397
Date of Presentation: June 23rd, 2014

Kristen R. Davis*1, Sarah L. Giesy1, Qiaoming Long1, Kevin J. Harvatine2, Laurie H. Glimcher3 and Yves R. Boisclair1
1Cornell University, Ithaca, NY, 2The Pennsylvania State, University Park, PA, 3Weill Cornell Medical College, New York, NY

Abstract

Lactation represents a metabolically demanding phase of mammalian life.  Over the course of lactation, the mouse mammary gland synthesizes over 30 grams of triglycerides and 12 grams of proteins.  The assembly and secretion of milk triglycerides and proteins involve the endoplasmic reticulum (ER), an organelle regulated in part by the transcription factor XBP1.  Recent work showed that adipose tissue XBP1 is essential for full activity of the lactating gland and that the lactogenic hormone prolactin induces the formation of its transcriptionally active form (spliced XBP1 or sXBP1).   To address the direct role of XBP1 in lactation, we obtained mice harboring a null XBP1 mutation in mammary epithelial cells (ΔXPB1MEC) by crossing floxed XBP1 mice with mice carrying the Cre recombinase gene under the control of the ovine beta lactoglobulin promoter.  When bred, ΔXPB1MEC females had normal pregnancies and gave birth to the same number of pups as WT mice.  On lactation day 5 (L5), however, XBP1 expression was reduced by over 90% in MEC isolated from ΔXPB1MEC females.  To study lactation, litters were equalized to 9 pups and weighed daily from L1 to L14, and milk was collected at L14.  ΔXPB1MEC litters suffered a growth deficit as early as L3 and exhibited a 74% reduction in weight gain by L14.  The protein content of ΔXPB1MEC milk was reduced by 21%, whereas the profile of fatty acids synthesized in MEC or contributed by either diet or adipose tissue was unaffected.   Whole mount analysis of L14 mammary gland, however, revealed a substantially smaller MEC compartment in ΔXPB1MEC mice.   To identify the timing of this effect, whole mount analysis was performed at pregnancy day 14 (P14), P18, L1 and L5 on a second cohort of WT and ΔXPB1MEC mice.  The MEC compartment was similar in both genotypes at P14, P18 and L1 but obviously smaller at L5 in ΔXPB1MEC mice.  To understand why the epithelial compartment failed to expand at L5, indices of proliferation and apoptosis were obtained using Ki67 and Tunnel immunohistochemistry assays, respectively.  Proliferation at L5 was lower, whereas apoptosis was higher in ΔXPB1MEC than WT mice.  Quantitative real time RT-PCR was performed on total RNA isolated from MEC of both genotypes at L5.  Absence of XBP1 had no effect on the expression of genes involved in lipid or protein synthesis (e.g., Fasn, Scd1 and Csn) but caused increased expression of markers of ER stress (e.g., Ddit3 and Hspa5).  By electron microscopy, both morphology and abundance of the ER compartment were substantially disrupted at L5 in MEC of ΔXPB1MEC mice.   Collectively, these results show that ablation of XBP1 in mammary epithelial cells impairs lactation.  This impairment relates to a failure to develop a normal ER compartment in early lactation and to an imbalance of proliferative and apoptotic signals such that expansion and activity of the secretory epithelium is reduced.

 

Nothing to Disclose: KRD, SLG, QL, KJH, LHG, YRB