XBP1 Regulates the Biosynthetic Capacity of the Mammary Gland During Lactation by Controlling Epithelial Expansion and Endoplasmic Reticulum Formation

• Published in: Endocrinology (Philadelphia) Vol. 157; no. 1; pp. 417 - 428
• Main Authors: Davis, Kristen R, Giesy, Sarah L, Long, Qiaoming, Krumm, Christopher S, Harvatine, Kevin J, Boisclair, Yves R
• Format: Journal Article
• Language: English
• Published: United States Endocrine Society 01.01.2016
• Subjects: Animals; Apoptosis; Biomarkers - metabolism; Cell Proliferation; Crosses, Genetic; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum - ultrastructure; Endoplasmic Reticulum Stress; Epithelial Cells - cytology; Epithelial Cells - metabolism; Epithelial Cells - secretion; Epithelial Cells - ultrastructure; Female; Lactation - metabolism; Lactose - biosynthesis; Lactose - secretion; Mammary Glands, Animal - cytology; Mammary Glands, Animal - metabolism; Mammary Glands, Animal - secretion; Mammary Glands, Animal - ultrastructure; Mice, Knockout; Mice, Transgenic; Microscopy, Electron, Transmission; Milk Proteins - biosynthesis; Milk Proteins - metabolism; Regulatory Factor X Transcription Factors; Transcription Factors - genetics; Transcription Factors - metabolism; Triglycerides - biosynthesis; Triglycerides - secretion; X-Box Binding Protein 1
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/en.2015-1676

Cells composing the mammary secretory compartment have evolved a high capacity to secrete not only proteins but also triglycerides and carbohydrates. This feature is illustrated by the mouse, which can secrete nearly twice its own weight in milk proteins, triglycerides and lactose over a short 20-day lactation. The coordination of synthesis and export of products in other secretory cells is orchestrated in part by the transcription factor X-box binding protein 1 (XBP1). To assess the role of XBP1 in mammary epithelial cells (MEC), we studied floxed XBP1 female mice lacking (wild type; WT) or expressing the Cre recombinase under the control of the ovine β-lactoglobulin promoter (ΔXBP1MEC). Pregnant ΔXBP1MEC females had morphologically normal mammary development and gave birth to the same number of pups as WT mice. Their litters, however, suffered a weight gain deficit by lactation day 3 (L3)3 that grew to 80% by L14. ΔXBP1MEC dams had only modest changes in milk composition (−21% protein, +24% triglyceride) and in the expression of associated genes in isolated MEC. By L5, WT glands were fully occupied by dilated alveoli, whereas ΔXBP1MEC glands contained fewer, mostly unfilled alveoli and retained a prominent adipocyte population. The smaller epithelial compartment in ΔXBP1MEC glands was explained by lower MEC proliferation and increased apoptosis. Finally, endoplasmic reticulum ribbons were less abundant in ΔXBP1MEC at pregnancy day 18 and failed to increase in abundance by L5. Collectively, these results show that XBP1 is required for MEC population expansion during lactation and its ability to develop an elaborate endoplasmic reticulum compartment.