Mammary ductal epithelium controls cold-induced adipocyte thermogenic program
Sympathetic activation during cold exposure increases adipocyte thermogenesis via the expression of mitochondrial protein uncoupling protein 1 (UCP1)1. The propensity of adipocytes to express UCP1 is under a critical influence of the adipose microenvironment and varies between sexes and among various fat depots2-7. Here, we report that cold-induced adipocyte UCP1 expression in a female mouse subcutaneous white adipose tissue (scWAT) is regulated by mammary gland ductal epithelial cells in the adipose niche. Single-cell RNA-sequencing (scRNA-seq) shows that glandular luminal epithelium subtypes express transcripts that encode secretory factors in controlling adipocyte UCP1 expression under cold conditions. We term luminal epithelium secretory factors as “mammokines”. Using whole-tissue immunofluorescence 3D visualization, we reveal previously undescribed sympathetic nerve-ductal points of contact. We show sympathetic nerve-activated mammary ducts limit adipocyte UCP1 expression via lipocalin 2. Both in vivo and ex vivo ablation of mammary ductal epithelium enhances cold-induced scWAT adipocyte thermogenic gene program. Since the mammary duct network extends throughout most scWATs in female mice, females show markedly less scWAT UCP1 expression, fat oxidation, energy expenditure, and subcutaneous fat mass loss compared to male mice, implicating sex-specific roles of mammokines in adipose thermogenesis. These results show a previously uncharacterized role of sympathetic nerve-activated glandular epithelium in adipocyte UCP1 expression and suggest an evolutionary function of mammary duct luminal cells in defending glandular adiposity during cold exposure. Overall, our findings highlight mammary gland epithelium as a highly active metabolic cell type and implicate a broader role of mammokines in controlling female adipose metabolism, mammary gland physiology, and systemic energy homeostasis. Overall design: 10X Genomics single cell sequencing was performed on the SVF fourth inguinal white adipose tissue (iWAT) depot mgWAT from 8-week old mice exposed to cold stress (4°C) or room temperature for 24 hr.