ScRNA-seq Reveals a Role of Mammary Luminal Epithelium in Adipocyte Adaptations

Almost four decades of research suggest a dynamic role of ductal epithelial cells in adipocyte adaptation in mammary gland white adipose tissue (mgWAT), but factors that mediate such communication are not known. Here, we identify a complex intercellular crosstalk in mgWAT revealed by single-cell RNA-seq (scRNA-seq) and comprehensive data analysis suggest that epithelial luminal cells during cold exposure undergo major transcriptomic changes that lead to the expression of an array of genes that encode for secreted factors involved in adipose metabolism such as Adropin (Enho), neuregulin 4 (Nrg4), angiopoietin-like 4 (Angptl4), lipocalin 2 (Lcn2), milk fat globule-EGF factor 8 (Mfge8), Insulin-like growth factor-binding protein 1 (Igfbp1), and haptoglobin (Hp). To define the mammary epithelial secretome, we coin the phrase “mammokines”. We validated our cluster annotations and cluster-specific transcriptomics using eight different adipose scRNA-seq datasets including Tabula Muris and Tabula Muris Senis. In situ mRNA hybridization and ex vivo isolated mgWAT luminal cells show highly localized expression of mammokines in mammary ducts. Trajectory inference demonstrates that cold-exposed luminal cells have similar transcriptional profiles to lactation post-involution (PI), a phase defined by reappearance and maintenance of adipocytes in the mammary gland. Concomitantly, we found that under cold exposure female mgWAT maintains more adipogenic and less thermogenic potential than male scWAT and ex vivo removal of luminal epithelial cells from mgWAT markedly potentiates beige adipocyte differentiation. Conditioned media from isolated mammary epithelial cells treated with isoproterenol suppressed thermogenesis in differentiated beige/brown adipocytes and treatment of beige/brown differentiated adipocyte with mammokine LCN2 suppresses thermogenesis and increases adipogenesis. Finally, we find that mice lacking LCN2 show markedly higher cold-dependent thermogenesis in mgWAT than controls, and reconstitution of LCN2 in the mgWAT of LCN2 knockout mice promotes inhibition of thermogenesis. These results show a previously unknown role of mammary epithelium in adipocyte metabolism and suggest a potentially redundant evolutionary role of mammokines in maintaining mgWAT adiposity during cold exposure. Our data highlight mammary gland epithelium as a highly active metabolic cell type and mammokines could have broader implications in mammary gland physiology and lipid metabolism.