Seipin-adipogenin controls lipid storage in fat cells.

Obesity is characterized by the accumulation of triacylglycerols in lipid droplets of adipocytes (fat cells) and the expansion of adipose tissue (1, 2). Adipocytes arise from stem cells through adipogenesis, a process driven by several transcription factors, particularly peroxisome proliferator–activated receptor gamma (PPAR?). During the differentiation of white adipocytes, numerous small lipid droplets initially form before coalescing into a single giant unilocular lipid droplet. By contrast, brown adipocytes retain multiple small lipid droplets. These morphological changes are critical, as the number and size of lipid droplets determine lipid storage capacity and the rate at which lipids can be broken down. Despite their importance, the molecular mechanisms governing lipid droplet biogenesis and expansion during adipocyte development remain incompletely understood. On page 593 of this issue, Li et al. (3) identify adipogenin as a molecular switch that shifts the emphasis from generating new lipid droplets to expanding existing ones during adipogenesis. [ABSTRACT FROM AUTHOR]