Inducible brown/beige adipocytes in retro-orbital adipose tissues

Beige adipocytes and brown adipocytes can generate heat by using mitochondrial uncoupling protein 1 (Ucp1), a thermogenic protein. Browning/beiging is the emergence of beige adipocytes in white adipose tissues (WAT) for cold acclimatization. Here we show the existence of brown/beige adipocytes in retro-orbital WAT in mice. Histologically, Ucpl-positive cells with multilocular lipid droplets were abundant in retro-orbital WAT of immature mice; those cells decreased in number with age. However, Ucpl-positive adipocytes with multilocular lipid droplets emerged in retro-orbital WAT in adult mice, due to cold exposure as short as 3 h. Consistent with this observation, the expression level of Ucpl mRNA was enhanced in tissues upon cold exposure. Furthermore, eye surface temperature remained within a physiological range during cold challenge. RT-qPCR suggested a mixed phenotype of brown and beige adipocytes in retro-orbital WAT. Transmission electron microscopic observation showed multiple lipid droplets and numerous mitochondria with high cristae density in retro-orbital WAT cells from both control and cold-exposed mice. Our results suggest that warming of the orbital cavity by browning/beiging in retro-orbital WAT is a protective mechanism against cold cataract caused by lowered lens temperature.