Forskolin induced remodeling of lipid droplets in rat adipocytes

Adipose tissue is the main energy reserve of the body. When energy is required, adipocyte triglycerides stored in lipid droplets (LDs) are broken down by lipase, and free fatty acids are released to supply the physiological need. Intracellular LDs are active metabolic organelles in mammalian cells, particularly in adipocytes. The present study was aimed to investigate the morphological changes of LDs and the alternation of LD-associated perilipin family proteins during long-term lipolysis stimulated by forskolin. Primary differentiated adipocytes derived from epididymal fat pads of Sprague-Dawley (SD) rats were incubated in the presence or absence of 1 μmol/L forskolin for 24 h. Content of glycerol released to the culture medium was determined by a colorimetric assay and served as an index of lipolysis. Morphological changes of LDs were observed by Nile red staining. The mRNA level of perilipin family genes was detected by quantitative real-time PCR. The protein level and subcellular localization were examined by immunoblotting and immunofluorescence staining, respectively. The results showed that forskolin induced sustained lipolysis in differentiated adipocytes. The morphology of LDs changed in a time-dependent manner. Large clustered LDs became gradually smaller in size and eventually disappeared; in contrast, peripheral micro-LDs increased gradually in number until the cytoplasm was filled with numerous micro-LDs. The protein level of the perilipin family proteins showed obvious alternation. Mature adipocytes physiologically expressed a very low level of Plin2 protein, whereas in adipocytes stimulated with lipolytic forskolin, the protein and mRNA levels of Plin2 were significantly increased, and the increased Plin2 was specifically bound to the surface of LDs. During chronic stimulation of forskolin, the mRNA level of Plin3 was unchanged, but the mRNA levels of Plin1, Plin4 and Plin5 were significantly decreased. These results suggest that the morphology of LDs and perilipin family proteins on the surface of LDs are significantly altered during long-term lipolysis stimulated by forskolin, representing a dynamic process of the remodeling of LDs.