Perilipin is a protein produced and secreted by adipose cells. This protein is localized exclusively to the surface of lipid droplets. In response to lypotic stimuli, perilipin is phosphorylated by protein kinase A. Once activated, perilipin has inhibitory effects on hormone-sensitive lipase (HSL), a protein that mediates the hydrolysis of triacylglycerol, the major form of stored energy in the body. Perilipin expression is limited to adipocytes and steroidogenic cells.