Mammary glands of adipophilin-null mice produce an amino-terminally truncated form of adipophilin that mediates milk lipid droplet formation and secretion.

Adipophilin (ADPH), a member of the perilipin family of lipid droplet-associated proteins, is hypothesized to mediate milk lipid formation and secretion. Unexpectedly, the fat content of milk from ADPH-null mice was only modestly lower than that of wild-type controls, and neither TIP47 nor perilipin appeared to fully compensate for ADPH loss. This prompted us to investigate the possibility that the mutated ADPH gene was not a genuine null mutation. ADPH transcripts were detected in ADPH-null mammary tissue by quantitative real-time PCR, and C-terminal-specific, but not N-terminal-specific, ADPH antibodies detected a single lower molecular weight product and immunostained cytoplasmic lipid droplets (CLDs) and secreted milk fat globules in ADPH-null mammary tissue. Furthermore, stable cell lines expressing cDNA constructs corresponding to the ADPH-null mutation produced a product comparable in size to the one detected in ADPH-null mammary glands and localized to CLDs. Based on these data, we conclude that ADPH-null mice express an N-terminally truncated form of ADPH that retains the ability to promote the formation and secretion of milk lipids.