DIFFERENTIATIONAL REGULATION AND PHOSPHORYLATION OF THE FATTY-ACID-BINDING PROTEIN FROM RAT MAMMARY EPITHELIAL-CELLS

From the soluble protein fraction of lactating rat mammary epithelial cells, fatty acid-binding protein (FABP) was isolated by immunoaffinit y chromatography. After digestion with trypsin, peptides were characte rized with time-of-flight mass spectrometry and revealed identity with corresponding peptides derived from the heart-type FABP isolated from rat heart. In addition, by electrospray mass spectrometry the molecul ar mass has been determined to 14683.9 +/- 3 Da, further corroborating the identity. The content of FABP in mammary glands from virgin, preg nant and lactating rats was evaluated using two-dimensional gel electr ophoresis and a FABP-specific immunosorbent assay. In the two-dimensio nal gels FABP was the apparently most abundant cytosolic protein in ma mmary epithelial cells from rats in late pregnancy as well as from lac tating rats. The content of FABP was 59 +/- 19 mu g/mg (n = 11) of sol uble proteins from the fully differentiated lactating mammary gland as determined by ELISA. This value represented an 80-fold increase compa red with the FABP content of mammary gland from virgin rats, and is co mparable with the level found in rat heart. Upon stimulation with insu lin a small fraction of FABP was phosphorylated in lactating mammary e pithelial cells. In conclusion, these findings indicate that the FABPs from rat mammary gland and heart are identical and further suggest th at in mammary gland this FABP may play a role in signal transduction d ownstream from the insulin receptor.