ASSOCIATION AND COEXPRESSION OF FATTY-ACID-BINDING PROTEIN AND GLYCOPROTEIN CD36 IN THE BOVINE MAMMARY-GLAND

The involvement of glycoprotein CD36 and fatty-acid-binding protein (F ABP) in cellular growth, differentiation, lipid transport and metaboli sm led us to examine the possible biochemical and physiological relati onship(s) between these two proteins. We investigated three aspects of this relationship. We first attempted to identify any physical comple x formed between CD36 and FABP in bovine milk fat globule membranes. T hese membranes are the product of mammary gland secretory epithelial c ells. The second aspect studied was the effect of synthetic peptide an alogs to the C-terminus (amino acid residues 121-131) of bovine mammar y gland FABP on cell proliferation, as a result of the interaction of these peptides with the ectodomain of CD36. Finally, mammary gland CD3 6 and FABP coexpression was defined at different stages of lactation a nd during involution. Immunoprecipitation, Western immunoblotting with anti-FABP and anti-CD36, Northern-blot analysis and a mammary epithel ial cell proliferation assay demonstrated that: (a) bovine milk fat gl obule membranes contain the complex of CD36 and FABP, and that this co mplex is, most likely, formed as a result of FABP binding to the cytop lasmic segments of CD36; (b) synthetic analog of the C-terminus of FAB P with the sequence Val-Thr-Cys, identical to the sequence found in th e CD36-binding domain of thrombospondin, was a more potent inhibitor o f bovine mammary gland epithelial cell proliferation than a synthetic peptide with the Val-Cys-Thr sequence; (c) the expression of FABP and CD36 is related to the state of mammary cell differentiation, since it reaches its maximum during lactation and declines during the involuti onary period.