The effect of two bovine beta-casein peptides on various functional properties of porcine macrophages and neutrophils: differential roles of protein kinase A and exchange protein directly activated by cyclic AMP-1.

The effects of two bovine beta-casein peptides on the urokinase plasminogen activator (u-PA) system and superoxide anion (SA) production by porcine macrophages and neutrophils activated by phorbol myristate acetate (PMA) were investigated. Macrophages and neutrophils were obtained from fourteen weaned piglets and were cultured in vitro for 24 h with or without one of two chemically synthesised peptides: tripeptide leucine-leucine-tyrosine (residues 191-193 of beta-casein) (LLY) and hexapeptide proline-glycine-proline-isoleucine-proline-asparagine (residues 63-68 of beta-casein). Following incubation, cells were stimulated with 80 microM-PMA. Total cell-associated u-PA, membrane-bound u-PA, free u-PA binding sites along with SA production were determined after stimulation with PMA. Both peptides suppressed the u-PA system and SA production of PMA-stimulated macrophages isolated from piglets during weeks 1-2 after weaning. Only the tripeptide LLY suppressed the u-PA system and SA production of PMA-stimulated neutrophils during the same time period. None of the peptides tested had any effect (P>0.05) on the u-PA system and SA production of PMA-stimulated macrophages and neutrophils isolated from the same piglets during weeks 5-6 after weaning. Thus, peptides are effective only in the early post-weaning period. Using cyclic AMP analogues that are highly specific activators of protein kinase A (PKA) or exchange protein directly activated by cyclic AMP-1 (Epac-1), we found that activation of PKA, but not Epac-1, was responsible for the downregulation of the u-PA system, whereas activation of PKA and/or Epac-1 was responsible for the downregulation of SA system in both macrophages and neutrophils.