Stimulation of cellular sphingomyelin import by the chemokine connective tissue-activating peptide III

The selective import of phospholipids into cells could be mediated by prote ins secreted from the cells into the extracellular compartment. We observed that the supernatants obtained from suspensions of thrombin-activated plat elets stimulated the exchange of pyrene (py)labeled sphingomyelin between l ipid vesicles in vitro. The proteins with sphingomyelin transfer activity w ere purified and identified as the chemokine connective tissue-activating p eptide III (CTAP-III) and platelet basic protein. Isolated CTAP-III stimula ted the exchange of py-sphingomyelin between lipid vesicles but did not aff ect the translocations of py-labeled phosphatidylcholine and phosphatidylet hanolamine. CTAP-III rapidly increased the transfer of py-sphingomyelin fro m low density lipoproteins into peripheral blood lymphocytes, other immune cells, and fibroblasts, In the presence of heparin, CTAP-III was unable to insert sphingomyelin into the peripheral blood lymphocytes. The activation energy of the py sphingomyelin transfer suggested that the translocation pr oceeded entirely in a hydrophobic environment, [H-3]Sphingomyelin transferr ed to the cells by CTAP-III was hydrolyzed to [H-3]ceramide and [H-3]sphing osine after activation with tumor necrosis factor a. The generation of the [H-3]sphingolipid messengers was catalyzed by acid sphingomyelinase. Our re sults identify CTAP-III as the first mediator of the selective (endocytosis -independent) cellular import of sphingomyelin allowing the paracrine modul ation of the sphingolipid signaling.