Further studies on the biological activities of the CFU-S inhibitory tetrapeptide AcSDKP. I. The precise point of the cell cycle sensitive to AcSDKP. Studies on the effect of AcSDKP on GM-CFC and on the possible involvement of T-lymphocytes in AcSDKP response.

In order to further investigate the mechanisms of spleen colony-forming unit (CFU-S) inhibition by the tetrapeptide acetyl-N-Ser-Asp-Lys-Pro (AcSDKP), the following related subjects were studied: 1) the effects of AcSDKP on the kinetics of granulocyte and macrophage precursor cells (granulocyte-macrophage colony-forming cells; GM-CFC); 2) the precise point in the cell cycle of CFU-S that is sensitive to AcSDKP; and 3) the role of lymphocytes in the chain of events leading to the inhibition of CFU-S entry into the cell cycle. The effects of AcSDKP on CFU-S and GM-CFC were tested using the spleen colony assay and methylcellulose culture technique, respectively; the cell cycle kinetic status was determined by the tritiated thymidine suicide technique. Nude mice were studied to assess the role of T-lymphocytes in the inhibitory phenomenon. Our results indicate that: 1) there is no inhibitory effect of AcSDKP on GM-CFC in vitro or in vivo; 2) AcSDKP is active at only the G0 or early G1 phases; and 3) AcSDKP activity is not modulated by T cells.