N-Acetyl-Ser-Asp-Lys-Pro is a natural and specific substrate for the N-terminal site of ACE. N-Acetyl-Ser-Asp-Lys-Pro is an endogenous tetrapeptide secreted by bone marrow and is ubiquitously found in plasma and various tissues. N-Acetyl-Ser-Asp-Lys-Pro is degraded specifically by ACE, and its plasma level rises substantially during ACE inhibitor therapy. N-Acetyl-Ser-Asp-Lys-Pro inhibits the proliferation of isolated cardiac fibroblasts but significantly stimulates the proliferation of vascular smooth muscle cells. Flow cytometry of rat cardiac fibroblasts treated with N-Acetyl-Ser-Asp-Lys-Pro shows significant inhibition of the progression of cells from G0/G1 phase to S phase of the cell cycle. In cardiac fibroblasts transfected with a Smad-sensitive luciferase reporter construct, N-Acetyl-Ser-Asp-Lys-Pro decreases luciferase activity by 55%. Moreover, phosphorylation and nuclear translocation of Smad2 is decreased in cardiac fibroblasts treated with N-Acetyl-Ser-Asp-Lys-Pro. N-acetyl-seryl-aspartyl-lysyl-proline is a negative regulator of hematopoietic stem cell proliferation. N-acetyl-seryl-aspartyl-lysyl-proline is involved in the control of hematopoietic stem cell proliferation by preventing their recruitment into S-phase. N-acetyl-seryl-aspartyl-lysyl-proline appears to exert this function by blocking the action of a stem cell-specific proliferation stimulator and acts selectively on quiescent progenitors. N-Acetyl-Ser-Asp-Lys-Pro inhibits collagenase expression and activation is associated with increased expression of TIMP-1 and TIMP-2. N-Acetyl-Ser-Asp-Lys-Pro does not alter collagenase or gelatinase activity in cardiac fibroblasts under basal conditions, but blunts the IL-1β-induced increase in total collagenase activity. Similarly, N-Acetyl-Ser-Asp-Lys-Pro normalizes the IL-1β-mediated increase in MMP-2 and MMP-9 activities and MMP-13 expression. N-Acetyl-Ser-Asp-Lys-Pro prevents hypertension-induced inflammatory cell infiltration, collagen deposition, nephrin downregulation and albuminuria, which could lead to renoprotection in hypertensive mice.