Effect of N-acetyl-seryl-aspartyl-lysyl-proline on DNA and collagen synthesis in rat cardiac fibroblasts

N-Acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural inhibitor of p luripotent hematopoietic stem cell entry into the S phase of the cell cycle and is normally present in human plasma. Ac-SDKP is exclusively hydrolyzed by ACE, and its plasma concentration is increased 5-fold after ACE inhibit ion in humans. We examined the effect of 0.05 to 100 nmol/L Ac-SDKP on 24-h our-H-3-thymidine incorporation (DNA synthesis) by cardiac fibroblasts both in the absence and presence of 5% FCS. Captopril (1 mu mol/L) was added in all cases to prevent the degradation of Ac-SDKP. Treatment of cardiac fibr oblasts with 5% FCS increased thymidine incorporation from a control value of 12 469+/-594 to 24 598+/-1051 cpm (P<0.001). Cotreatment with 1 nmol/L A c-SDKP reduced stimulation to control levels (10 373+/-200 cpm, P<0.001), W e measured hydroxyproline content and incorporation of H-3-proline into col lagenous fibroblast proteins and found that Ac-SDKP blocked endothelin-1 (1 0(-8) mol/L)-induced collagen synthesis in a biphasic and dose-dependent ma nner, causing inhibition at low doses, whereas high doses had little or no effect. It also blunted the activity of p44/p42 mitogen-activated protein k inase in a biphasic and dose-dependent manner in serum-stimulated fibroblas ts, suggesting that the inhibitory effect of DNA and collagen synthesis may depend in part on blocking mitogen-activated protein kinase activity. Part icipation of p44/p42 in collagen synthesis was confirmed, because a specifi c inhibitor for p44/p42 activation (PD 980591 25 mu mol/L) was able to bloc k endothelin-I-induced collagen synthesis, similar to the effect of Ac-SDKP , The fact that Ac-SDKP inhibits DNA and collagen synthesis in cardiac fibr oblasts suggests that it may be an important endogenous regulator of fibrob last proliferation and collagen synthesis in the heart. Ac-SDKP may partici pate in the cardioprotective effect of ACE inhibitors by limiting fibroblas t proliferation (and hence collagen production), and therefore it would red uce fibrosis in patients with hypertension.