To explore the effect(s) of growth hormone signaling on thrombosis, we studied signal transduction and transcription factor 5 (STAT5)-deficient mice and found markedly reduced survival in an in vivo thrombosis model. These findings were not explained by a compensatory increase in growth hormone secretion. There was a modest increase in the activity of several procoagulant factors, but there was no difference in the rate or magnitude of thrombin generation in STAT5-deficient mice relative to control.However, thrombin-triggered clot times were markedly shorter, and fibrin polymerization occurred more rapidly in plasma from STAT5-deficient mice. Fibrinogen depletion and mixing studies indicated that the effect on fibrin polymerization was not due to intrinsic changes in fibrinogen, but resulted from changes in the concentration of a circulating plasma inhibitor. While thrombin-triggered clot times were significantly shorter in STAT5-deficient animals, reptilase-triggered clot times were unchanged. Accordingly, while the rate of thrombin-catalyzed release of fibrinopeptide A was similar, the release of fibrinopeptide B was accelerated in STAT5-deficient plasma versus control. Taken IntroductionThrombosis-related diseases such as myocardial infarction, stroke, and venous thromboembolism account for substantial morbidity and mortality worldwide. We recently demonstrated that thrombosis susceptibility is mediated, in part, by sex-specific patterns of growth hormone (GH) secretion and subsequent effects on expression of coagulation-related genes in the liver. 1 GH is a pleiotropic hormone that is synthesized and secreted by the anterior pituitary gland and has diverse effects on its target tissues. 2,3 Hepatic GH signaling occurs via the type I cytokine receptor, growth hormone receptor, and activation of its primary downstream effectors, the Janus kinase 2 (JAK2) and signal transduction and transcription factor 5 (STAT5). Mice deficient in both STAT5A and STAT5B are largely insensitive to GH and consequently, have an impaired insulin-like growth factor 1-mediated feedback inhibition and dysregulated GH secretion. [4][5][6][7][8] STAT5 is also an integral component of interleukin, erythropoietin, and prolactin signaling pathways. Furthermore, STAT5 can be activated independently of JAK2 by receptor tyrosine kinases and other mechanisms. 9,10 STAT5 has thus been shown to be a central component of GH signaling, as well as other important cytokine signaling pathways.Previously, we demonstrated that GH-deficient little mice are protected from thrombosis in vivo. 1 Considering the important role of STAT5 in GH signaling, we aimed to study the role of STAT5 in thrombosis. In this study, we characterized 2 mouse models of STAT5 deficiency. We predicted that mice deficient in STAT5 would have a decrease in thrombosis susceptibility, as seen in GH-deficient little mice. Surprisingly, we found that the genetic loss of STAT5 resulted in increased susceptibility to thrombosis, in vivo, and shortened clotting times, in vitro. These...