Elimination of Platelet Factor 4 (PF4) from Platelets Reduces Atherosclerosis and Increases HDL Cholesterol in C57Bl/6 and ApoE^−/− Mice.

There is a presence of circulating, activated platelets in blood of patients with atherosclerosis, coronary disease and hypercholesterolemia. Upon activation, platelets release a large amount of platelet factor 4 (PF4), a platelet specific chemokine. Our laboratory has previously demonstrated several potentially proatherogenic properties of PF4 including alteration of LDL metabolism and cellular trafficking, and activation of NFkB, a proinflammatory transcription factor involved in atherosclerosis. We have also localized PF4 to human atherosclerotic lesions. However, to date, no direct in vivo evidence for the involvement of PF4 in atherogenesis. In the current study, we have bred PF4^−/− mice onto two athero-susceptible backgrounds, WT-C57Bl/6(WT) and apoE^−/−, to examine the importance of PF4 in atherogenesis. PF4^−/− and PF4^−/−apoE^−/− (DKO) mice are viable and healthy, with no spontaneous bleeding disorders. In order to induce atherosclerosis, WT and PF4^−/− mice were fed an atherogenic Paigen diet for 30 weeks (Study 1), while apoE^−/− and DKO mice were fed a high fat Western style diet for 10 weeks (Study 2). Examination of lesions in the aortic roots of Study 1 animals demonstrated a 5-fold reduction in PF4^−/− compared to WT mice (p = 0.008). Measurement of cholesterol levels demonstrated similar total and non-HDL cholesterol levels in WT and PF4^−/− mice. However, HDL cholesterol was significantly increased in PF4^−/− mice compared to WT (2.5-fold, p = 0.001). Examination of apoE^−/− mice (Study 2) demonstrated similar changes, with DKO mice demonstrating a 2.7-fold reduction in aortic atherosclerosis (measured by the en face method; p = 0.03) and a 1.7-fold increase in HDL cholesterol (p = 0.02) compared to apoE^−/− mice. Although platelet counts were increased by ~30% in mice lacking PF4, the activation state of the platelets in our mice at sacrifice (WT vs PF4^−/− and apoE^−/− vs DKO) were similar as measured by both p-selectin expression and annexin V binding. These data demonstrate, for the first time, that the platelet specific chemokine PF4 promotes atherosclerotic lesion development in vivo.