Inhibition of human umbilical vein endothelial cell proliferation by the CXC chemokine, platelet factor 4 (PF4), is associated with impaired downregulation of p21(Cip1/WAF1)

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 o n epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independ ent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cel l cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E-cyclin-dependent kinase 2 (cdk2) activity with resulting attenu ation of retinoblastoma protein phosphorylation. PF4-dependent downregulati on of cyclin E-cdk2 activity was associated with increased binding of the c yclin-dependent kinase inhibitor, p21(Cip1/WAF1), to the cyclin E-cdk2 comp lex. Analysis of total cellular p21(Cip1/WAF1) showed that in the presence of PF4, p21(Cip1/WAF1) levels were sustained at time points when p21(Cip1/W AF1) was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21(Cip1/WAF1 ) and provide the first evidence for interference with cell cycle mechanism s by a chemokine. (C) 1999 by The American Society of Hematology.