Mildly elevated unconjugated bilirubin is associated with reduced platelet activation-related thrombogenesis and inflammation in Gilbert's syndrome.

Gilbert's syndrome (GS) is associated with a mild unconjugated hyperbilirubinemia, increased circulating antioxidant capacity, and reduced cardiovascular disease (CVD) risk. The current study investigated whether mildly elevated circulating unconjugated bilirubin (UCB) is negatively associated with multiple thrombotic risk factors including platelet activity, hemostatic function, and inflammation in individuals with GS. Blood samples were collected from matched GS and control subjects (14 per group). Activation-dependent platelet surface marker expression of PAC-1 (binds to GPIIb/IIIa surface receptors on activated platelets) and CD62P (marker for P-selectin released from activated degranulated platelets) was assessed in adenosine diphosphate (ADP)-stimulated platelets using flow cytometry. Exogenous agonists, ADP, collagen, and arachidonic acid (AA), were used to stimulate platelet aggregation. A statistically significant decrease in the expression of P-selectin (P = 0.030) on activated platelets was observed in GS subjects. Collagen and AA-induced platelet aggregation were significantly (P = 0.018; P = 0.032 for respective agonists) reduced in GS versus control group. Elevated UCB (P = 0.001) and high density lipoprotein (P = 0.033) in addition to reduced low density lipoprotein (P = 0.024) and high sensitive C-reactive protein (P = 0.043) were also observed in GS when compared to the control group. Reduced P-selectin expression suggests decreased platelet activation-dependent degranulation, while reduced platelet aggregation by AA and collagen indicates a quantitative decrease in platelet aggregation consequently targeting the cyclooxygenase-1 and GP VI pathways, respectively. These findings are the first to demonstrate that the activation of platelets is mildly inhibited in individuals with GS, an effect that might contribute to protection from platelet hyperactivation-induced thrombosis and thus cardiovascular mortality in individuals with benign hyperbilirubinemia.