NOS3 and pulmonary arterial hypertension: HDL-4 incorporates the previously described HDL3b and HDL3c subclasses.8 15 These have been implicated in several biological pathways relevant to PAH, including upregulation of endothelial nitric oxide synthase promoting vasodilatation,32 anti-inflammatory effects through decreased expression of endothelial adhesion molecules, decreased oxidation of LDL, antiapoptotic effects on the endothelium and activation of fibrinolysis.33