Abstract
The enhanced production of monocytes expressing pro-inflammatory markers such as the integrin CD11b in patients with hypercholesterolemia may promote vascular inflammation and exacerbate atherogenesis. The objective of the present study was to determine whether hypercholesterolemia stimulates the production of CD11b(+) monocytes in bone marrow, and whether the renin-angiotensin system participates in this process and thus provides a target for therapeutic intervention. The dietary induction of hypercholesterolemia in adult male cynomolgus monkeys was accompanied by increased bone marrow cellularity and elevated peripheral blood and bone marrow monocyte CD11b expression. Isolated bone marrow CD34(+) hematopoietic stem cells (HSCs) evaluated by in vitro functional assays exhibited enhanced myeloproliferative capacity and differentiation into CD11b(+) monocytes. Treatment of hypercholesterolemic monkeys with the angiotensin II AT(1) receptor blocker losartan for 15 weeks reduced bone marrow cellularity, suppressed peripheral blood and bone marrow monocyte CD11b expression, and normalized CD34(+) cell function assays. All variables returned to pretreatment levels 6 weeks after discontinuation of losartan treatment. Hypercholesterolemia was associated with increased CD34(+) cell AT(1) receptor expression and an exaggerated in vitro myeloproliferative response to angiotensin II stimulation that positively correlated to plasma LDL concentrations. In vitro exposure to native low-density lipoproteins (LDL) also increased CD34(+) cell AT(1) receptor expression and the myeloproliferative response to angiotensin II stimulation in a dose-dependent and receptor-mediated manner. Our data provide support for a positive regulatory role of plasma LDL on AT(1) receptor-mediated HSC differentiation and the production of pro-atherogenic monocytes. LDL-regulated HSC function may explain in part hypercholesterolemia-induced inflammation as well as the anti-inflammatory and anti-atherosclerotic effects of AT(1) receptor blockers.