Deficiency of Abca1 and Abcg1 mediated cholesterol efflux pathways in smooth muscle cells enhances vasoconstriction but does not affect atherosclerosis

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): VIDI grant from Netherlands Organization of Scientific Research (NWO). Rosalind Franklin Fellowship from the University Medical Center Groningen. Both to Marit Westerterp, PhD Smooth muscle cells (SMCs) regulate blood flow distribution and blood pressure via vasoconstriction mediated by α-adrenergic receptors (α-ARs). Plasma membrane cholesterol may affect α1-AR signaling, but consequences for SMC-mediated vasoconstriction are unclear. Cholesterol loading promotes SMC-to-macrophage transition in vitro, which may enhance atherosclerotic plaque vulnerability. The cholesterol transporters ATP Binding Cassette A1 and G1 (ABCA1/ABCG1) are highly expressed by SMCs and mediate cholesterol efflux to apolipoprotein A1 and high-density lipoprotein (HDL), respectively. The role of ABCA1/ABCG1-mediated cholesterol efflux pathways in SMC-mediated vasoconstriction and atherogenesis remains poorly understood. We generated mice with SMC-specific Abca1/Abcg1 deficiency on the low-density lipoprotein receptor deficient (Ldlr-/-) background by crossbreeding Abca1fl/flAbcg1fl/flLdlr-/- mice with Myh11-CreERT2 transgenic mice and feeding them tamoxifen-diet. To induce SMC cholesterol accumulation and atherogenesis, we fed Myh11-CreERT2Abca1fl/flAbcg1fl/flLdlr-/- and Myh11-CreERT2Ldlr-/- mice Western-type diet (WTD) for 16 weeks. Combined SMC-Abca1/Abcg1 deficiency increased vasoconstriction in aortic rings induced by the α1-AR agonist phenylephrine, with reversal by methyl-β-cyclodextrin, substantiating its cholesterol-dependency. Unexpectedly, SMC-Abca1/Abcg1 deficiency induced urinary bladder enlargement by 20-fold, resembling bladder outlet obstruction (BOO). This was reversed by the α1-AR antagonist tamsulosin, indicating its dependence on bladder neck SMC constriction. Moreover, SMC-Abca1/Abcg1 deficiency decreased SMC markers and increased macrophage- and fibroblast-markers in the bladder wall, enhancing collagen deposition, consistent with SMC transdifferentiation. However, after 16 weeks WTD, SMC-Abca1/Abcg1 deficiency did not affect atherosclerotic lesion size, fibrous cap thickness, necrotic core, collagen, or macrophage content, suggesting that SMCs in atherosclerotic plaques were not affected. This may be due to low Abca1/Abcg1 expression in intimal compared to medial SMCs. We uncover a new role of SMC cholesterol efflux pathways in suppressing α1-AR mediated vasoconstriction and bladder SMC transdifferentiation, decreasing BOO. Our data may provide a mechanistic link for the association between BOO and diabetes in humans, particularly because diabetes is associated with decreased cholesterol efflux. SMC-Abca1/Abcg1 deficiency did not affect atherosclerotic lesion size or plaque composition, presumably due to low Abca1/Abcg1 expression in intimal SMCs of atherosclerotic lesions, as shown in mice and humans.