PDGFRβ Induces Ribosome Biogenesis in Aortic Smooth Muscle Cells

Abstract

IntroductionArterial fibrosis (AF) is an important determinant of cardiovascular health. AF results in a thickened arterial wall and loss of compliance which is a direct result of excessive extracellular matrix (ECM) accumulation. Platelet Derived Growth Factor Receptor β (PDGFRβ) activation in vascular smooth muscle cells (VSMCs) results in AF that manifests with increased VSMC proliferation and increased collagen synthesis and deposition in the arteries. However, the mechanisms by which PDGFRβ activation results in AF are not fully elucidated.ObjectiveWe sought to investigate whether PDGFRβ regulates ribosome biogenesis (RB), and the extent to which increased RB downstream of PDGFRβ activation in VSMC mediates the AF‐associated structural and functional changes in mouse aorta.MethodsWe used Pdgfrb+/D849V mice (designated Bk) as a genetic model for AF where VSMC specific SM22Cre is used to drive expression of a conditional Pdgfrb knock‐in allele that encodes a constitutively active PDGFRβ in VSMC. In order to reduce ribosome content we crossed Bk mice with Rpl24Bst mice with a hypomorphic Rpl24 allele resulting in a 40% reduction in Ribosomal Protein L24. Arterial structure was evaluated in tissue sections. Ultrasound imaging was used to measure structural and functional arterial changes. We also serum‐starved rat aortic smooth muscle (A7r5) and 3T3 fibroblast cell lines and then stimulated with PDGF‐BB. RB was evaluated by measuring ribosomal proteins using Western Blot and assessing rRNA as RNA/DNA ratio using a fluorometric assay.ResultsCompared to wild type, Bk aortas were significantly larger with increased diameter and wall thickness and were significantly stiffer with reduced distension. Secondary to the arterial changes, Bk mice showed decreased cardiac contractility at 6 months of age that was not apparent at 3 months, but cardiac output did not decrease. Stimulating serum‐starved 3T3 fibroblasts and A7r5 smooth muscle cell lines with PDGF‐BB resulted in RB measured as increased ribosomal proteins and RNA/DNA ratio. In Bk mice, VSMC consisted of significantly more ribosomal proteins (RPL26, RPL10A, RPL17, RPS3 and RPS6, p>0.05) and increased transcript and RNA/DNA ratio (150%, p>0.05). Altering ribosomal proteins using Rpl24Bst partially attenuated some of these changes.ConclusionsThis study highlights the novel concept that RB is an important factor in AF. We found that RB and ribosome content is increased in VSMC in response to PDGFRβ activation and that altering ribosomes using Rpl24Bst mutation attenuated the arterial changes associated with PDGFRβ activation. It can be expected that RB will prove to be instrumental for AF in VSMC, however a few unanswered questions still remain. First, what are the downstream signaling events by which PDGFRβ stimulates RB? And second, what are the mechanisms by which increased ribosome content results in increased vascular ECM deposition? In my future research, I will investigate the signaling pathways downstream of PDGFRβ that result in RB. I will also identify specific transcripts that are targeted by ribosomes in response to PDGFRβ activation and will test the efficacy of ribosome‐modulating interventions in inhibiting PDGFRβ‐induced AF.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.