Date Published: March 30, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Tristan T. Hays, Ben Ma, Ning Zhou, Shaunrick Stoll, William J. Pearce, Hongyu Qiu.
Aortic stiffening is an independent risk factor that underlies cardiovascular morbidity in the elderly. We have previously shown that intrinsic mechanical properties of vascular smooth muscle cells (VSMCs) play a key role in aortic stiffening in both aging and hypertension. Here, we test the hypothesis that VSMCs also contribute to aortic stiffening through their extracellular effects. Aortic stiffening was confirmed in spontaneously hypertensive rats (SHRs) vs. Wistar‐Kyoto (WKY) rats in vivo by echocardiography and ex vivo by isometric force measurements in isolated de‐endothelized aortic vessel segments. Vascular smooth muscle cells were isolated from thoracic aorta and embedded in a collagen I matrix in an in vitro 3D model to form reconstituted vessels. Reconstituted vessel segments made with SHR VSMCs were significantly stiffer than vessels made with WKY VSMCs. SHR VSMCs in the reconstituted vessels exhibited different morphologies and diminished adaptability to stretch compared to WKY VSMCs, implying dual effects on both static and dynamic stiffness. SHR VSMCs increased the synthesis of collagen and induced collagen fibril disorganization in reconstituted vessels. Mechanistically, compared to WKY VSMCs, SHR VSMCs exhibited an increase in the levels of active integrin β1‐ and bone morphogenetic protein 1 (BMP1)‐mediated proteolytic cleavage of lysyl oxidase (LOX). These VSMC‐induced alterations in the SHR were attenuated by an inhibitor of serum response factor (SRF)/myocardin. Therefore, SHR VSMCs exhibit extracellular dysregulation through modulating integrin β1 and BMP1/LOX via SRF/myocardin signaling in aortic stiffening.
Aortic stiffening is a fundamental component of aging‐related vascular diseases (McEniery, Wilkinson & Avolio, 2007; Wallace et al., 2007). Increased aortic stiffness, whatever the underlying cause, is also an independent predictor of outcomes of cardiovascular diseases in the elderly. It is well known that hypertension is a highly age‐related human disease. Despite a widely held belief that increased aortic stiffness in hypertensive patients is largely a manifestation of long‐standing hypertension‐related damage, a recent statement from the American Heart Association (AHA) asserts that aortic stiffening is a cause rather than a consequence of hypertension in middle‐aged and older individuals (Townsend et al., 2015). This new concept further clarifies the cause and effect relationship between aortic stiffening and hypertension in aged individuals. Additionally, our previous studies in monkey have demonstrated that aortic stiffening is also strongly associated with aging (Qiu et al., 2007). Specifically, our recent studies with atomic force microscopy (AFM) have demonstrated similar characteristics of aortic vascular smooth muscle cells (VSMCs) in both aging and hypertension, indicating that VSMC‐mediated regulation is a fundamental basis of aortic stiffening in both conditions (Qiu et al., 2010; Zhou, Lee, Stoll, Ma, Costa, et al. 2017; Zhou, Lee, Stoll, Ma, Wiener, et al. 2017). However, the underlying mechanisms are not fully understood. It is conceivable that, in addition to intracellular effects, VSMCs are able to contribute to aortic stiffening via extracellular effects. However, it is difficult to discern the extracellular effects of VSMCs in intact aortic tissue in vivo. Our previous study successfully distinguished the role of VSMCs from the extracellular matrix (ECM) in aortic stiffness in vitro utilizing a three‐dimensional (3D) tissue model reconstituted system consisting of isolated VSMCs and collagen (Qiu et al., 2010). This model also provides the necessary simplicity to characterize interactions between VSMCs and the surrounding ECM and explore the molecular mechanisms mediating these changes.
This study integrated in vivo, ex vivo, and in vitro experiments to identify the individual components that contribute to aortic stiffness. We first determined the contributions of the crucial medial layer to aortic stiffening, which is independent of endothelium and adventitia. Second, use of a 3D reconstituted vessel revealed that VSMCs are essential for aortic stiffening, independent of the ECM in the medial layer. The data from both ex vivo native tissues and in vitro reconstituted tissues complemented our previous studies of single VSMCs (Zhou, Lee, Stoll, Ma, Wiener, et al. 2017) and emphasize the importance of VSMCs in aortic stiffening.
H.Q. conceived and designed the study. H.Q., T.H., B.M., and W.P. designed and performed ex vivo and in vitro experiments, analyzed the data, and wrote the manuscript; N.Z. and S.S designed, performed, and analyzed the in vivo experiments. All authors discussed and analyzed the results and reviewed and approved the final version of the manuscript.