Flow-Induced Vascular Remodeling in the Mouse

Axl, a receptor tyrosine kinase, mediates flow-induced vascular remodeling

Intima-media thickening (IMT) in response to hemodynamic stress is a physiological process that requires coordinated signaling among endothelial, inflammatory, and vascular smooth muscle cells (VSMC). Axl, a receptor tyrosine kinase, whose ligand is Gas6, is highly induced in VSMC after carotid injury. Because Axl regulates cell migration, phagocytosis and apoptosis, we hypothesized that Axl would play a role in IMT. Vascular remodeling in mice deficient in Axl (Axl(-/-)) and wild-type littermates (Axl(+/+)) was induced by ligation of the left carotid artery (LCA) branches maintaining flow via the left occipital artery. Both genotypes had similar baseline hemodynamic parameters and carotid artery structure. Partial ligation altered blood flow equally in both genotypes: increased by 60% in the right carotid artery (RCA) and decreased by 80% in the LCA. There were no significant differences in RCA remodeling between genotypes. However, in the LCA Axl(-/-) developed significantly smaller intima+media compared with Axl(+/+) (31+/-4 versus 42+/-6x10(-6) microm3, respectively). Quantitative immunohistochemistry of Axl(-/-) LCA showed increased apoptosis compared with Axl(+/+) (5-fold). As expected, p-Akt was decreased in Axl(-/-), whereas there was no difference in Gas6 expression. Cell composition also changed significantly, with increases in CD45+ cells and decreases in VSMC, macrophages, and neutrophils in Axl(-/-) compared with Axl(+/+). These data demonstrate an important role for Axl in flow-dependent remodeling by regulating vascular apoptosis and vascular inflammation.

Role of nuclear Ca2+/calmodulin-stimulated phosphodiesterase 1A in vascular smooth muscle cell growth and survival

In response to biological and mechanical injury, or in vitro culturing, vascular smooth muscle cells (VSMCs) undergo phenotypic modulation from a differentiated "contractile" phenotype to a dedifferentiated "synthetic" one. This results in the capacity to proliferate, migrate, and produce extracellular matrix proteins, thus contributing to neointimal formation. Cyclic nucleotide phosphodiesterases (PDEs), by hydrolyzing cAMP or cGMP, are critical in the homeostasis of cyclic nucleotides that regulate VSMC growth. Here, we demonstrate that PDE1A, a Ca2+-calmodulin-stimulated PDE preferentially hydrolyzing cGMP, is predominantly cytoplasmic in medial "contractile" VSMCs but is nuclear in neointimal "synthetic" VSMCs. Using primary VSMCs, we show that cytoplasmic and nuclear PDE1A were associated with a contractile marker (SM-calponin) and a growth marker (Ki-67), respectively. This suggests that cytoplasmic PDE1A is associated with the "contractile" phenotype, whereas nuclear PDE1A is with the "synthetic" phenotype. To determine the role of nuclear PDE1A, we examined the effects loss-of-PDE1A function on subcultured VSMC growth and survival using PDE1A RNA interference and pharmacological inhibition. Reducing PDE1A function significantly attenuated VSMC growth by decreasing proliferation via G1 arrest and inducing apoptosis. Inhibiting PDE1A also led to intracellular cGMP elevation, p27Kip1 upregulation, cyclin D1 downregulation, and p53 activation. We further demonstrated that in subcultured VSMCs redifferentiated by growth on collagen gels, cytoplasmic PDE1A regulates myosin light chain phosphorylation with little effect on apoptosis, whereas inhibiting nuclear PDE1A has the opposite effects. These suggest that nuclear PDE1A is important in VSMC growth and survival and may contribute to the neointima formation in atherosclerosis and restenosis.

Age-related increase in wall stress of the human abdominal aorta: an in vivo study

BACKGROUND

The regulation of wall stress in the abdominal aorta (AA) of humans might be of specific interest, because the AA is the most common site for aneurysm formation in which wall stress seems to be an important pathophysiological factor. We studied the age-related changes in wall stress of the AA in healthy subjects, with the common carotid artery (CCA) as a comparison.

METHODS

A total of 111 healthy subjects were examined with B-mode ultrasonography to determine the lumen diameter and intima-media thickness (IMT) in the AA and the CCA.

RESULTS

Aortic IMT was affected by age in men and by both age and lumen diameter in women. Carotid IMT was affected by age and pulse pressure in both men and women. Wall stress was higher in the AA than in the CCA (P < .001), and men had higher wall stress than women in both the AA (P < .001) and the CCA (P < .05). Furthermore, wall stress was constant during life in the CCA of men and women and in the AA of women. In the male aorta, however, wall stress increased with age (P < 0.01).

CONCLUSIONS

Arterial diameters increase with age, and a compensatory thickening of the arterial wall prevents the circumferential wall stress from increasing. However, this compensatory response is insufficient in the male AA and results in an increase in stress with age. These findings might explain the propensity for aneurysms to develop in the AA of men.

Interleukin-18 and macrophage migration inhibitory factor are associated with increased carotid intima-media thickening

OBJECTIVE

Carotid intima-media thickening (IMT) is a form of vascular remodeling that has a strong genetic component. Recently, we discovered that in response to decreased carotid blood flow SJL mice developed the largest intima among 5 inbred strains. Because the SJL strain is prone to autoimmune diseases, we hypothesized that inflammation contributed to IMT in SJL mice.

METHODS AND RESULTS

We compared vascular remodeling (induced by 2 weeks of low flow) in 2 strains with small IMT (C3H/HeJ and C3HeB/FeJ) versus 2 strains with large IMT (FVB/NJ and SJL/J). Quantitative immunohistochemistry showed a dramatic increase in inflammatory cells per intima area in SJL compared with other strains. Microarray profiling of inflammatory gene mRNAs from carotids showed significant increases in interleukin (IL)-18 and Mif gene expression in SJL compared with C3HeB/FeJ mice. Increased expression of these genes was confirmed by quantitative reverse-transcription polymerase chain reaction and immunohistochemistry. Furthermore, greater cell proliferation in the intima of SJL accounted for increased intima-media thickening, whereas a higher level of apoptosis and a lower level of proliferation were observed in C3HeB/FeJ mice.

CONCLUSIONS

The present study indicates that increased expression of Mif and IL-18 cytokines is associated with intima-media thickening in SJL mice, likely by stimulating inflammation and proliferation.

Hemodynamic conditions alter axial and circumferential remodeling of arteries engineered ex vivo

We previously demonstrated that growth and remodeling was stimulated in arteries elongated ex vivo using step increases in axial strain. Viability and vasoactivity were similar to fresh arteries, however there was a substantial decrease in the ultimate circumferential stress. To test the hypothesis that the subphysiological perfusion conditions (i.e., low pressure and flow) previously used caused the reduction, arteries were subjected to the identical elongation protocol (50% increase over 9 days) while being perfused with physiological levels of flow, viscosity and pulsatile pressure. A significant increase in unloaded length was achieved by elongation under both perfusion conditions, although the increase was less under physiological (7 +/- 1%) than under subphysiological conditions (19 +/- 2%, p < 0.005). When length at physiological stress was estimated using mechanical testing data the values were similar. The ultimate circumferential stress of arteries elongated under physiological conditions was increased (33%), whereas the ultimate axial stress was decreased (50%) as compared with arteries elongated under subphysiological conditions. Elongated arteries under both perfusion conditions showed significant increases in proliferation and collagen mass, and similar viability and appearance to fresh arteries. These data suggest that there is substantial cross-talk between perfusion conditions and axial strain that modulates arterial remodeling and length.

Urokinase-induced smooth muscle cell migration requires PI3-K and Akt activation

OBJECTIVE

To examine the role of the phospho-inositol-3'-kinase (PI3-K)-akt signaling axis during smooth muscle cell (SMC) migration in response to the aminoterminal fragment of urokinase (ATF).

BACKGROUND

Urokinase (uPA) is involved in vessel remodeling and mediates smooth muscle cell migration. Migration in response to urokinase is dependent on ATF. The role of PI3-K/akt signaling during migration in response to the uPA fragments is not understood.

METHODS

Murine arterial SMCs were cultured in vitro. Linear wound and Boyden microchemotaxis assays of migration were performed in the presence of ATF with and without the PI3-K inhibitors (Wortmannin, Wn [10 nm] and LY294002, LY [10 microm]) and an akt inhibitor (aktI, [10 microm]). Western blotting was performed for akt, ERK1/2, and GSK3beta phosphorylation after cells were stimulated with ATF in the presence and absence of the inhibitors. Statistics were analyzed by one-way ANOVA.

RESULTS

Both PI3-K and akt inhibitors blocked the migratory response to ATF in both assays. ATF induced time-dependent increases in akt phosphorylation at both S472 and T308 sites and ERK1/2 phosphorylation. Activation of akt and ERK1/2 was inhibited by Wn and LY. Manumycin A, a ras inhibitor, did not inhibit activation of akt but did inhibit ERK1/2 activation. Activation of akt and the dephosphorylation of its downstream kinase GSK3beta were inhibited by the akt inhibitor. Direct inhibition of akt did not influence ERK1/2 activation and inhibition of ERK1/2 did not influence akt activation.

CONCLUSION

ATF mediated migration is PI3-K dependent and activates two separate pathways: ERK1/2 and akt. ATF induces akt phosphorylation through a PI3K-mediated but ras-independent mechanism while both ras and PI3K are required for ERK1/2 activation. Defining key signaling pathways is vital to regulate vessel remodeling.

Plasminogen Activator Expression Correlates with Genetic Differences in Vascular Remodeling

Intima-media thickening (IMT) of the carotid artery, a form of vascular remodeling, correlates well with coronary artery disease risk in humans. Vascular remodeling in response to blood flow is a complex process that critically involves altered cell matrix interactions. To gain insight into these events, we performed partial carotid ligation (left carotid (LCA) = low flow and right carotid (RCA) = high flow) in 2 inbred mouse strains: C57Bl/6J (C57) and FVB/NJ (FVB). To evaluate the role of the 2 major matrix-degrading systems, plasminogen activators (PAs) and matrix metalloproteinases (MMPs), we compared the expression of u-PA, t-PA, MMP-2 and MMP-9 in ligated carotids of C57 and FVB mice. The extent of remodeling was greater in response to low LCA than high RCA flow. Despite a similar decrease in LCA flow in both strains, maximal IMT volume was greater in FVB (82 +/- 7 x 10(-6) microm(3)) than in C57 (38 +/- 4 x 10(-6) microm(3)) after ligation. Among PAs and MMPs, increased expression of t-PA and u-PA correlated with increased IMT (p < 0.0005 and p < 0.001, respectively). MMP-2, MMP-9 and tissue inhibitors of metalloproteinase-2 expression also increased, but did not differ between strains. In summary, flow-induced IMT of the carotid is genetically determined and correlates with t-PA and u-PA expression in 2 inbred mouse strains.

Impact of IL-1β on flow-induced outward arterial remodeling

BACKGROUND

Flow reduction upregulates arterial wall interleukin 1beta (IL-1beta), and IL-1beta independently modulates intimal hyperplasia under low flow conditions. We hypothesized that IL-1beta expression is also augmented under high flow, and outward remodeling occurs by way of IL-1beta-dependent mechanisms.

METHODS

Carotid artery (CA) flow was surgically augmented in rabbits (n = 20). CAs were harvested at 1, 3, 7, and 14 days, and assayed via quantitative reverse transcriptase-polymerase chain reaction. IL-1 receptor I null mice (KO) and wild-type controls underwent unilateral CA ligation and harvest 4 weeks later to assess the impact of increased flow on the contralateral CA (n = 82).

RESULTS

The rabbit model led to an immediate 36% increase in contralateral flow (P = .01) with an 80% increase at 14 days (P = .016) with subsequent positive remodeling. High flow induced IL-1beta messenger RNA expression (114-fold at 1 day, P < .05), with levels remaining elevated through 14 days (61-fold, P < .05). In murine experiments, CA ligation resulted in a 44% increase in contralateral flow. Wild-type and KO animals responded with equivalent 83% and 78% increases in luminal area (P = .87).

CONCLUSIONS

Positive and negative perturbations of arterial blood flow induce IL-1beta in a time-dependent fashion. However, as opposed to intimal hyperplasia after flow reduction, positive arterial remodeling in response to increased flow occurs via IL-1beta independent mechanisms.

Strain-Dependent Vascular Remodeling

Background— Atherosclerosis of the carotid artery, called intima-media thickening (IMT), is a form of vascular remodeling that is an important predictor for cardiovascular events and has a strong genetic component.

Methods and Results— Recently, we established a mouse model of vascular remodeling based on partial ligation of the carotid, which is relevant to the “Glagov phenomenon.” We hypothesized that there would be genetically determined differences in outward remodeling and IMT induced by carotid flow alterations. We compared vascular remodeling among 5 inbred strains of mice. Despite similar changes in flow among the strains in the left carotid artery (LCA), we observed dramatic differences in remodeling of the partially ligated LCA relative to control. The smallest IMT volume (26±3 μm 3 ) was found in C3H/HeJ mice, and the largest were in SJL/J (59±10 μm 3 ) and FVB/NJ (81±6 μm 3 ). Shear stress did not differ after ligation among strains. Lumen area decreased only when stenosis was ≥55%. IMT correlated significantly with outward remodeling among inbred strains (except C3H). There were significant strain-dependent differences in remodeling index (measured as vessel area/IMT), which suggest fundamental alterations in sensing or transducing hemodynamic signals among strains. Among hemodynamic factors, low shear stress and high heart rate were predictive for IMT. Specifically, heart rate (bpm: C3H, 592±6; SJL, 649±6; FVB, 683±7) but not systolic blood pressure (mm Hg: C3H, 116±2; SJL, 119±1; FVB, 136±1) was predictive.

Conclusions— The present study indicates that performing a genetic cross of these strains and total genome scan should identify genes that mediate vascular remodeling.