In the present study, we demonstrate that SMC-specific PPARγ deficiency augments AngII-induced atherosclerosis in male LDL receptor −/− mice. Interestingly, Pio administration attenuates AngII-induced atherosclerosis only in wild type mice but not in SMC-specific PPARγ-deficient mice, which characterizes SMC-specific PPARγ as the key molecular target for the ligand-mediated attenuation of atherosclerosis.
SMC-specific PPARγ deficiency augmented AngII-induced atherosclerosis only in male mice. This is in agreement with the study of Li and colleagues, in which the attenuation of atherosclerosis by a PPARγ ligand was only observed in male LDL receptor −/− mice.1
The basis for these gender differences have not been defined.
Pio administration activates PPARγ in both Cre0/0 and Cre+/0 genotypes, which was evidenced by increased PPARγ expression observed in peritoneal macrophages and other tissues. Previous in vitro
studies demonstrated that TZDs inhibited SMC proliferation and induced apoptosis through PPARγ dependent mechanisms.14
In the present study, Pio administration attenuates AngII-induced atherosclerosis only in Cre0/0 mice, but not in mice with SMC-specific PPARγ deficiency. Considering that SMC proliferation constitutes an important cellular mechanism for atherosclerosis initiation,15
our findings demonstrated not only SMC-specific PPARγ as an endogenous inhibitor of atherosclerosis, but also established that TZDs exert anti-atherosclerotic effects through this pathway.
Pio administration significantly suppresses AngII-induced SBP in both genotypes. This result indicates that Pio-mediated SBP lowering effect is independent of SMC-specific PPARγ. In support of this observation, a recently published paper using both SM22-Cre+ and Tie2-Cre+ PPARγ flox mice, showed that TZD-mediated the SBP lowering effects via PPARγ expressed in endothelium.16
Since endothelial PPARγ is intact, Pio administration attenuates AngII-induced SBP in both Cre0/0 and Cre+/0 groups in our study.
SMC-specific PPARγ deficiency or Pio administration did not influence aneurysm formation in LDL receptor −/− mice, which is contrary to a recent publication in which Pio reduced suprarenal aortic expansion in AngII-infused ApoE−/− mice.4
The differences may be due to the lower dose used in the present study.4
Our dietary delivery was estimated to be ~20 mg/kg/day, while the drinking water delivery in the study of Golledge et al.4
was estimated to be 50 mg/kg/day. In another study, rosiglitazone attenuated AngII-induced AAA formation in ApoE−/− mice, which was mainly associated with decreased expression of inflammatory mediators.3
The basis for the inconsistent effects of TZDs on AngII-induced AAAs is unclear.
To further understand the mechanism by which Pio mediates its effect via SMC-PPARγ on atherosclerosis, we examined the effect of AngII on MCP-1 production in cultured Cre+ and PPARγL+
SMCs. Interestingly, AngII activates MCP-1 production only in Cre+/0 and PPARγL+
SMCs, but not in control SMCs, suggesting that endogenous SMC-PPARγ regulates AngII-induced MCP-1 production. In addition, Pio had no effect on AngII-induced MCP-1 production in Cre+/0 SMCs which is consistent with this TZD requiring interaction with PPARγ to reduce AngII-induced atherosclerosis. The specificity of this pathway was demonstrated by the continued induction of MCP-1 secretion in PPARγL+
cells during IFNγ incubation that signals via CD74 pathway in SMCs.17
This SMC-PPARγ dependent effect of AngII is localized to SMCs that is not reflected by plasma concentrations of MCP-1.
In summary, this study provides evidence that lack of PPARγ in vascular SMCs results in significant increases in atherosclerosis associated with increased MCP-1 production. Furthermore, the study reveals that SMC-specific PPARγ expression is a novel mediator of ligand-mediated attenuation of atherosclerosis.
NOVELTY AND SIGNIFICANCE
What is Known?
- PPARγ, a nuclear receptor, is a target of therapeutic interventions to augment insulin sensitivity.
- PPARγ expression in macrophages moderates the development of experimental atherosclerosis.
- Activation of PPARγ by thiazolidinediones (TZDs) suppresses SMC proliferation.
- TZDs, PPARγ agonists, attenuate atherosclerosis in male mice.
What new information does this article contribute?
- Pio-induced attenuation of atherosclerosis depends upon PPARγ in SMC.
- Selective deficiency of - PPARγ in SMC augments AngII-aggravated atherosclerosis
PPARγ is a nuclear receptor that is highly expressed in many of cell types involved in vascular pathologies, including macrophages, endothelial cells and smooth muscle cells (SMCs). The (TZDs agoinsts of PPARγ have been shown to inhibit the development of atherosclerosis in male animals. Currently, it is unclear whether the beneficial effects of TZDs could be attributed to PPARγ agonism in a specific cell type. In vitro, TZDs inhibit SMC proliferation and migration, the key events that promote intimal hyperplasia during atherogenesis; however, the contribution of SMC PPARγ to the anti-atherogenic effects of TZD has not been assessed. Because TZDs regulate SMC proliferation, which is a key step in the development of atherosclerosis, we hypothesized that SMC-specific PPARγ is responsible for the beneficial effects of TZD on atherosclerosis. By generating SMC-specific PPARγ deficient mice, we show that SMC-specific PPARγ plays a critical role in the development of angiotensin IIinduced atherosclerosis. We demonstrate that PPARγ expression in SMCs is required for the reduction in AngII-induced atherosclerosis by pioglitazone. This is the first study to report that pioglitazone exerts its beneficial effect on atherosclerosis via a SMC-specific PPARγ-dependent mechanism.