Chronic inflammation is present in atherosclerosis that is common in patients with hypertension, where the key-facilitating role of Ang II is well recognized. In this study, we provided evidence that Ang II is capable to induce CRP generation in VSMCs both in vitro and in vivo, which is mediated predominately through AT1R in the VSMCs, and followed by MAPK activation. Together with the finding that CRP upregulates Ang II receptor expression in VSMCs [13], our study demonstrated that there is a crosstalk between Ang II and CRP, which deepened the understanding of the important role of VSMCs and Ang II-elicited inflammatory process in atherosclerosis.
Hepatocytes are believed to be the major contributor of circulating CRP in plasma, however, other tissues such as kidney, adipocytes, neurons and VSMCs may be able to generate CRP [7,14]. Although there are contradictory results that inferred CRP protein found in the hyperlipidemic atherosclerosis lesions was essentially derived from the deposition of blood circulation rather than synthesized de novo by vascular cells [15], there is growing evidence suggesting that vascular secretion of CRP is involved in the atherosclerosis and plaque formation [16]. In our study, it is suggested that CRP in the artery wall was most probably produced by VSMCs, since an increased colocalization of CRP and -actin in the VSMC plasma in Ang II-infused rat aortic artery walls were detected. Moreover, in situ hybridization analysis identified the existence of CRP mRNA signals in the VSMCs layers in the aortic artery, which strongly suggested that VSMCs are the potential source for the CRP protein detected in the artery walls. In addition, in Ang II-infused rats, since there was a significant increase of plasma CRP, we hypothesized that Ang II-induced CRP generation from VSMCs may partly, if not all, contribute to the elevated serum CRP, although it cannot be excluded that Ang II may stimulate hepatocytes and other types of cells generating CRP.
The possible explanation for the discrepancy of vessel wall CRP origination may be raised by the different animals and treatments used in these studies, since there are considerable variations of CRP between different species with respect to ligand-binding specificity and glycosylation characters [17]. For instance, CRP of rabbit and human being is not glycosylated and does not bind to pneumococcal C-polysaccharide, which is different from rat CRP, since rat CRP is a glycoprotein bearing a single complex oligosaccha-ride on each polypeptide subunit and binds to pneumococcal C-polysaccharide [18], which may greatly influence the features of distribution and deposition of CRP in the vessel walls.
It is supported by a multiple of studies that renin-angiotensin system activation plays an important role in atherosclerosis [19,20]. As one of Ang II’s major targets, VSMCs express AT1 receptors, through which Ang II exerts its activities, such as vasoactive regulation, cell proliferation and oxidative production. VSMCs are also capable of releasing proinflammatory cytokines including interleukin-6, interleukin-18, adhesion molecules and chemokines in response to Ang II stimulation [21], suggesting its potential proinflammatory role in atherosclerosis. Infusion of Ang II into hypercholesterolemic mice dramatically accelerates the atherosclerotic process, leading to the development of extensive atherosclerotic plaque formation [22,23]. In the investigation, given that CRP involves multiple roles in the atherogenesis, and the local produced CRP is more important in the inflammatory process in the artery walls, our finding that Ang II is capable of upregulating CRP mRNA level and CRP release, thus, strengthened the association between Ang II and VSMCs in the chronic inflammation process of atherosclerosis in artery walls.
Ang II binding AT1 receptors of VSMCs are linked to MAPK-dependent pathways [24,25]. MAPK pathway has been found to be critical for Ang II-induced DNA synthesis [26,27]. It is also known that exogenous administration of Ang II is usually accompanied by increased MAPK/ERK activity in vascular tissues [28]. In addition, production of Ang II-enhanced proinflammatory cytokines is believed to be associated with NF-kappa B activation, which may be the downstream event of ERK activation in VSMCs [29]. In accord with it, the present study demonstrated that Ang II-induced CRP generation by rat VSMCs was regulated by MAPK signaling pathway through the AT1 receptor, because PD098059, which specifically inhibits the upstream activator of ERK1/2, almost completely abolished the increase of Ang II-induced CRP mRNA and protein. In this pathway, AT2 receptor seemed to be insignificant, since its specific antagonist PD123319 did not alter the effect of Ang II on CRP both in mRNA and in protein.
In conclusion, the study demonstrates that Ang II possesses the property to induce the production of CRP in VSMCs, in which AT1 receptor and MAPK signal pathway are involved. Regarding the emerging pivotal role of CRP in the local chronic inflammation of artery wall and its multiple detrimental effects on VSMC proliferation and endothelial dysfunction, our finding underscores the importance of Ang II-facilitated inflammation of VSMCs in the process of atherosclerosis. The present study may also offer a direct explanation of the clinical finding that AT1 receptor blockers significantly decrease the serum level of CRP [10,19].
