Myocardial migration by fibroblast progenitor cells is blood pressure dependent in a model of angII myocardial fibrosis

Abstract

Activation of the renin-angiotensin system (RAS) is thought to promote myocardial fibrosis. However, it is unclear whether this physiological fibrotic response results from chronic hemodynamic stress or from direct cellular signaling. Male C57B/6 mice were randomly assigned to receive angiotensin II (AngII) (2.0 lg kg À1 min À1 ), AngII+hydralazine (6.9 lg kg À1 min À1 ) or saline (control) via osmotic pumps for 7 days. Blood pressure was measured via noninvasive plethysmography. Hearts were harvested and processed for analysis. Cellular infiltration and collagen deposition were analyzed using histological staining. Molecular mediators were assessed using quantitative RT-PCR. As previously described, animals that received AngII developed hypertension and multifocal cellular infiltration by SMA + /CD133 + fibroblast progenitors followed by collagen deposition. The coadministration of hydralazine with AngII completely inhibited the hypertensive effects of AngII (Pp0.01) and resulted in minimal cellular infiltration and minimal collagen deposition. These findings were in the context of persistent RAS activation, which was evidenced by elevation in serum aldosterone levels in animals that received AngII or AngII+hydralazine compared with animals that received saline. At the molecular level, infusion of AngII resulted in the significant upregulation of profibrotic factors (connective tissue growth factor-7.8±0.7 fold), proinflammatory mediators (TNFa-4.6±0.8 fold; IL-1b-6.4±2.6 fold) and chemokines (CCL2-3.8 ± 1.0 fold; CXCL12-3.2 ± 0.4 fold), which were inhibited when hydralazine was also infused. We provide evidence that myocardial infiltration by fibroblast progenitor cells secondary to AngII and the resultant fibrosis can be prevented by the addition of hydralazine. Furthermore, the beneficial effects of hydralazine were observed while maintaining RAS activation, suggesting that the mechanism of fibrosis is blood pressure dependent.

Renin Inhibition and AT1R blockade improve metabolic signaling, oxidant stress and myocardial tissue remodeling

“…Our additional observation that combination led to no greater reductions in fibrosis suggest that mechanism to promote fibroblasts in this model are more dependent on hemodynamic that hormonal factors, per se. Thereby, our findings are contrary to the premise that greater reductions in aldosterone would then lead to greater reductions in fibrosis in this transgenic model and corroborate recent work in Ang II infused mice suggesting that fibroblast migration and interstitial fibrosis may be pressor-dependent processes[42]. …”

Nonclassical Resident Macrophages Are Important Determinants in the Development of Myocardial Fibrosis

“…In the wellestablished AngII-dependent model of hypertension, we and others have demonstrated an absence of tissue necrosis and associated polymorphonuclear cell infiltrates, suggesting that this model may not require the mass influx of classical macrophages. 7,15,16,21,23 We have also characterized that early myocardial infiltration in AngII-exposed hearts is largely unaffected in Ccr2 À/À mice, suggesting redundancy in chemokines or implicating Cx3cr1-dependent nonclassical macrophage recruitment. 21 Despite this body of evidence it remained to be demonstrated whether macrophages were mediating the cellular changes observed in the myocardium and the ECM remodeling after AngII infusion.…”

Connective tissue growth factor expression after angiotensin II exposure is dependent on transforming growth factor-β signaling via the canonical Smad-dependent pathway in hypertensive induced myocardial fibrosis

“…Mice were randomly assigned to either saline (vehicle) or Ang-II (2.8 mg/kg/d; Sigma-Aldrich, Oakville, ON, Canada) through the use of subcutaneously implanted Alzet osmotic minipumps (Alzet Corp., Palo Alto, CA) as previously described. 21 , 22 In brief, isoflurane with oxygen was used to anesthetize mice. A mid-scapular skin incision (approximately 1–2 cm) was made to implant the osmotic minipump subcutaneously.…”

“…In animal models, our group has focused our attention on the initial few days post-angiotensin II (Ang-II) infusion and demonstrated that significant inflammation and fibrosis development were associated with increased expression of TGF-β and CTGF in the myocardium. 18 , 21 – 23 Highlighting the importance of time-dependent events leading up to fibrosis, our previous work also demonstrated a marked elevation in CTGF gene expression prior to significant TGF-β production that remains to be fully characterized. 18 …”