Background:
The small GTPase Rac1 is a multifunctional protein that contributes to cardiac remodeling by e.g. transcriptionally regulating pro-hypertrophic and pro-fibrotic factors and by its involvement in oxidative stress generation. We have recently shown that the Gi-dependent activation of Rac1 via guanine nucleotide exchange factor (GEF) Tiam1 is essential to α1-adrenoceptor driven hypertrophy in neonatal rat cardiomyocytes (NRCM).
Objective:
To further explore the function of Tiam1 in cardiac pathophysiology in vivo and in vitro by using genetic and pharmacological approaches.
Methods and Results:
We found that Tiam1 mRNA is upregulated in cardiac tissue of patients with heart failure, and in experimental models of chronic catecholamine exposure. Mice subjected to α/β-adrenergic stimulation via a combination of isoprenaline (ISO) and phenylephrine (ISO/PE, 30 mg/kg*d for each drug) showed a significant upregulation of cardiac Tiam1 mRNA after 48-96h of exposure, paralleling hypertrophic growth measured as the increase of heart weight to tibia length ratios (HW/TL). Further investigation revealed that Tiam1 is upregulated in cardiac fibroblasts, but not endothelial cells or cardiomyocytes. To analyze the contribution of Tiam1 to hypertrophic remodeling in vivo, mice with a global Tiam1 depletion (Tiam1-KO) were subjected to 7d and 28d of chronic ISO/PE infusions. In accordance with earlier in vitro data, Tiam1-KO were markedly resistant to cardiac hypertrophy compared to WT animals. Moreover, in contrast to WT, Tiam1-KO showed preserved systolic function after 28d of exposure and no sign of lung congestion. Interestingly, protection from hypertrophy did not correlate with an attenuated expression of fetal genes; instead, we observed a significant lower induction of genes associated with cardiac fibroblast activation such as periostin, collagen1a1 and TGFβ during the phase of hypertrophic growth (6-96 h).
To investigate Tiam1 depletion or inhibition in cardiac fibroblast pathophysiology, where Tiam1 has been described to mediate Rac1 activation in response to angiotensin II (AngII) type 1 receptor stimulation, we isolated adult cardiac fibroblasts from WT and Tiam1KO mice, and applied the Tiam1 inhibitor NSC 23766 (10 µM). Tiam1 inhibition or depletion significantly impaired Ang II-induced PAK- and p38-, but not ERK1/2-phosphorylation. When subjected to 28 d of chronic Ang II (1 mg/kg*d) perfusion, Tiam1KO showed delayed hypertrophy and limited concentric cell growth, but were unprotected against ventricular fibrosis and diastolic dysfunction.
Conclusion:
Tiam1 deficiency protects against catecholamine-induced cardiac hypertrophy and systolic dysfunction in vivo. This protective effect appears to involve attenuated fibroblast activation during the early stages of hypertrophic growth.