Rac-Induced Left Ventricular Dilation in Thyroxin-Treated ZmRacD Transgenic Mice: Role of Cardiomyocyte Apoptosis and Myocardial Fibrosis

Abstract

The pathways inducing the critical transition from compensated hypertrophy to cardiac dilation and failure remain poorly understood. The goal of our study is to determine the role of Rac-induced signaling in this transition process. Our previous results showed that Thyroxin (T4) treatment resulted in increased myocardial Rac expression in wild-type mice and a higher level of expression in Zea maize RacD (ZmRacD) transgenic mice. Our current results showed that T4 treatment induced physiologic cardiac hypertrophy in wild-type mice, as demonstrated by echocardiography and histopathology analyses. This was associated with significant increases in myocardial Rac-GTP, superoxide and ERK1/2 activities. Conversely, echocardiography and histopathology analyses showed that T4 treatment induced dilated cardiomyopathy along with compensatory cardiac hypertrophy in ZmRacD mice. These were linked with further increases in myocardial Rac-GTP, superoxide and ERK1/2 activities. Additionally, there were significant increases in caspase-8 expression and caspase-3 activity. However, there was a significant decrease in p38-MAPK activity. Interestingly, inhibition of myocardial Rac-GTP activity and superoxide generation with pravastatin and carvedilol, respectively, attenuated all functional, structural, and molecular changes associated with the T4-induced cardiomyopathy in ZmRacD mice except the compensatory cardiac hypertrophy. Taken together, T4-induced ZmRacD is a novel mouse model of dilated cardiomyopathy that shares many characteristics with the human disease phenotype. To our knowledge, this is the first study to show graded Rac-mediated O2·− results in cardiac phenotype shift in-vivo. Moreover, Rac-mediated O2·− generation, cardiomyocyte apoptosis, and myocardial fibrosis seem to play a pivotal role in the transition from cardiac hypertrophy to cardiac dilation and failure. Targeting Rac signaling could represent valuable therapeutic strategy not only in saving the failing myocardium but also to prevent this transition process.

Cardiomyocyte-specific overexpression of an active form of Rac predisposes the heart to increased myocardial stunning and ischemia-reperfusion injury

“…Table 1 summarizes morphologic and baseline in vivo cardiac parameters in adult WT and TG mice. Consistent with previous studies (19,20), there were no differences in the body weight (BW) between young adult (5-8 mo) WT and TG mice, and it was 30.7 Ϯ 0.9 g and 31.3 Ϯ 1.3 g in WT and TG mice, respectively (n ϭ 22/group, P ϭ NS). In vivo echocardiographic parameters between anesthetized WT and TG mice were comparable for ventricular dimensions and systolic LV fractional shortening between the two strains as reported (19,20); however, heart rates in TG mice tended to be lower than WT mice (504 Ϯ 16 vs. 549 Ϯ 16 beats/min, n ϭ 10 -12/group, P ϭ 0.066).…”

“…Consistent with these findings, it has been reported that increased endogenous myocardial Rac/NADPH oxidase activity in aging rats causes cardiomyocyte hypertrophy and cardiac remodeling (57). Interestingly, the activation of myocardial ZmRacD along with endogenous Rac expression with thyroxine treatment led to cardiac dilation and severe systolic dysfunction in young adult transgenic mice (19,20). Thus a potential molecular link exists between increased Rac activity and myocardial susceptibility to cardiac pathologies.…”

“…However, our novel ZmRacD-TG mouse model (19,20) is unique compared with Rac1 TG model (49), because, ZmRacD TG mice gradually develop cardiac phenotype with aging and provide an executable window to investigate the pathological conditions involved in the progression of Rac-induced cardiovascular diseases. At basal levels of transgene expression, increased myocardial Rac expression, Rac-GTPase activity and oxidative stress were present in all transgenic mice (19); but only the older mice (Ͼ18 mo of age) exhibited cardiac hypertrophy and a moderate decrease in systolic function.…”

“…Western blots were performed to determine the relative cardiac expression of Rac1, gp91 phox , and Pak1 in both WT and TG mice as described previously (19,20). Briefly, hearts were homogenized and equal amounts (50 g) of protein extract were separated by SDS-PAGE on polyacrylamide gel.…”

Stiffness of Left Ventricular Cardiac Fibroblasts Is Associated With Ventricular Dilation in Patients With Recent-Onset Nonischemic and Nonvalvular Cardiomyopathy

“…As shown in Figure 4, CFBs derived from the LV showed significantly decreased cell stiffness when compared with CFBs derived from the RV as indexed by decreased Young's modulus (3,383±390 Pa vs. 4,850±692 Pa; P=0.01, mean ± standard in LV dilation, 25 we quantified the total collagen content of RV and LV EMBs. In contrast to our results on single-cell stiffness, the levels of both RV and LV fibrosis did not correlate with RV and LV dilation, respectively.…”