Differential Role of G Protein–Coupled Receptor Kinase 5 in Physiological Versus Pathological Cardiac Hypertrophy

Rationale : G protein–coupled receptor kinases (GRKs) are dynamic regulators of cellular signaling. GRK5 is highly expressed within myocardium and is upregulated in heart failure. Although GRK5 is a critical regulator of cardiac G protein–coupled receptor signaling, recent data has uncovered noncanonical activity of GRK5 within nuclei that plays a key role in pathological hypertrophy. Targeted cardiac elevation of GRK5 in mice leads to exaggerated hypertrophy and early heart failure after transverse aortic constriction (TAC) because of GRK5 nuclear accumulation. Objective : In this study, we investigated the role of GRK5 in physiological, swimming-induced hypertrophy (SIH). Methods and Results : Cardiac-specific GRK5 transgenic mice and nontransgenic littermate control mice were subjected to a 21-day high-intensity swim protocol (or no swim sham controls). SIH and specific molecular and genetic indices of physiological hypertrophy were assessed, including nuclear localization of GRK5, and compared with TAC. Unlike after TAC, swim-trained transgenic GRK5 and nontransgenic littermate control mice exhibited similar increases in cardiac growth. Mechanistically, SIH did not lead to GRK5 nuclear accumulation, which was confirmed in vitro as insulin-like growth factor-1, a known mediator of physiological hypertrophy, was unable to induce GRK5 nuclear translocation in myocytes. We found specific patterns of altered gene expression between TAC and SIH with GRK5 overexpression. Further, SIH in post-TAC transgenic GRK5 mice was able to preserve cardiac function. Conclusions: These data suggest that although nuclear-localized GRK5 is a pathological mediator after stress, this noncanonical nuclear activity of GRK5 is not induced during physiological hypertrophy.

Expression of mutant human epidermal receptor 3 attenuates lung fibrosis and improves survival in mice

Neuregulin-1 (NRG-1), binding to the human epidermal growth factor receptor HER2/HER3, plays a role in pulmonary epithelial cell proliferation and recovery from injury in vitro. We hypothesized that activation of HER2/HER3 by NRG-1 would also play a role in recovery from in vivo lung injury. We tested this hypothesis using bleomycin lung injury of transgenic mice incapable of signaling through HER2/HER3 due to lung-specific dominant-negative HER3 (DNHER3) expression. In animals expressing DNHER3, protein leak, cell infiltration, and NRG-1 levels in bronchoalveolar lavage fluid increased after injury, similar to that in nontransgenic littermate control animals. However, HER2/HER3 was not activated, and DNHER3 animals displayed fewer lung morphological changes at 10 and 21 days after injury ( P = 0.01). In addition, they contained 51% less collagen in injured lungs ( P = 0.04). Transforming growth factor-β1 did not increase in bronchoalveolar lavage fluid from DNHER3 mice compared with nontransgenic littermate mice ( P = 0.001), suggesting that a mechanism for the decreased fibrosis was lack of transforming growth factor-β1 induction in DNHER3 mice. Severe lung injury (0.08 units bleomycin) resulted in 80% mortality of nontransgenic mice, but only 35% mortality of DNHER3 transgenic mice ( P = 0.04). Thus inhibition of HER2/HER3 signaling protects against pulmonary fibrosis and improves survival.

Transgenic Rabbits Expressing Human Apolipoprotein A-I in the Liver

Human apolipoprotein A-I (apo A-I) transgenic rabbits were created by use of an 11-kb genomic human apo A-I construct containing a liver-specific promoter. Five independent transgenic lines were obtained in which human apo A-I gene had integrated and was expressed. Plasma levels of human apo A-I ranged from 8 to 100 mg/dL for the founder and up to 175 mg/dL for the progeny. Rabbit apo A-I levels were substantially decreased in the transgenic rabbits. HDL cholesterol (HDL-C) levels were higher in two of the five transgenic rabbit lines than in controls (line 20 versus nontransgenic littermate, HDL-C=80±7 versus 37±6 mg/dL; line 8 versus nontransgenic littermate, HDL-C=54±16 versus 35±6 mg/dL). This resulted in less atherogenic lipoprotein profiles, with very low (VLDL+LDL-C)/HDL-C ratios. HDL size and protein and lipid compositions were similar between transgenic and littermate nontransgenic rabbits. However, a large amount of pre-β apo A-I–containing lipoproteins was observed in the plasma of the highest human apo A-I expressor. Cell cholesterol efflux was evaluated with the incubation of whole serum from transgenic and control rabbits. Cell cholesterol efflux was highly correlated with HDL cholesterol, with apo A-I, and with the presence of pre-β apo A-I–containing lipoproteins. These rabbits will be an extremely useful model for the evaluation of the effect of increased hepatic apo A-I expression on atherosclerosis.