| Objective Myocardial infarction (MI) continues to be a major health care problem. Left ventriclular (LV) remodeling after MI is associated with disability and death. A cascade of molecular events is involved in the cardiac remodeling process post MI. The serine/threonine kinase Pim-1 plays a critical role in the transduction of mitogenic signals and is rapidly induced by a variety of growth factors and cytokines. Pim-1 was recently identified as a cardiomyocyte survival regulator downstream of Akt in the myocardium. The protective effects of Akt in the myocardium have been intensively studied; but the cardioprotective role of Pim-1 as a signaling protein has not been fully clarified. The present study aims to clone and sequence PIM-1 mRNA and examine the Pim-1 associated survival proteins and its association with the strains and apoptosis in the remodeling myocardium in a large animal model.Methods Thirteen male Dorsett hybrid sheep bred for laboratory use were randomly divided to two groups: Non-MI(N=5) and MI(N=8). MI of 25% LV mass was created in MI group by ligation of coronary arterys. Echocardiograms and hemodynamic data were collected. The post-MI LV regional movement and deformation were monitored with sonomicrometry and quantified using areal end-systolic strain and remodeling strain. Total RNA was extracted from ovine heart tissues, and then first-strand cDNA was synthesized. The ovine PIM-1 cDNA was amplified by PCR using specific primers and PCR products were sequenced. Data were analyzed by softwares. Myocardial tissue samples were harvested at 10 weeks after MI from the remote and adjacent regions of the infarct for analyses of Pim-1 and Pim-1 associated survival signaling proteins and apoptosis.Results Both the LVESV and the LVEDV increased significantly, while the EF decreased over 10 weeks after MI. An increase in the length of wall motion abnormality was observed. Likewise, the ratio of wall motion abnormality to endocardial circumference significantly increased. There is no statistically significant change in heart rate, mean arterial pressure, and cardiac output during the whole course of the study. The end-systolic strains of the infarct and the adjacent regions decreased significantly after MI, while the remodeling strains of the infarct and the adjacent regions increased significantly after MI. The cDNA (1341bp) of ovine Pim-1 with the coding sequences for two Pim-1 kinase isoforms, 33 and 44 kDa, was first identified. It is highly conserved. Its 5'-UTR is involved in translational regulation. Both the isoforms expressed in heart tissue and the overall Pim-1 expression were highly controlled at multiple molecular levels. Pim-1, as well as Pim-1 mediated survival signaling proteins, Bcl-2, Bcl-xL, and phospho-Bad (Ser112), were upregulated in the adjacent region at 10 weeks post-MI and correlated positively with the degree of the remodeling. The tissue from the adjacent region exhibited significant increase in Akt expression compared to that from the normal control tissue. The ratio of Hsp90 and Hsp70 bound to Pim-1 significantly increased in the tissue from the adjacent region compared to that in the normal control tissue. However, the apoptotic activities also increased with remodeling strain.Conclusions The sheep MI model created by ligation of coronary arterys is a representative cardiac remodeling model. The sonomicrometry array showed that the process of remodeling is inhomogeneous and the end-systolic and remodeling strain are region-specific. PIM-1 mRNA is highly conserved and encodes 33kD and 44kD Pim-1 by utilizing alternative translation initiation codons. Its 5"-UTR is involved in translational regulation. Pim-1 is regulated in multiple levels. The adjacent region experienced higher remodeling strain and exhibited significant upregulation of Pim-1 related survival signals. A positive correlation between the survival proteins expressions and the regional remodeling strain is observed. The results suggest that following an MI, the myocytes in this region attempted to survive by increasing the expressions of the survival signals, however, the intrinsic elevations of the survival signals were not sufficient to protect myocytes from apoptosis. |
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