Aging influences cardiac mitochondrial gene expression and cardiovascular function following hemorrhage injury

Abstract

Cardiac dysfunction and mortality associated with trauma and sepsis increase with age. Mitochondria play a critical role in the energy demand of cardiac muscles, and thereby on the function of the heart. Specific molecular pathways responsible for mitochondrial functional alterations after injury in relation to aging are largely unknown. To further investigate this, 6- and 22-month-old rats were subjected to trauma-hemorrhage (T-H) or sham operation and euthanized following resuscitation. Left ventricular tissue was profiled using our custom rodent mitochondrial gene chip (RoMitochip). Our experiments demonstrated a declined left ventricular performance and decreased alteration in mitochondrial gene expression with age following T-H and we have identified c-Myc, a pleotropic transcription factor, to be the most upregulated gene in 6- and 22-month-old rats after T-H. Following T-H, while 142 probe sets were altered significantly (39 up and 103 down) in 6-month-old rats, only 66 were altered (30 up and 36 down) in 22-month-old rats; 36 probe sets (11 up and 25 down) showed the same trend in both groups. The expression of c-Myc and cardiac death promoting gene Bnip3 were increased, and Pgc1-α and Ppar-α a decreased following T-H. Eleven tRNA transcripts on mtDNA were upregulated following T-H in the aged animals, compared with the sham group. Our observations suggest a c-myc-regulated mitochondrial dysfunction following T-H injury and marked decrease in age-dependent changes in the transcriptional profile of mitochondrial genes following T-H, possibly indicating cellular senescence. To our knowledge, this is the first report on mitochondrial gene expression profile following T-H in relation to aging.

Figure 1

Left ventricular performance decreases with aging and T-H. Six- and 22-month-old rats were subjected to sham or T-H procedure and left ventricular performance (+ and −dP/dt) was measured. dP/dt values expressed as mean > SE. Panel A, >dP/dt and panel B, −dP/dt. See “Materials and Methods” section for experimental details.

Figure 2

Mitochondrial gene expression changes following T-H. Changes in expression of a subset of mitochondrial genes following T-H in two different age groups (6- and 22- months). Upregulated (A) and downregulated (B) genes in each group are shown as bars. Bars represent mean fold change with a P < 0.05.

Figure 3

Pgc-1α and Bnip3 protein expression changes following T-H. Left ventricular tissues from 6-month-old rats subjected to T-H or sham operation were tested for Pgc-1α (A) and Bnip3 (B) by Western blot. A representative result from two sham and two T-H rats are given. The bar graph represents mean and standard error of three experiments using tissues from four sham and four T-H rats.

Figure 4

Alteration of tRNA transcripts on mtDNA following T-H. Transcripts of tRNAs that demonstrated a significant upregulation in 22-month-old rats following T-H. Expression changes in each animal are represented as a heat map (see “Materials and Methods” for details).

Figure 5

Upregulation of c-Myc following T-H. Left ventricular tissues from rats subjected to T-H or sham operation were tested by Western blot (A and B) or real time PCR (C) for c-Myc expression. The bands were quantified by densitometry (C).

Figure 6

Regulation of Pgc-1α and mitochondrial function by c-Myc, T-H and aging. Aging and T-H are known to reduce Pgc-1α activity. Our results indicate that c-Myc expression increases following T-H in both age groups and this transcription factor might be the mediator for decreased Pgc-1α following injury, resulting in diminished mitochondrial biogenesis and function.