Age-dependent cellular and molecular mechanisms of functional alterations in left ventricle structure and function

Changes in left ventricle (LV) structure and function occur with aging. While the precise cellular and molecular mechanisms altered during the aging process remain poorly defined, the extracellular matrix (ECM) functions both to support myocytes and signal changes. To test the hypothesis that aging induces ECM changes, we evaluated LV function by echocardiography, myocardial protein levels by immunoblotting, and myocardial fibroblast function by isolating and characterizing fibroblast phenotypes. CB6F1 male mice from three age groups: young (1–2 months), middle age (15 months), and old (23 months) were analyzed for LV function (n = 6 each group), protein expression (avg relative intensity±SD; n = 3 each group), and fibroblast function (n = 3–6 each group). Myocardial extracts were immunoblotted for MMPs 2, 3, 7, 8, 9, 12, 13, and 14, TIMPs 1, 2, 3, 4, collagen I and III, and integrins α1, α2, α3, α4, α5, αv, and β1. LV function was lower in middle age and old mice. Posterior wall thickness was higher in the older groups, indicative of hypertrophy. At sacrifice, older mice had elevated body weight, LV weight, and LV to tibia length ratios. MMP-3 levels increased from 1±1 to 43±13 (p<0.01 vs young) and 20±13 (p = n.s.) in the middle age and old groups. The middle age group also had increased MMPs 7, 12, TIMP-1, and α3 integrin and decreased α4 integrin (all p<0.05). Total TIMP-1 levels increased from 116±58 in the young mice to 1664±266 (p<0.001 vs young) and 1710±120 (p<0.001 vs young) in the middle age and old groups. Older mice also had increased MMP-7 and decreased TIMP-4 and α4 integrin (all p<0.05). Mac-3 levels were 83±48, 219±163, and 114±46 for young, middle age, and old mice, suggesting that macrophage infiltration may be higher in the older mice (p = n.s.). Prolyl-4-hydroxylase levels were 2±0 for young mice, 401±363 for middle age mice (p = n.s.), and 313±212 for old mice (p = n.s.). All mice from the middle age and old groups had higher levels than any individual young mouse, indicating that fibroblasts accumulate with aging. Proliferation was 14% and 28% lower for fibroblasts from middle age and old mice. Collagen synthesis rates were over 20% lower for fibroblasts isolated from middle age and old mice, indicating reduced fibroblast function. Proliferation rates correlated negatively with the increased posterior and septal wall thickness (r = −0.71, p<0.01; r = −0.62, p = 0.02, respectively). These data demonstrate significant changes in specific cellular and molecular mechanisms regulating ECM structure and function. Changes within the ECM may cause increased myocardial susceptibility to physiological stimuli and may facilitate adverse LV remodeling.