Thyroid hormone induces myocardial matrix degradation by activating matrix metalloproteinase-1

Abstract

Hyperthyroid patients develop left ventricular hypertrophy associated with alterations of several cardiac parameters such as heart rate, cardiac output, cardiac contraction and hemodynamic overload leading to cardiac complications. Although cardiac hypertrophy and contractile abnormality occur, interstitial fibrosis in the heart usually does not take place in hyperthyroid condition. Therefore, in the present study, the mechanism regulating myocardial extracellular matrix (ECM) remodeling in hyperthyroid condition was investigated. Cardiac hypertrophy was developed in Sprague–Dawley rats by administration of 3,5,3′-triiodo-l-thyronine (triiodothyronine, 8 μg/100g BW, ip, SID) and glucocorticosteroid, dexamethasone (DEX, 35 μg/100g BW, po, SID), which is also an inducer of hypertrophy for 15 days. Heart/Body weight ratio and atrial and brain natriuretic peptide mRNAs were significantly increased in both triiodothyronine- and DEX-treated rats compared to control. Collagens-I and -III deposition in the left ventricular sections was reduced in triiodothyronine-treated rats, whereas in DEX-treated animals those were increased compared to control. While mRNA and protein levels of procollagens-I and -III were increased with triiodothyronine (p < 0.01), the levels of mature collagens-I and -III were decreased. The levels of the mature collagens were increased with DEX compared to control. MMP-1 activity in the serum and left ventricle was higher with reduced levels of TIMPs-3 and -4 in the left ventricle of triiodothyronine-treated rats. The results suggest that accelerated breakdown of collagens-I and -III by MMP-1 due to suppression of the endogenous TIMPs plays an important role in regulating the ECM in myocardium of hyperthyroid rat.

Introduction

The main components of the extracellular matrix (ECM) in heart are fibrillar collagens-I and -III (Medugorac, 1982). These collagens ensure structural integrity of the adjoining myocytes which is important for translating myocyte shortening into left ventricular (LV) pump function (Spinale et al., 2000). Collagen matrix in the heart is a dynamic structure as reflected by continuous synthesis and degradation of matrix proteins. Degradation of matrix proteins is caused by matrix metalloproteinases (MMPs), which belongs to a family of proteolytic enzymes (Shapiro, 1998). The family of MMPs consists of more than 20 different zinc-containing Ca²⁺-dependent endopeptidases which cause proteolysis of different kinds of matrix proteins including collagens, gelatins, fibronectin and laminins (Spinale et al., 2000, Creemers et al., 2001, Illman et al., 2003). In mammalian myocardium, MMP-1 has been demonstrated to play an important role in the degradation of interstitial collagens (Spinale, 2002). MMP-1 proteolyzes collagens-I, -II, -III, -VII and basement membrane components (Spinale et al., 2000, Nagase and Okada, 1997), and MMP-1 activity is regulated by the endogenous tissue inhibitors of the metalloproteinases (TIMPs). Therefore, the actual rate of MMP-1-mediated proteolysis of matrix proteins is dependent on the balance between MMPs and TIMPs (Gomez et al., 1997).

Modulation of ECM components is one of the crucial events inducing remodeling of the left ventricle in hypertrophied myocardium (Swynghedauw, 1999). Ventricular hypertrophy is an adaptive response of the myocardium to various mechanical and hormonal stimuli. To compensate for the increased workload, initially the size of the individual cardiomyocytes is increased but eventually hypertrophic growth of the myocardium becomes maladaptive (Feldman et al., 1993). In combination with cellular hypertrophy enhanced collagen synthesis and interstitial fibrosis occur causing myocardial stiffness and contractile dysfunction (Weber and Brilla, 1991).

Contractile dysfunction associated with cardiac hypertrophy occurs in experimental animal models of hyperthyroidism as well as humans suffering from chronic hyperthyroidism (Klein and Ojamaa, 2001, Degens et al., 2003, De et al., 2004, Shohet et al., 2004). The heart is very sensitive to changes in serum thyroid hormone level, and increased serum thyroid hormone (triiodothyronine) level is associated with a number of changes, including increased resting heart rate, alteration in myocardial contractility, as well as the development of LV hypertrophy (Fadel et al., 2000). While increased collagen deposition (fibrosis) within the LV myocardium is a common feature associated with LV hypertrophy (Brilla et al., 1996, Diez et al., 2002), a similar effect has not been reported for LV hypertrophy induced under hyperthyroid conditions (Yao and Eghbali, 1992). Mechanisms that regulate this differential form of ECM deposition/degradation in the LV of hyperthyroid animals remain unclear. Therefore, understanding the molecular mechanism of ECM regulation in hyperthyroid heart is of great importance.

The decrease in stability of collagen mRNA (Yao and Eghbali, 1992) and negative regulation of collagen I gene expression in response to thyroid hormone in primary cultured cardiomyocytes (Chen et al., 2000) have been suggested as possible causes for reduced interstitial fibrosis in the LV myocardium of hyperthyroid animals. Although the state of ECM depends upon the continuous synthesis, maturation, and degradation of collagens, cellular regulation of these important steps has not been studied in hyperthyroid-induced hypertrophied heart in vivo. In the present study we have examined the expression of collagens-I and -III genes as well as their protein levels with concomitant changes in MMP-1 and TIMPs in hyperthyroid-induced hypertrophied heart in vivo. Since excess steroid hormone also induces cardiac hypertrophy by inducing hypertension via the angiotensin pathway (Grunfeld and Eloy, 1987, Sato et al., 1994, Saruta, 1996), DEX-treated rat heart tissues were also used to compare the effect of triiodothyronine on collagen metabolism.