Jill E. Bishop

MECHANISMS OF TISSUE REPAIR: FROM WOUND HEALING TO FIBROSIS

To set the scene for this Directed Issue on Mechanisms of Tissue Repair of The International Journal of Biochemistry and Cell Biology, this introductory overview briefly describes the process of wound healing and highlights some of the key recent advances in this field of research. It emphasizes the importance of cell-cell and cell-matrix interactions, particularly relating to the role of cell surface adhesion molecules, and describes developments that have led to a better understanding of the dynamic nature of matrix turnover with reference to negative and positive mediators that regulate procollagen gene expression and protein production. An important component of this Directed Issue is concerned with the development of tissue fibrosis, which accompanies a number of disease states and demonstrates remarkable parallels with the normal wound healing process; excessive amounts of matrix are laid down but the resolution of scarring, which would be anticipated in wound healing, is impaired. The possible mechanisms involved in fibrosis are discussed here. Since cytokines play an important role in regulating cell function such as proliferation, migration and matrix synthesis, it is the balance of these mediators which is likely to play a key role in regulating the initiation, progression and resolution of wounds. Finally, this review highlights areas of tissue repair research in which recent developments have important clinical implications that may lead to novel therapeutic strategies.

Regulation of cardiovascular collagen synthesis by mechanical load

Time for primary review 20 days. The cardiovascular system is constantly exposed to mechanical perturbation from shear and tensile stresses. During development cardiovascular cells respond to changes in mechanical load; growing, dividing and laying down extracellular matrix. Changes in the normal levels of these forces then have further profound effects on these cells resulting in abnormal changes in cardiovascular structure and consequently function. These remodelling processes suggest that the mechanical environment is a key modulator of cell function. The importance of mechanical forces in the regulation of tissue growth, development and disease has been appreciated for many years. Early studies in the 1960–70s demonstrated, for example, the importance of mechanical load in skeletal muscle growth and development. It was determined that even in the presence of adequate nutrition, and with hormonal and neuronal control, skeletal muscle would not grow without mechanical stimulation [1–3]. The reverse is also true – disuse of a skeletal muscle leads to atrophy [4]. Similarly bone growth and remodelling is dependent on continuous stimuli of pressure and tension [5]. Growth of the lung is also partly regulated by mechanical forces [6]. The response of the cardiovascular system to mechanical stimuli is therefore not unique, however the ability of the cardiovascular system to respond to changes in physical forces by changing the physical properties of the cardiovascular tissues in an attempt to normalise these forces (see Fig. 1) – makes this reciprocal interaction between structure and function and the mechanical environment an extremely fascinating area of study. Fig. 1 Reciprocal relation between mechanical forces and cardiovascular remodelling The cardiovascular system responds to changes (Δ) in haemodynamics by cell hypertrophy, proliferation and extracellular matrix deposition. This tissue remodelling results in changes in the physical properties of the tissues which may be sufficient to counteract the altered … * Corresponding author. Tel.: +44-171-209-6972; fax: +44-171-209-6973

Changes in collagen metabolism in response to endothelin-1: Evidence for fibroblast heterogeneity

Endothelin-1 (Et-1) is a 21-amino acid peptide primarily synthesized by endothelial cells. It was originally classified as a potent vasoconstrictor but recent evidence suggests that it also possesses a wide variety of non-vascular actions. It stimulates fibroblast and smooth muscle cell proliferation and it has been shown to stimulate fibroblast collagen metabolism. However, studies on its ability to regulate collagen production remain incomplete, and its effect on post-translational processing of procollagen has not been studied. This report details the effect of Et-1 on the rates of procollagen synthesis and degradation in two fibroblast cell lines; human foetal lung (HFL-1) and whole foetal rat fibroblasts (Rat 2). Fibroblast cultures were incubated for 24 hr in the presence or absence of Et-1 before procollagen metabolism was determined by measuring hydroxyproline. Non-collagen metabolism was also determined in these cultures from the uptake of tritiated phenylalanine. Et-1 stimulated procollagen synthesis in HFL-1 fibroblasts and reduced synthesis in Rat 2 cells. The response was dose dependent with the greatest effect at 1.10(-6) M Et-1 for both cell types (155 +/- 6% of control (mean +/- SD, n = 6, P < 0.01) and 61 +/- 4% of control (n = 4, P < 0.01) for HFL-1 and Rat 2 fibroblasts, respectively). Non-collagen protein synthesis was increased to 148 +/- 5% of control (P < 0.05) at 1.10(-6) M Et-1. Non-collagen protein synthesis remained unaffected in the HFL-1 fibroblast cultures. Procollagen degradation, expressed as a proportion of total procollagen synthesis, was decreased in HFL-1 fibroblasts (control, 29 +/- 2%; Et-1, 1.10(-6) M; 21 +/- 2%; P < 0.01), and increased in Rat 2 fibroblasts (control 42 +/- 1%; Et-1, 1.10(-6) M; 49 +/- 1%; P < 0.01). Blocking of the EtA receptor for Et-1, using the receptor antagonist-BQ123, abolished the effect of Et-1 on procollagen metabolism in both cell types. These results suggest that different populations of fibroblasts exhibit heterogeneous responses to Et-1. It is concluded that Et-1 may play an important role in the extent and distribution of fibrosis seen in diseases associated with the overproduction of Et-1.

Raised blood pressure, not renin–angiotensin systems, causes cardiac fibrosis in TGR m(Ren2)27 rats

OBJECTIVES Elevated systemic arterial blood pressure is associated with left ventricular hypertrophy and fibrosis. It has been suggested that both circulating and local myocardial renin-angiotensin systems play a role in mediating these responses. Here we describe the natural history of ventricular hypertrophy and fibrosis in the transgenic (mRen2)27 rat--a monogenetic model--which has a high tissue expression of the murine renin transgene, and suffers severe hypertension. We further explored the relative contribution of both hypertensive burden and circulating and tissue renin-angiotensin systems to the fibrotic process. METHODS The transgenic rats were treated from 28 days old with (1) a hypotensive dose of the ACE inhibitor ramipril which inhibited both tissue and circulating ACE activity, (2) the calcium antagonist amlodipine, or (3) a non-hypotensive dose of ramipril which inhibited about 60% of tissue ACE activity with little effect on circulating ACE. Normotensive Sprague-Dawley rats were used as controls. RESULTS The transgenics developed left ventricular hypertrophy along with perivascular and interstitial fibrosis which became progressively worse up to 24 weeks of age. Both the high dose of ramipril and amlodipine prevented the hypertrophy and fibrosis, whereas tissue ACE inhibition without lowering blood pressure had no effect, and actually led to a worsening of the fibrosis by 24 weeks. CONCLUSIONS These results suggest that the development of left ventricular hypertrophy and fibrosis in the transgenic (mRen2)27 rat are regulated by blood pressure and not activity of the renin-angiotensin systems and that progression of fibrosis at 24 weeks involves a mechanism unrelated to local renin-angiotensin activity.