Transgenic expression of survivin in keratinocytes counteracts UVB-induced apoptosis and cooperates with loss of p53.

The inhibitor of apoptosis protein survivin has been implicated in both cell cycle control and apoptosis resistance. To discriminate between these different roles, we used transgenic expression of survivin in the skin as a model for cell proliferation, differentiation, and apoptosis. Transgenic mice expressing survivin under the control of a keratin-14 promoter developed normally, without histologic abnormalities of the skin or hair, epidermal hyperplasia, or developmental abnormalities of basal or suprabasal epidermis. Keratinocyte proliferation assessed under basal conditions, or after ultraviolet-B (UVB) irradiation, or phorbol ester stimulation was unchanged in survivin transgenic mice. In contrast, survivin expression inhibited UVB-induced apoptosis in vitro and in vivo (i.e., sunburn cell formation), whereas it did not affect Fas-induced cell death. When crossed with p53 knockout mice, transgenic expression of survivin in a p53(+/-) background substituted for the loss of a second p53 allele and further inhibited UVB-induced apoptosis. These data provide the first in vivo evidence that survivin inhibits apoptosis and suggest that this pathway may oppose the elimination of cancerous cells by p53.

Factor Xa is a fibroblast mitogen via binding to effector-cell protease receptor-1 and autocrine release of PDGF.

The coagulation cascade protease thrombin is a fibroblast mitogen, but the proliferative potential of other coagulation proteases is not known. In this study we show that factor Xa stimulated human fetal lung fibroblast DNA synthesis in a concentration-dependent manner from 1 nM onward with a fourfold increase at 200 nM. The mitogenic effect of factor Xa was confirmed using a colorimetric proliferation assay and direct cell counting. Factor Xa and thrombin had equivalent potencies, and their stimulatory effects followed a similar time course. Comparable results were also obtained with primary human adult fibroblasts derived from lung, kidney, heart, skin, and liver. Factor VIIa also stimulated fibroblast proliferation, but only at concentrations >10 nM, whereas factor IXa had no effect. To begin to address the mechanism by which factor Xa is acting, we show that human fibroblasts express effector-cell protease receptor-1 and that blocking antibodies to this receptor and the catalytic site of factor Xa inhibited its mitogenic effect. Furthermore, factor Xa upregulated platelet-derived growth factor-A (PDGF-A) mRNA expression, whereas PDGF-B could not be detected, and a blocking antibody to PDGF inhibited the mitogenic effect of factor Xa. We conclude that factor Xa acts as a fibroblast mitogen via binding to effector-cell protease receptor-1 and the autocrine release of PDGF.

Thrombin is a potent inducer of connective tissue growth factor production via proteolytic activation of protease-activated receptor-1.

The coagulation protease thrombin plays a critical role in hemostasis and exerts pro-inflammatory and pro-fibrotic effects via proteolytic activation of the major thrombin receptor, protease-activated receptor-1 (PAR-1). Connective tissue growth factor (CTGF) is a novel fibroblast mitogen and also promotes extracellular matrix protein production. It is selectively induced by transforming growth factor beta (TGF-beta) and is thought to be the autocrine agent responsible for mediating its pro-fibrotic effects. CTGF is up-regulated during tissue repair and in fibrotic conditions associated with activation of the coagulation cascade. We therefore hypothesized that coagulation proteases promote the production of CTGF by cells at sites of tissue injury. To begin to address this hypothesis, we assessed the effect of coagulation proteases on fibroblast CTGF expression in vitro, and we show that thrombin, at physiological concentrations, up-regulated CTGF mRNA levels 5-fold relative to base line (p < 0.01) in fetal fibroblasts and 7-fold in primary adult fibroblasts (p < 0.01). These effects were cycloheximide-insensitive and were not blocked with a pan-specific TGF-beta-neutralizing antibody. They were further paralleled by a concomitant increase in CTGF protein production and could be mimicked with selective PAR-1 agonists. In addition, fibroblasts derived from PAR-1 knockout mice were unresponsive to thrombin but responded normally to TGF-beta(1). Finally, factor Xa, which is responsible for activating prothrombin during blood coagulation, exerted similar stimulatory effects. We propose that coagulation proteases and PAR-1 may play a role in promoting connective tissue formation during normal tissue repair and the development of fibrosis by up-regulating fibroblast CTGF expression.

Thrombin stimulates fibroblast procollagen production via proteolytic activation of protease-activated receptor 1.

Thrombin is a multifunctional serine protease that has a crucial role in blood coagulation. It is also a potent mesenchymal cell mitogen and chemoattractant and might therefore have an important role in the recruitment and local proliferation of mesenchymal cells at sites of tissue injury. We hypothesized that thrombin might also affect the deposition of connective tissue proteins at these sites by directly stimulating fibroblast procollagen production. To address this hypothesis, the effect of thrombin on procollagen production and gene expression by human foetal lung fibroblasts was assessed over 48 h. Thrombin stimulated procollagen production at concentrations of 1 nM and above, with maximal increases of between 60% and 117% at 10 nM thrombin. These effects of thrombin were, at least in part, due to increased steady-state levels of alpha1(I) procollagen mRNA. They could furthermore be reproduced with thrombin receptor-activating peptides for the protease-activated receptor 1 (PAR-1) and were completely abolished when thrombin was rendered proteolytically inactive with the specific inhibitors d-Phe-Pro-ArgCH2Cl and hirudin, indicating that thrombin is mediating these effects via the proteolytic activation of PAR-1. These results suggest that thrombin might influence the deposition of connective tissue proteins during normal wound healing and the development of tissue fibrosis by stimulating fibroblast procollagen production.