Proliferation of Ito cells (fat-storing cells) in acute carbon tetrachloride liver injury. A light and electron microscopic autoradiographic study

Connective tissue growth factor (CTGF) promotes activated mesangial cell survival via up-regulation of mitogen-activated protein kinase phosphatase-1 (MKP-1)

Activated mesangial cells are thought to play a pivotal role in the development of kidney fibrosis under chronic pathological conditions, including DN (diabetic nephropathy). Their prolonged survival may enhance the development of the disease since they express increased amounts of growth factors and extracellular matrix proteins. CTGF (connective tissue growth factor) is one of the growth factors produced by activated mesangial cells and is reported to play a key role in the pathogenesis of DN. Previous studies have shown that addition of exogenous CTGF to HMCs (human mesangial cells) rapidly activates ERK1/2 (extracellular-signal-regulated kinase 1/2) MAPK (mitogen-activated protein kinase) and JNK (c-Jun N-terminal kinase) MAPK, but not the p38 MAPK, despite the activation of the upstream kinases, MKK3/6 (MAPK kinase 3/6). The aim of the present study was to investigate whether the lack of phosphorylated p38 MAPK by CTGF has an anti-apoptotic effect on activated HMCs. We show that in HMC CTGF induces the rapid transcriptional activation and synthesis of MKP-1 (MAPK phosphatase-1), a dual specificity phosphatase that dephosphorylates p38 MAPK. This in turn prevents the anti-apoptotic protein, Bcl-2, from being phosphorylated and losing its function, leading to the survival of the cells. Knockout of MKP-1 protein in mesangial cells treated with CTGF, using siRNA (small interfering RNA) or antisense oligonucleotides, allows p38 MAPK activation and induces mesangial cell death.

Synergism between hepatic injuries and a nonhepatotropic reovirus in mice. Enhanced hepatic infection and death

Reovirus type 1, after intravenous inoculation in the adult mouse, is secreted via bile into the intestine in an infectious form. Although reovirus type 1 is rapidly removed from systemic circulation by the liver and the lung, very few hepatocytes express reovirus antigen during infection. In intestinal cells, reovirus replicates selectively in the crypts. This site preference may be due to active cell proliferation in the crypts. We hypothesized that the state of the cell may affect virus replication and tested this hypothesis by using chemical and surgical means to increase hepatic mitotic activity. Adult mice were treated with carbon tetrachloride or surgical trauma, inoculated with reovirus type 1 intravenously, and subsequently killed. Virus antigen was identified using a highly specific immunohistochemical technique. Liver sections were stained using immunoperoxidase with specific rabbit antireovirus antibody. Hepatotoxin and surgical trauma increase reovirus antigen detection in both Kupffer cells and hepatocytes. Only the sequential administration of CCl4 and virus caused mortality at doses sublethal for each alone. These data demonstrate a synergism between hepatic injury and reovirus which results in a significant increase in the magnitude of viral infection and contributes to mortality. Such synergism may be important in idiopathic liver disease.

Activation of cultured rat hepatic lipocytes by Kupffer cell conditioned medium. Direct enhancement of matrix synthesis and stimulation of cell proliferation via induction of platelet-derived growth factor receptors

Hepatic lipocytes appear to be central to the pathogenesis of hepatic fibrosis, undergoing activation during inflammation to a matrix-producing, proliferative cell type. We have studied the activation process in culture by examining the response of lipocytes to conditioned medium from hepatic macrophages (Kupffer cells). Lipocytes exposed to Kupffer cell medium (KCM) exhibited cellular and nuclear enlargement associated with up to a threefold increase in collagen and total protein synthesis per cell. Cell proliferation was also stimulated as measured by [3H]thymidine incorporation and direct cell counting. The latter effect was serum dependent and inhibited by antibodies to platelet-derived growth factor (PDGF). Proliferation could be stimulated by recombinant PDGF, but only after preincubation of cells with KCM. These findings suggested that KCM was eliciting expression of the PDGF receptor in lipocytes, and this was confirmed by immunoblot analysis with antibodies to the PDGF receptor. DNA synthesis in lipocytes exposed to KCM occurred at 48 h, which reflected the time required for PDGF receptor expression (24 h) plus initiation of [3H]thymidine incorporation (24 h). These results indicate that KCM has multiple stimulatory effects on cultured lipocytes similar to activation of these cells observed in vivo.