Apoptotic hepatocyte DNA inhibits hepatic stellate cell chemotaxis via toll-like receptor 9

Abstract

Apoptosis of hepatocytes results in the development of liver fibrosis, but the molecular signals mediating this are poorly understood. Degradation and modification of nuclear DNA is a central feature of apoptosis, and DNA from apoptotic mammalian cells is known to activate immune cells via Toll-like receptor 9 (TLR9). We tested if DNA from apoptotic hepatocytes can induce hepatic stellate cell (HSC) differentiation. Our data show that apoptotic hepatocyte DNA and cytidine-phosphate-guanosine oligonucleotides induced up-regulation of transforming growth factor ␤1 and collagen 1 messenger RNA both in the human HSC line LX-2 and in primary mouse HSCs. These effects were opposed by TLR9 antagonists. We have recently shown that adenosine inhibits HSC chemotaxis, and we now show that apoptotic hepatocyte DNA also inhibits platelet-derived growth factor (PDGF)-mediated HSC chemotaxis.

Mechanisms of Hepatic Fibrogenesis

“…Although earlier models suggested that the pathways of activation were identical regardless of the disease, it is now clear that there are disease-specific pathways of fibrosis (see DiseaseSpecific Pathways of Hepatic Fibrosis section), and, moreover, that not all cytokine pathways are necessarily activated in parallel. For example, although PDGF stimulation may drive cellular proliferation in parallel with fibrogenic stimulation in some settings, TLR9 activation blocks PDGF-mediated migration while provoking fibrogenesis, 26 thereby providing a stop signal that allows activated cells to accumulate at sites of injury where they can deposit more scar.…”

“…25 This response to apoptotic hepatocytes in part reflects the interaction of hepatocyte DNA with Toll-like receptor 9 (TLR9) expressed on stellate cells. 26 A profibrogenic response also can be elicited by hepatocyte apoptosis after disruption of the anti-apoptotic mediator Bcl-xL, 27 and by Fas. 28 Thus, efforts to block hepatocyte apoptosis therapeutically are being developed as a potential antifibrotic strategy.…”

Liver fibrosis: mechanisms of immune-mediated liver injury

“…Following apoptotic hepatocyte DNA ligation, activated TLR9 has been shown to modulate the biology of HSC, resulting in inhibited cell migration and upregulated collagen production. 41 Furthermore, the development of biliary fibrosis is delayed in mice lacking TLR9. 42 …”

Apoptotic cell death takes its toll

“…20 In the current issue of HEPATOLOGY Watanabe et al made an important observation that CpG DNA from apoptotic hepatocytes inhibits stellate cell chemotaxis via TLR9 signaling. 17 Exposure of HSC to apoptotic DNA from hepatocytes or CpG DNA was also found to up-regulate TGF-␤1 and collagen-1 expression involving a TLR9 mediated pathway, as TLR9 antagonists blocked the up-regulation. This is the first time that TLR9 expression is described both in a HSC line and in primary stellate cells.…”

“…It was recently shown that hepatic stellate cells (HSC), which are at the center of the fibrogenic process, engulf apoptotic bodies, which triggers a profibrogenic response with upregulation of transforming growth factor (TGF)-␤1 and procollagen ␣ 1 (I) expression. 15,16 It is also possible that HSC engulf necrotic debris, and uptake DNA from apoptotic cells as this is suggested by Watanabe et al 17 Since HSC acquire a migratory phenotype upon transdifferentiation, it is of great interest, how they reach and recognize the area of cell injury. Platelet-derived growth factor (PDGF) is a major motogenic cytokine for stellate cells, and several studies described its crucial role in protrusion, lamellipodia formation and actin-myosin cytoskeleton rearrangement.…”