Retinoic acid signaling sensitizes hepatic stellate cells to NK cell killing via upregulation of NK cell activating ligand RAE1

Abstract

Hepatic stellate cells (HSCs) store 75% of the body's supply of vitamin A (retinol) and play a key role in liver fibrogenesis. During liver injury, HSCs become activated and susceptible to natural killer (NK) cell killing due to increased expression of the NK cell activating ligand retinoic acid early inducible gene 1 (RAE-1). To study the mechanism by which RAE-1 is upregulated in HSCs during activation, an in vitro model of cultured mouse HSCs was employed. RAE-1 was detected at low levels in quiescent HSCs but upregulated in 4- and 7-day cultured HSCs (early activated HSCs), whereas 21-day cultured HSCs (fully activated HSCs) lost RAE-1 expression. High levels of RAE-1 in 4- and 7-day cultured HSCs correlated with their susceptibility to NK cell killing, which was diminished by treatment with RAE-1 neutralizing antibody. Furthermore, retinoic acid (RA) and retinal dehydrogenase (Raldh) levels were upregulated in early activated HSCs compared with quiescent or fully activated HSCs. Blocking RA synthesis by the Raldh inhibitor or blocking RA signaling by the retinoic acid receptor antagonist abolished upregulation of RAE-1 whereas treatment with RA induced RAE-1 expression in HSCs. In conclusion, during activation, HSCs lose retinol, which is either secreted out or oxidized into RA; the latter stimulates RAE-1 expression and sensitizes early activated HSCs to NK cell killing. In contrast, fully activated HSCs become resistant to NK cell killing because of lack of RAE1 expression, leading to chronic liver fibrosis and disease.

Retinol Binding Protein-Albumin Domain III Fusion Protein Deactivates Hepatic Stellate Cells

“…To examine the possibility that albumin may modulate cytoplasmic levels of RA, HSCs-P2 were transiently transfected with expression plasmids for His-tagged albumin or R-III and the level of alltrans retinoic acid (ATRA), a biologically active form of vitamin A, was measured from whole-cell extracts. ATRA production was detected in control HSCs as previously reported (Radaeva et al, 2007), but disappeared in transfectant cells (Fig. 4A).…”

“…Reverse-phase HPLC (RP-HPLC) was carried out on an AKTA Explorer HPLC system (GE Healthcare Life Sciences, UK). The chromatographic conditions were as described previously (Radaeva et al, 2007): column, LC-ABZ…”

Deletion of the Collagen-specific Molecular Chaperone Hsp47 Causes Endoplasmic Reticulum Stress-mediated Apoptosis of Hepatic Stellate Cells

“…Isolation and Culture of HSCs-HSCs were isolated from Hsp47 flox/flox mice as described by Radaeva et al (38). The isolated HSCs were cultured on a plastic dish in low glucose DMEM supplemented with 10% FBS and antibiotic-antimycotic solution (Sigma-Aldrich) and maintained at 37°C in a humidified atmosphere with 5% CO 2 .…”

Mechanisms of Hepatic Fibrogenesis

“…Indeed, CD8 T cells appear to be more fibrogenic towards stellate cells than CD4 cells, 40 which could account for the increased rate of fibrosis in patients with HIV and HCV, in whom the CD4/CD8 ratio typically is reduced. 41 In contrast, natural killer cells play an important role in clearing activated stellate cells, 42,43 an activity enhanced by gamma interferon and retinoic acid 44 and abrogated by ethanol. 45 Intriguingly, stellate cells also are antigen-presenting cells, 46 and may contribute to the liver's immunotolerant properties through T-cell suppression.…”