Hepatic myofibroblast apoptosis is critical to resolution of liver fibrosis. We show that human hepatic myofibroblasts co-express p75(NTR) (p75 neurotrophin receptor) and sortilin, thus facilitating differential responses to mature and pro nerve growth factor (proNGF). Although mature NGF is proapoptotic, proNGF protects human hepatic myofibroblasts from apoptosis. Moreover, in recovery from experimental liver fibrosis, the decrease in proNGF parallels loss of hepatic myofibroblasts by apoptosis. Macrophage-derived matrix metalloproteinase 7 (MMP7) cleaves proNGF in a concentration-dependent manner, and its expression in the liver coincides with falling proNGF levels. To define the dominant effect of p75(NTR)-mediated events in experimental liver fibrosis, we have used a mouse lacking the p75(NTR) ligand-binding domain but expressing the intracellular domain. We show that absence of p75(NTR) ligand-mediated signals leads to significantly retarded architectural resolution and reduced hepatic myofibroblast loss by apoptosis. Lack of the ligand-competent p75(NTR) limits hepatocyte and oval cell proliferative capacity in vivo without preventing hepatic stellate cell transdifferentiation.
Conclusion: NGF species have a differential effect on hepatic myofibroblast survival. Our data suggest that cleavage of proNGF by MMP7 during the early phase of recovery from liver fibrosis alters the pro/mature NGF balance to facilitate hepatic myofibroblast loss. Whereas fibrosis develops in the absence of p75(NTR) signaling, the dominant effects of loss of p75(NTR) ligand-mediated events are the retardation of liver fibrosis resolution via regulation of hepatic myofibroblast proliferation and apoptosis, and the reduction of hepatocyte and oval cell proliferation.