PSMP/MSMP promotes hepatic fibrosis through CCR2 and represents a novel therapeutic target

C-C motif chemokine receptor 2 (CCR2) has been recognized as a promising target for the treatment of liver fibrosis. PC3-secreted microprotein (PSMP)/microseminoprotein (MSMP) is a novel chemotactic cytokine and its receptor is CCR2. In the present study we investigated the expression and role of PSMP in liver fibrosis/cirrhosis.

PSMP expression was studied in patients with fibrosis/cirrhosis and in 3 murine models of liver fibrosis, including mice treated with carbon tetrachloride (CCl₄), bile-duct ligation, or a 5-diethoxycarbonyl-1,4-dihydrocollidine diet. The role of PSMP was evaluated in Psmp^-/- mice and after treatment with a PSMP antibody in wild-type mice. The direct effects of PSMP on macrophages and hepatic stellate cells were studied in vitro.

In this study, we found that PSMP was highly expressed in fibrotic/cirrhotic tissues from patients with different etiologies of liver disease and in the 3 experimental mouse models of fibrosis. Damage-associated molecular pattern molecules HMGB-1 and IL-33 induced hepatocytes to produce PSMP. PSMP deficiency resulted in a marked amelioration of hepatic injury and fibrosis. In CCl₄-induced hepatic injury, the infiltration of macrophages and CCR2⁺ monocytes into the liver was significantly decreased in Psmp^-/- mice. Consistent with the decreased levels of intrahepatic macrophages, proinflammatory cytokines were significantly reduced. Moreover, adeno-associated virus-8 vectors successfully overexpressing human PSMP in Psmp^-/- mouse livers could reverse the attenuation of liver injury and fibrosis induced by CCl₄ in a CCR2-dependent manner. Treatment with a specific PSMP-neutralizing antibody, 3D5, prevented liver injury and fibrosis induced by CCl₄ in mice. At the cellular level, PSMP directly promoted M1 polarization of macrophages and activation of LX-2 cells.

PSMP enhances liver fibrosis through its receptor, CCR2. PSMP is a potentially attractive therapeutic target for the treatment of patients with liver fibrosis.

Our present study identifies the essential role of the protein PSMP for the development and progression of liver fibrosis in humans and mice. PSMP promotes liver fibrosis through inflammatory macrophage infiltration, polarization and production of proinflammatory cytokines, as well as direct activation of hepatic stellate cells via its receptor CCR2. A PSMP antibody can significantly reduce liver fibrosis development in vivo. These findings indicate that PSMP is a potential therapeutic target and its antibody is a potential therapeutic agent for the treatment of liver fibrosis.