Role of GPNMB in hepatic steatogenesis and cancer

NAFLD is characterized by lipid deposition in liver cells and is progressed to NASH and HCC. Hepatocyte (HC)-derived Glycoprotein non-metastatic melanoma B (Gpnmb) was recently reported as a regulator of fat metabolism in adipose tissue, however, its role in the pathogenesis of NASH-related HCC are not clear. We aim to functionally investigate Gpnmb during liver steatogenesis and HCC. A NASH-based HCC mouse model, STAM was selected. In this model, NASH and HCC stages were analyzed. We also investigated publically available patient cohorts i.e. GSE48452 (NASH) and GSE14520 (HCC) datasets. An in vitro steatosis model was induced by oleic acid (OA) in primary mouse hepatocytes (PMHC) and AML-12. GPNMB expression was manipulated by overexpression (OE) and knockdown (KD) approaches. Steatosis induction and lipid metabolic targets was assessed upon modulation of GPNMB by PCR. Cell growth and death of Huh7 cells was analysed by MTT and caspase-3 assay, PCR, WB and time-lapse imaging. Comparative analysis of patient cohorts and STAM mouse model identifies GPNMB as a consistently upregulated gene. IHC staining shows that GPNMB protein localizes in HC in healthy and NASH, and in HC-derived tumour cells in HCC tissues. In vitro, OA induces GPNMB in PMHC and AML-12 cells. Lipid accumulation increased upon KD of Gpnmb, and decreased by OE as measured by triglyceride (TG) level in OA-treated pMHC and AML-12 cells. We analyzed GPNMB dependent gene expression alterations of critical players in hepatic lipogenesis i.e. SREBP-1c, PPAR-α, PPARγ, Fasn and Scd1, as well as targets involved in FA oxidation as Cpt1 and Acox1. In line with TG accumulation, Gpnmb KD increases SREBP-1c, PPAR-α, PPARγ, Fasn and Scd1 mRNA expression in OA-treated PMHC and AML12. However, Cpt1 and Acox1 are also upregulated. In case of GPNMB OE, we obtained complementary results. Preliminary data using Huh7 cells indicate that GPNMB OE suppresses cell proliferation and induces apoptosis. Mechanistically, GPNMB OE facilitates cell death via inhibition of AKT phosphorylation. GPNMB is a consistently upregulated target in NASH and HCC. In contrast to previous reports, GPNMB is expressed in HC and HC-derived cancer cells, instead of macrophages. In fatty liver, GPNMB is upregulated to tone down lipogenesis, therefore, it seems that GPNMB has a protective role against liver fat toxicity. In liver cancer cells, GPNMB acts as a tumor suppressor by providing cytostatic effects.