The Novel Role Of Osteopontin In Liver Injury, Carcinogenesis And Progression

Osteopontin (OPN) is a phosphorglycoprotein. There are two types of OPN,secreted OPN (sOPN) and intracellular OPN (iOPN). sOPN is an extracellular proteinand participates in several physiological and pathological process, for example, immuneregulation, hepatotoxicity, tumor progression and metastasis. iOPN is a shortened proteinthat lacks the16amino acids of N-terminal of sOPN and mainly localizes to cytoplasm,it takes part in cell duplication, migration and functions as an innate receptor molecular.OPN has also been researched in tumor growth and progression. However, its role duringhepatocarcinogenesis is poorly known.In our previous work, wild type (WT) and OPN knock out (Opn-/-) mice wereadminstrated diethylnitrosamine (DEN), a carcinogen. Opn-/-mice had heavier loads ontumor formation and inflammatory response than WT controls.Here, our further study found under DEN administration, much cellular debris ofnecrotic hepatocytes was released, which stimulated adjacent macrophages, activatedTLR (Toll like receptor) signaling pathway and produced proinflammatory cytokines. Inthis model，iOPN, not sOPN, was more critical. When macrophages from Opn-/-micewere transfected with iOPN-pcDNA3.0or sOPN-pcDNA3.0, the former efficientlyinhibited produciton of Interleukin-6(IL-6). Meanwhile, through coimmunoprecipitationand confocal microscope, iOPN was found to colocalize with myeloid differentiationprimary response gene88(MyD88) under stimulation of cellular debris released bynecrotic hepatocytes, and reduced dissociation of IL-1receptor-associated kinase1(IRAK1) from MyD88complex, then suppressed proinflammatory cytokines productionand lessened tumorigenesis.During Acetaminophen (APAP)-induced liver injury, alanine transarninase (ALT)level was decreased significantly, while mRNA level of cytochromes was elevated inOpn-/-mice than in WT controls. Therefore, in early stage of injury, OPN in hepatocytescould decrease liver injury and increase expression of cytochromes to facilitate APAPmetabolism. By immunofluorescence assay, WT livers exhibited more infiltration ofmacrophages than Opn-/-livers, which could be blocked by anti-OPN antibodies.Consistent with macrophage infiltration, WT livers exhibited heavier production of proinflammatory cytokines. Thus, pruduction of proinflammatory cytokines inmacrophages mainly contributed to sOPN in this model. These results suggested that inAPAP-induced liver injury, OPN had a dual role in different stages.Autophagy is a process which involves in bulk degradation of proteins andorganelles in cytoplasm. Under stress, such as low oxygen or nutrient deprivation,autophagy can supply energy for survival of tumor cells. Here, we used lentivirusinfection system and found sOPN could promote autophagy in hepatocellular carcinoma(HCC): more conversion of microtubule-associated protein1light chain3(LC3) I to II,more GFP-LC3dots and more autophagosome formation. Through sphere formationassay, we found tumor cells formed a larger amount of spheres while they had a higherexpression of OPN under nutrient deprivation, and these phenomenons could be blockedby autophagy inhibitor. Then, we used chemoresistant cells to detect side population (SP)cells, which partially represented stem-like characteristic. Tumor cells, which had ahigher expression of OPN, had an obviously larger SP fraction, and autophagy inhibitorcould decrease these fractions as well.In conclusion, OPN had disparate roles in different liver disease. In DEN-inducedHCC, iOPN in macrophages negatively regulated TLR signaling pathway, lessenedproduction of proinflammatory cytokines and decreased tumorigenesis. In APAP-inducedliver injury, sOPN in macrophages facilitated production of proinflammatory cytokines.In HCC cell lines, sOPN promoted autophagy to increase proliferation of tumor cells andmaintain stemness for chemoresistance. Therefore, OPN, from different cells and indifferent forms, played diverse roles in tumorigenesis and progression. Duringanti-cancer therapy, more individual treatment should be taken into consideration.