PRMT5–mediated symmetric dimethylation of SHBs at Arg169 stabilizes SHBs and promotes angiogenesis and tumor growth

The small hepatitis B surface antigen (SHBs), an oncogenic protein encoded by hepatitis B virus (HBV), contributes to hepatocellular carcinoma. However, the post-translational regulatory mechanisms of SHBs remain incompletely defined. Here, we show that SHBs is selectively modified by symmetric dimethylarginine (SDMA) in hepatoma cells, whereas monomethylation and asymmetric dimethylation are not detected. Site mapping using arginine-to-lysine substitutions identifies Arg169 as the predominant symmetric dimethylation site. Mechanistically, SHBs physically associates with protein arginine methyltransferase 5 (PRMT5), and PRMT5 increases SHBs SDMA levels in cells; conversely, PRMT5 silencing or pharmacologic inhibition reduces SHBs SDMA. An in vitro methylation assay further demonstrates that PRMT5 directly catalyzes symmetric dimethylation of SHBs, which is abolished by the R169K mutation. Functionally, PRMT5 increases SHBs protein abundance and prolongs SHBs half-life, whereas the R169K mutant exhibits reduced stability and is largely insensitive to PRMT5. To evaluate angiogenic activity, conditioned media from SHBs-expressing cells promotes tube formation and migration of endothelial cells, effects that are markedly attenuated by the R169K mutation. In nude mouse xenografts, SHBs expression accelerates tumor growth and increases CD31-positive microvessel density compared with the R169K mutant. Collectively, these results identify PRMT5-mediated symmetric dimethylation of SHBs at Arg169 as an important determinant of SHBs stability and SHBs-driven angiogenesis and tumor growth.