De novo expression of glutamine synthetase during transformation of hepatic stellate cells into myofibroblast-like cells.

The expression of glutamine synthetase (GS) was studied in cultured quiescent hepatic stellate cells (HSC) and during their transformation into myofibroblast-like cells. GS mRNA was detectable in quiescent HSC (1-day culture); however, the enzyme protein was not expressed, as assessed by Western blot analysis, immunocytochemistry and the absence of detectable enzyme activity. Similar findings were obtained after 2 days of culture; in addition, the mRNA levels had dropped by about 70%, but they increased again thereafter during the process of HSC transformation in culture, as indicated by the expression of alpha-smooth-muscle actin. In parallel with the accumulation of alpha-smooth-muscle actin, GS was expressed, as shown by Western blot analysis and immunocytochemistry, and enzyme activity increased from undetectable levels in quiescent cells to 0.13+/-0.01 micromol/h per mg of cell protein within 7-14 days. This value compares with GS activity in liver parenchymal cells of 0.57+/-0.03 micromol/h per mg of cell protein. The findings suggest that activation of HSC results in the de novo expression of GS protein and activity, and this may serve as another marker of HSC transformation.