IDENTIFICATION OF HISTONE H2A.X AS A GROWTH-FACTOR SECRETED BY AN ANDROGEN-INDEPENDENT SUBLINE OF MOUSE MAMMARY-CARCINOMA CELLS

Shionogi carcinoma 115 (SC 115) cells and Chiba subline 2 (CS 2) cells are clones of an androgen-responsive mouse tumor cell line and its au tonomous subline, respectively, We have shown previously that CS 2 cel ls produce a heparin-binding growth factor that stimulates the growth of SC 115 cells as well as the growth of themselves. In this study, a growth factor was purified from serum-free conditioned media of CS 2 c ells cultured without testosterone. A heparin-binding fr action showed growth- promoting activity on SC 115 cells and BALB/3T3 cells. The am ino acid sequence analysis revealed that the components were identical to histones H2A.1 and H2A.X. Since histone H2A purified from bovine t hymus had almost no growth-promoting activity on SC115 cells, histone H2A.X was assumed to be a growth factor. cDNA of histone H2A.X was clo ned from a library of CS 2 cells, and its sequence was confirmed. The expressed product of histone H2A.X cDNA in Escherichia coli showed rem arkable stimulatory effects on growth of SC 115 cells cultured in the absence of testesterone, These results indicate that histone H2A.X is secreted from CS 2 cells cultured without testosterone and plays a rol e as a growth factor.