Liquid medium cultures of three human cell lines (B-lymphoma, myeloma, and squamous lung carcinoma) with population-doubling times (PDT) and cloning efficiencies (CE) in the range of 32-43 h and 0.01-5.6%, respectively, were exposed to 5-azacytidine (5-azaC) for 3 d. The doses used (1-3 microM) were found to be nontoxic as measured by cell growth in liquid and semisolid agar medium and to be nonmutagenic as measured by the rate of generation of ouabain- and 6-thioguanine-resistant cell variants. After 5-azaC treatment, cell samples were subsequently harvested every day and assayed for their CE in semisolid agar medium. For each cell line, 30 to 42 individual clones were harvested at the day of maximal CE and expanded in liquid culture medium. PDT and CE were determined for each subclone about every 6 wk for 12 mo. The majority of the subclones had unaltered PDT and CE compared to the original lines. However, several clones had profoundly changed proliferative activity with PDT on approximately 12-14 h and/or CE 5 to greater than 50%. Some of the clones with altered growth properties reverted to PDT and/or CE values of untreated clones. However, a few clones of each line had stable alterations with PDT on 12-14 h and CE 5 to greater than 50%; these clones were all significantly hypomethylated. It is concluded that the human gene repertoire does contain genes that appropriately activated can result in growth properties with very short PDT and high CE (and comparable to animal cell lines), and that this activation may be obtained by 5-azaC treatment. It is conceivable that the procedure here described to alter growth properties of human cell lines may be applied to experimental situations, where alterations of cell growth properties are desired.

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