Recovery of HeLa cell population growth after treatment with 5,6‐dichloro‐1‐β‐D‐ribofuranosylbenzimidazole (DRB)

Dose‐response curves for the inhibition of heterogeneous nuclear RNA (hnRNA) synthesis in HeLa cells by 5,6‐dichloro‐1‐β‐D‐ribofuranosylbenzimidazole (DRB; 5–100 μM; 30 min) are biphasic and indicate the existence of two subpopulations of hnRNA molecules, one highly sensitive and the other completely resistant, as previously reported for molecules >1,000 nucleoides long (Tamm et al., 1976; Sehgal et al., 1976a). In the short‐term experiments, the drug‐sensitive synthesis of hnRNA was inhibited 50% at a DRB concentration of ∼7 μM, and 70% at 20 μM, whereas drug‐resistant synthesis, which comprises ∼20% of total, continued at DRB concentrations as high as 100 μM. After 24 hr of DRB treatment in medium containing 5% fetal calf serum, the increase in cell number in the exponentially growing population was inhibited by only 42% at 20 μM DRB, and the formation of colonies of ≥ ten cells was not decreased. DRB at 40 μM concentration decreased population growth by 76% and colony formation by 63%. Treatment with 60 μM DRB was sufficient to prevent a net increase in cell number and to reduce colony formation by 78%. After termination of treatment, the time required for the surviving population to begin rapid proliferation was directly related to the concentration of DRB used to treat cells and to the duration of treatment. After 24‐hr treatment with 40 μM DRB, cultures recovered within 1 day, whereas after 60 μM DRB, 3–4 days were required. After 40‐hr treatment with 60 μM DRB, 5–6 days were required for recovery, and after 80 μM DRB, 9–11 days. During the “dormant” period the cell number ranged from 15 to 60% of the initial number and was fairly stable for given conditions. After the “dormant” period, recovery was rapid. The population growth rate in cultures undergoing treatment with DRB is directly related to serum concentration; however, the recovery rate during the post‐treatment period is unaffeced by serum concentration.