Inhibition of cell proliferation by interferons

Abstract

The inhibition of proliferation of human lymphoblastoid Daudi cells in culture by human interferons is characterized by a change in the kinetics of labeling of different size classes of newly synthesized DNA. Initially, labeled precursors are incorporated exclusively into small DNA (Okazaki fragments) in both control and interferon-treated cells, as revealed by alkaline sucrose gradient centrifugation. In the interferon-treated cells, there is enhanced labeling of this small DNA after short periods of incorporation and slower conversion to larger DNA size classes, in comparison with the DNA of control cells. This effect is apparent after 12 h of interferon treatment, coincident with the onset of the inhibition of cell proliferation. It becomes progressively more marked up to 4 days, by which time cell growth has ceased completely. Experiments using bromodeoxyuridine as a density label and analysis of radioactive DNA on CsCl/Cs2SO4 gradients also reveal that some newly replicated DNA may be unstable and may turn over within a few hours of its synthesis. The label derived from DNA breakdown is efficiently reincorporated into newly synthesized molecules. It is suggested that interferon treatment inhibits DNA replication by activating DNA turnover rather than by directly inhibiting synthesis. This effect, together with the progressive retardation of conversion of Okazaki fragments to larger DNA, may lead to the eventual failure of cell proliferation.