Growth inhibition of mitogen-stimulated fibroblasts induced by double-stranded RNA depends on cell density

Polyinosinic-polycytidylic acid poly(I:C)], a synthetic double-stranded RNA, is an inhibitor of mitogen-induced proliferation of normal fibroblasts. We show that this inhibition depends strongly on cell density. While cultures with densities at or above confluence are completely inhibited by poly(I:C) in their proliferative response to epidermal growth factor (EGF), the proliferation of sparse (subconfluent) cultures is only delayed. Conditioned medium from dense fibroblasts exposed to poly(I:C) inhibits EGF stimulation of sparse cells, indicating that the inhibition is, at least in part, mediated by a factor released from the cells. Preincubation of quiescent cultures with poly(I:C) renders the cells refractory to the inhibitory effects of poly(I:C). This desensitization correlates with a decreased production of the inhibitor. Since the inhibition of mitogenic stimulation by poly(I:C) is completely overcome by antisera recognizing interferon-beta (IFN-beta) and interleukin-6 (IL-6), we tested the effect of IL-6 and IFN-beta on EGF mitogenicity. None of the available IL-6 preparations had any effect on cell cycle entry. IFN-beta caused a dose-dependent delay of cell division but did not affect the density-dependent proportion of cells entering the cell cycle in response to EGF. Thus, IFN-beta cannot be the sole mediator of the poly(I:C)-induced inhibition. In the presence of dexamethasone, poly(I:C) did not inhibit EGF mitogenis. Indeed, the combined presence of poly(I:C) and dexamethasone did more than just restore the density-dependent control levels of EGF stimulation; most cells entered the cell cycle even at extremely high cell densities. Thus, poly(I:C) in combination with dexamethasone could deactivate the cell density-dependent negative control of proliferation.