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.
Böhmer
R M
RM
Melbourne Tumor Biology Branch, Ludwig Institute for Cancer Research, Victoria, Australia.
Forsberg
K
K
Westermark
B
B
D001685:Biological Factors / Q000378:metabolism; D002452:Cell Count; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D003907:Dexamethasone / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000494:pharmacology; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D007370:Interferon Type I / Q000494:pharmacology; D015850:Interleukin-6 / Q000494:pharmacology; D011070:Poly I-C / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D012867:Skin / Q000166:cytology
D001685:Biological Factors; D007370:Interferon Type I; D015850:Interleukin-6; D004815:Epidermal Growth Factor; D003907:Dexamethasone; D011070:Poly I-C