Density-dependent inhibition of fibroblast growth is overcome by pure mitogenic factors

Abstract

CELL density is of interest because it may have a critical role in the regulation of growth, development, and in malignant transformation. In cell culture, crowding slows and eventually stops cell proliferation of a variety of cells maintained on a solid surface^1–5. This property is usually lost after malignant transformation^1–5. Two major hypotheses have evolved to explain this phenomenon. The contact inhibition hypothesis holds that specific surface receptors are activated by cell–cell contact and generate a negative signal which halts further growth^1–3. The humoral hypothesis proposes that high cell density limits the availability of medium components, particularly of growth factors present in serum^4,5. Although much experimental evidence has been adduced in support of the humoral hypothesis, most experiments in which the effect of cell density on growth has been thoroughly investigated were performed in the presence of whole serum as the source of growth factors. The chemical complexity of serum leaves open the possibility that some of its effects may be exerted by molecules that interfere with inhibitory cell–cell contacts. Thus, definitive evidence for a humoral hypothesis in growth regulation in culture is still lacking. To circumvent this objection we have examined the effect of cell density in a model system in which chemically defined molecules rather than whole serum stimulate DNA synthesis and cell division in sparse or crowded cells arrested in G₀/G₁. We report here that density dependent inhibition of fibroblast growth is overcome by pure mitogenic factors.