Insulin-like growth factor-1 (IGF-1), insulin, and epidermal growth factor (EGF) are survival factors for density-inhibited, quiescent Balb/c-3T3 murine fibroblasts

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Abstract
The great majority of murine Balb/c‐3T3 fibroblasts in density‐inhibited, quiescent cultures disintegrate and die rapidly when cells are deprived of serum in the medium. Platelet‐derived growth factor (PDGF, 5 ng/ml) used alone and insulin‐like growth factor (IGF‐1, 40 ng/ml) + epidermal growth factor (EGF, 10 ng/ml) prevent most of this cell death and all three factors used together protect close to all cells in the confluent monolayer as determined by counting trypsinized cell suspensions in a Coulter counter. IGF‐1 used alone affords a high level of protection during the first 5 hours of incubation in serum‐free medium but the protective effect declines subsequently unless EGF is also present. EGF alone has little protective activity. The survival‐promoting activity of PDGF used alone or of PDGF + EGF + IGF‐1 is not significantly decreased by selective inhibition of messenger precursor RNA transcription with 5,6‐dichloro‐1‐β‐D‐ribofuranosylbenzimidazole (DRB,20 or 40 μM), which prevents G1 traverse of the cells mediated by the combination of the three growth factors. DRB also does not interfere with the early protective effect of IGF‐1 + EGF, but decreases the late protective effect of this growth factor combination. DRB by itself decreases cell viability in the absence of growth factors or serum. In these experiments viability was assayed by neutral red uptake by using an automated microplate reader. Inhibition of protein synthesis with cycloheximide (CHX, 1 or 5 μg/ml) over a 20‐hour period was associated with decreased survival of cells protected by IGF‐1 + EGF or PDGF + EGF + IGF, but also with decreased survival of cells incubated in the absence of growth factors or serum. The decrease in survival was somewhat more marked when IGF + EGF was present than when PDGF + EGF + IGF‐1 was present. Insulin (1,500 ng/ml) mimics the action of IGF‐1 (40 ng/ml). The cell survival‐enhancing activities of growth factors are concentration dependent. The evidence presented indicates that PDGF, EGF, and IGF‐1 (or insulin) act through distinctive mechanisms in affording protection of cells against death. The short‐term protective effects of the growth factors are independent of gene expression and may be mediated via metabolic events. Long‐term protection may be dependent on gene expression, especially in the case of IGF‐1 + EGF.