A distinct characteristic of the quiescent state of human dermal fibroblasts in contracted collagen gel as revealed by no response to epidermal growth factor alone, but a positive growth response to a combination of the growth factor and saikosaponin b1.

It is known that fibroblasts cultured in a reconstituted collagen gel contract the gel, resulting in a high density of collagen fibrils comparable to that in dermis. Our previous study indicated that human fibroblasts in the contracted collagen gel did not proliferate in the presence of 10% serum even though there was no apparent cell-cell contact. We interpreted this cell growth inhibition as being caused by a high level of cell-collagen fibril interactions or cell-matrix contact inhibition. In the present study, the effect of epidermal growth factor (EGF) on fibroblast proliferation in the contracted collagen gel was compared with that on cells in other quiescent states. Non-dividing cells at confluency on a plastic dish or on collagen gel responded to the added EGF and multiplied, while the cells in the contracted collagen gel did not show any growth response to EGF at concentrations up to 100 ng/ml. Binding assay of [125I]-EGF to the cells showed that the number of binding sites and the binding constant obtained from Scatchard analysis were essentially unchanged in the contracted collagen gel, indicating that EGF receptors were not masked by collagen fibrils but that the signal transduction after binding of EGF was blocked. The block in the signal transduction was suppressed by the addition of saikosaponin b1. These results suggested that the quiescent fibroblasts in the contracted collagen gel were in a distinct state from previously known quiescent states of cultured cells, namely quiescent states due to cell-cell contact inhibition at confluency or to deficiency of growth factors. The mechanism of the effect of saikosaponin b1, which has a potent saponin activity, is discussed.