Failure of platelet-derived growth factor plus insulin to stimulate sustained proliferation of Swiss 3T3 cells : requirement for hydrocortisone, prostaglandin E1, lipoproteins, fibronectin and an unidentified component derived from serum

Platelet-derived growth factor (PDGF) has been reported to be a potent mitogen for Swiss 3T3 cells made quiescent by growth to saturation density in high serum, and its activity is further potentiated by high levels of insulin, which alone have little effect. We show here that this is not the case for sparse 3T3 cells made quiescent by plating in low serum. Under these conditions, insulin alone is at least as effective as PDGF and frequently more so. Together, the response is no more than additive at best, and in many cases less than additive, the combined effect being no greater than for insulin alone. Instead, we find that optimal mitogenic stimulation requires the additional presence, besides PDGF and insulin, of hydrocortisone, prostaglandin E1 and an unidentified, non-dialysable component contained in serum treated with dithiothreitol (DTT) to inactivate endogenous growth factors. Interestingly, overnight pretreatment of the cells with hydrocortisone alone potentiates the subsequent response to PDGF + insulin, i.e. pretreatment induces a long-term memory that persists after the removal of the hydrocortisone from the medium. In short-term (24h) thymidine incorporation assays, the combination of PDGF, insulin, hydrocortisone, prostaglandin E1 and DTT-serum, is as effective as optimal levels of whole serum, but is unable to sustain longer-term proliferation (measured over 6 days). For this, high- and low-density lipoproteins, fibronectin and, to some extent epidermal growth factor (EGF), are also necessary.