| Summary: | I found that SV40 transformation induced the loss of several specific growth factor requirements. In particular, SV40 transformed 3T3 fibroblasts had a significantly reduced growth requirement for insulin. A reduced insulin requirement was also observed in several other transformed cell lines. Dose response studies with insulin and insulin-like growth factors indicated that the mitogenic response to insulin is in all probability mediated by IGF-I receptors, and that the reduced insulin requirement observed in transformed fibroblasts actually reflects the loss of a strong IGF-I requirement. IGF-I is under strict pituitary-growth hormone control in vivo, and mediates many if not all of the growth promoting effects of growth hormone. A reduced IGF-I requirement may allow transformed fibroblasts to escape from this major humoral regulatory system. SV40 transformed cells also displayed a significantly diminished requirement for platelet-derived growth factor (PDGF). The loss of this particular growth factor requirement was found to be closely associated with the loss of density-dependent growth inhibition. Cell lines transformed by temperature sensitive mutants of SV40 exhibited a temperature sensitive loss of the PDGF requirement, indicating that SV40 T-antigen mediates this effect. Results pertaining to the temperature sensitivity of the insulin requirement were inconclusive. I found that SV40 could directly reduce the insulin requirement of 3T3 cells in a transformation assay based upon the stringent insulin requirement of 3T3 cells for colony formation. Several such insulin-transformants were isolated and characterized. Although all of these transformants expressed SV40 T-antigen, some of them retained anchorage-dependence and/or a partial PDGF requirement, unlike transformants obtained in the standard density of anchorage assays. A revertant of SV40 transformed 3T3 cells was found to have regained a very strong dependence upon insulin. This dependence was not overcome by re-transformation with Kirsten Murine Sarcoma virus, although retransformation did obviate both its density-dependent growth inhibition and PDGF requirement. Kirsten transformed 3T3 normally display a greatly reduced insulin requirement, indicating that this particular revertant may have suffered a cellular mutation that prevents transforming viruses from induced the reduced insulin requirement.
|
|---|
