We have constructed a recombinant retrovirus carrying the dbl transforming gene and infected monolayers of PC12 cells, a rat pheochromocytoma-derived cell line able to differentiate into neuron-like cells upon treatment with nerve growth factor (NGF). Clonal populations of cells infected with the dbl recombinant virus do not differentiate in the presence of NGF. Fibroblast growth factor (FGF) or cAMP, two other inducers of neuronal differentiation of uninfected PC12 cells, also failed to induce PC12 cells infected with the dbl recombinant virus to differentiate. Analysis of these clones has shown that cells carrying the transforming dbl gene no longer express the NGF receptor. Moreover. these cells have also lost TH and CAT activity, two of the neuronal markers characteristic of PC12 cells. PC12 cells expressing the dbl oncogene product also have an altered expression of integrins. We have expressed at high levels the dbl protein in SF9 insect cells utilizing a baculovirus expression vector system. When the soluble fraction of the infected insect cells was microinjected into Xenopus oocytes. we observed germinal vesicles breakdown in about 50% of the treated oocytes. The same results were obtained when the oocytes were microinjected with the in vitro transcribed dbl mRNA. Moreover, extracts of oocytes microinjected with dblencoding RNA showed activation of Hl kinase activity.
