Proliferation of human malignant astrocytomas is dependent on Ras activation

Overexpression and activation of receptor tyrosine kinases, such as platelet derived growth factor receptors (PDGFRs) and epidermal growth factor receptor (EGFR), leads to proliferation of human malignant astrocytoma cells. Although oncogenic mutations affecting Pas are not prevalent in human malignant astrocytomas, we have investigated whether levels of activated Ras.GTP might be elevated in these tumors secondary to the mitogenic signals originating from activated receptor tyrosine kinases. In support of this hypothesis high levels of Ras.GTP, similar to those found in oncogenic Pas transformed fibroblasts, were present in four established human malignant astrocytoma cell lines which express PDGFRs and EGFR, and 20 operative malignant astrocytoma specimens. Stimulation of PDGFR's and EGFR's induced tyrosine phosphorylation of the Shc adaptor protein and its association with Grb2, suggesting a mechanism by which Pas may be activated in human malignant astrocytoma cells. Furthermore, blocking Pas activation by expression of the Ha-Ras-Asn17 dominant-negative mutant, or by farnesyl transferase inhibitors, decreased in vitro proliferation of the human astrocytoma cell lines. These results support the hypothesis that proliferative signals from receptor tyrosine kinases expressed by human malignant astrocytoma cells utilize the Pas mitogenic pathway. Pharmacological inhibitors of the Ras pathway may therefore be of therapeutic value in these presently terminal tumors.