MUTANT ALPHA-SUBUNITS OF G12 AND G13 PROTEINS INDUCE NEOPLASTIC TRANSFORMATION OF RAT-1 FIBROBLASTS

Mutationally activated alpha subunits of two G proteins, G(s) and G(i2 ,)) induce neoplastic transformation of fibroblasts and are found in h uman tumors. Here we report that mutationally activated alpha subunits of two other G proteins, G12 and G13, induce neoplastic transformatio n of Rat-1 fibroblasts and NIH3T3 fibroblasts. Constitute activation o f these alpha subunits resulted from replacement by leucine of glutami ne-229 and glutamine-226 in alpha 12 and alpha 13, respectively. Trans ient expression of mutant alpha 12 and alpha 13 cDNAs induced focus fo rmation in Rat-1 cells and NIH3T3 cells, and stable expression of thes e mutant proteins in Rat-1 cells accelerated growth rate, induced grow th in soft agar, and increased DNA synthesis. Mitogen-activated protei n (MAP) kinase activity, stimulated by EGF, was increased in Rat-1 cel ls that expressed mutant alpha 12 or alpha 13. The MAP kinase cascade plays a role in mediating neoplastic transformation induced by other G TPases, including ras and the alpha subunit of G(i2). Therefore, we pr opose that the MAP kinase cascade is an effector pathway affected by a lpha 12 and alpha 13 and may contribute to neoplastic transformation b y these mutant proteins. We predict that activating somatic mutations in alpha 12 and alpha 13 genes will be found in human tumors, as is th e case for mutationally activated alpha subunits of G(s) and G(i2).