TRANSFORMING RAS PROTEINS ACCELERATE HORMONE-INDUCED MATURATION AND STIMULATE CYCLIC-AMP PHOSPHODIESTERASE IN XENOPUS OOCYTES

Transforming Harvey (Ha) ras oncogene products accelerated the time course of Xenopus oocyte maturation induced by insulin, insulinlike growth factor 1, or progesterone. The transforming constructs, [Val-12]Ha p21 and [Val-12, Thr-59]Ha p32, displayed equal potency and efficacy in their abilities to accelerate the growth peptide-induced response. Normal Ha p21 was only 60% as powerful and one-fifth as potent as the mutants containing valine in the 12 position. In contrast, two nontransforming constructs, [Val-12, Ala-35, Leu-36, Thr-59]Ha p21 and [Val-12, Thr-59]Ha(term-174) p21, had no effect on the time course of hormone-induced maturation. Effects of the transforming ras proteins on hormone-induced maturation correlated with their abilities to stimulate in vivo phosphodiesterase activity measured after microinjection of 200 μM cyclic [3H]AMP. When p21 injection followed 90 min of insulin treatment, there was no increase in phosphodiesterase activity over that measured after hormone treatment or p21 injection alone, but additive effects of p21 and insulin on enzyme activity were observed during the first 90 min of insulin treatment. Even though normal Ha p21 and transforming [Val-12, Thr-59]Ha p21 stimulated oocyte phosphodiesterase to equal levels when coinjected with substrate at the initiation of the in vivo assay, the transforming protein elicited a more sustained stimulation of enzyme activity. These results suggest that stimulation of a cyclic AMP phosphodiesterase activity associated with insulin-induced maturation is involved in the growth-promoting actions of ras oncogene products in Xenopus oocytes.