Lactogenic hormones rapidly activate p21(ras)/mitogen-activated protein kinase in Nb2-11C rat lymphoma cells

Lactogenic hormone-dependent Nb2-11C cells proliferate in response to prolactin (PRL) or human growth hormone (hGH). We have investigated the activation of p21(ras) and mitogen-activated protein kinase (MAP-kinase) by hGH in lactogen-dependent Nb2-11C and in autonomous hormone-independent Nb2-SP rat lymphoma cells. Exposure of Nb2-11C cells to hGH resulted in a dose-dependent activation of p21(ras) and of MAP-kinase. Activation occurs at physiological hGH concentration and with a rapid onset (~1 min) reaching maximal level at 10-20 min. In contrast, in Nb2-SP autonomous lactogen-independent cells, p21(ras) and MAP-kinase are constitutively activated and a challenge with lactogenic hormone had a modest additional activating effect. TPA, an activator of protein kinase C, enhanced p21(ras) and MAP-kinase activity in Nb2-11C cells but failed to induce proliferation. The mechanism of activation of p21(ras) in Nb2-11C cells by lactogenic hormones involves both an increased binding of guanine nucleotides to p21(ras) as well as an increase in GTP/GDP + GTP ratio. In summary, we have demonstrated here that activation of the p21(ras)/MAP-kinase pathway follows PRL receptor activation but is not sufficient for the lactogenic hormone-dependent mitogenesis.