Cyclic AMP activates Ras

Abstract

In addition to protein kinase A (PKA), cAMP regulates the activity of cAMP-gated channels and Rap1-speci®c guanine nucleotide exchange factors. We tested the hypothesis that the targets of cAMP might also include regulators of the Ras protooncogene. In rat thyroid cells, thyrotropin (TSH) stimulates proliferation through a cAMP-mediated pathway that requires Ras activity. Interference with Ras impairs DNA synthesis stimulated by TSH as well as cAMP elevating agents and analogs, demonstrating that the requirement for Ras lies downstream of cAMP. Although cAMP stimulates proliferation, microinjection of the puri®ed PKA catalytic subunit failed to do so, suggesting that factors in addition to PKA are required for cAMP-stimulated cell cycle progression. When added to thyroid cells expressing human Ha-Ras, TSH rapidly and markedly increased the proportion of GTP-bound Ras. Ras activity was increased within 1 min of TSH addition, maximal at 5 ± 15 min, and declined to basal levels 30 ± 60 min after hormone treatment. Cyclic AMP elevating agents elicited similar e ects on Ras, indicating that TSH activates Ras through a cAMP-mediated pathway. Although cAMP-mediated, Ras activation by TSH and cAMP was independent of PKA activity. Moreover, cAMP-stimulated Ras activation was not impaired by tyrosine kinase inhibitors. These results indicate that cAMP activates targets in addition to PKA in thyroid cells, and that these targets may include regulators of Ras. The ability of cAMP elevating agents to activate Ras in addition to PKA may explain the inability of the PKA catalytic subunit to stimulate DNA synthesis in thyroid cells. Oncogene (2000) 19, 3609 ± 3615.

On the mitogenic properties of Rap1b: cAMP-induced G 1 /S entry requires activated and phosphorylated Rap1b

“…A recently identified family of Ras and Rap guanine nucleotide exchange factors can be activated by direct binding of cAMP, independent of PKA action (31,32). In fact, Ras can be activated by cAMP in a PKA-independent manner and it seems to be required for TSH action (28,30,33). However, expression of a constitutively active Ras mutant does not fully mimic the mitogenic action of TSH, but instead induces dedifferentiation (34)(35)(36)(37).…”

“…The mitogenic responses induced by TSH and cAMP require protein kinase A (PKA) activity (28). However, microinjection of the PKA catalytic subunit is not sufficient to stimulate mitogenesis in thyroid cells, suggesting that cAMP engages events in addition to stimulation of PKA for its mitogenic action (29,30). A recently identified family of Ras and Rap guanine nucleotide exchange factors can be activated by direct binding of cAMP, independent of PKA action (31,32).…”

Specificity of Co-Promoting Effects of Caffeine on Thyroid Carcinogenesis in Rats Pretreated with N-Bis(2-hydroxypropyl)nitrosamine

“…Further studies are required for the mechanisms underlying co-promoting effects of caffeine and ID on thyroid carcinogenesis. TSH activates the ras protooncogene through a cAMPmediated pathway in rat thyroid cells (Tsygankova et al, 2000), and cAMP may act as a positive signal for FRTL-5 thyroid cells and lung tissue proliferation (Jin et al, 1986;Dere and Rapoport, 1986). Caffeine, a phosphodiesterase inhibitor, has a stimulatory effect on intracellular cAMP production, which functions as a second messenger and mediates all the intercellular effects of TSH (Kohn et al, 1985;Dumont et al, 1992).…”

Enhanced sensitivity to apoptosis in Ras-transformed thyroid cells

“…This provides what may be the ®rst example of opposing e ects of Ras and Rap1 on the regulation of cell survival, similar to their contrasting e ects on thyroid di erentiation (Tsygankova et al, 2001). As TSH is capable of activating both Ras (Tsygankova et al, 2000) and Rap1 (Tsygankova et al, 2001) in WRT cells, the balance in the activity of Rasand Rap1-mediated signaling may be an important determinant of thyroid cell homeostasis.…”