cAMP Inhibition of Akt Is Mediated by Activated and Phosphorylated Rap1b

Abstract

Rap1b has been implicated in the transduction of the cAMP mitogenic signal. Rap1b is phosphorylated and activated by cAMP, and its expression in cells where cAMP is mitogenic leads to an increase in G 1 /S phase entry and tumor formation. The PCCL3 thyroid follicular cells represent a differentiated and physiologically relevant system that requires thyrotropin (TSH), acting via cAMP, for a full mitogenic response. In this model system, cAMP stimulation of DNA synthesis requires activation and phosphorylation of Rap1b by the cAMPdependent protein kinase A (PKA). This scenario presents the challenge of identifying biochemical processes involved in the phosphorylation-dependent Rap1b mitogenic action. In thyroid cells, Akt has been implicated in the stimulation of cell proliferation by TSH and cAMP. However, the mechanism(s) by which cAMP regulates Akt activity remains unclear. In this study we show that in PCCL3 cells 1) TSH inhibits Akt activity via cAMP and PKA; 2) Rap1b is required for cAMP inhibition of Akt; and 3) transduction of the cAMP signal into Akt requires activation as well as phosphorylation of Rap1b by PKA.

Gα and Gβγ Require Distinct Src-dependent Pathways to Activate Rap1 and Ras

“…Data presented here identify a requirement for Ras because the activation by isoproterenol of AKT was independent of PKA and unaffected by SrcS17A. This is consistent with a recent report suggesting that cAMP/PKA/Rap1 does not activate AKT (77).…”

BCR/ABL and IL-3 activate Rap1 to stimulate the B-Raf/MEK/Erk and Akt signaling pathways and to regulate proliferation, apoptosis, and adhesion

“…Previous reports have implicated Rap1 as a positive or a negative regulator of Akt activation in various types of cells stimulated with different stimuli (Tsygankova et al, 2001;Wang et al, 2001;Lou et al, 2002;Christian et al, 2003). Although the molecular mechanisms underling these effects have not been elucidated, Rap1 has been proposed to bind PI3K through its effector-binding region, which is identical with that of Ras (Downward, 1997;Bos et al, 2001).…”

“…Although the molecular mechanisms underling these effects have not been elucidated, Rap1 has been proposed to bind PI3K through its effector-binding region, which is identical with that of Ras (Downward, 1997;Bos et al, 2001). Different effects of Rap1 on Akt activation may be due to differences in phosphorylation of Rap1, which has been shown to play crucial roles in modulation of the PI3K/Akt signaling pathway (Tsygankova et al, 2001;Lou et al, 2002). Although the molecular mechanism has remained to be examined, the present study strongly implicates Rap1 as a positive regulator for activation of the PI3K/Akt signaling pathway, which plays important roles in regulation of cell proliferation and apoptosis (Brazil et al, 2004).…”

“…Thus, depending on cell types, Rap1 may activate the MEK/Erk signaling pathway through B-Raf or may reduce its activation through its inhibitory effect on Raf-1. Similarly, recent studies have shown that Rap1 may also interact with the Ras effector PI3K and show a negative or positive effect on activation of the PI3K/Akt pathway depending on cell types and stimuli (Tsygankova et al, 2001;Wang et al, 2001;Lou et al, 2002;Christian et al, 2003). In our previous study, we found that IL-3 as well as Epo activates b1 integrin through Rap1 and induces hematopoietic cell adhesion, which agrees with recent studies showing that Rap1 plays important roles in regulation of cell adhesion through various integrins (Arai et al, 2001;Bos et al, 2001;Stork and Dillon, 2005).…”

The small GTPase Rap1b negatively regulates neutrophil chemotaxis and transcellular diapedesis by inhibiting Akt activation

“…7 B) or fMLP alone (Fig. 7 C), as shown in other cells (Lou et al, 2002). On the other hand, MAPK signaling Representative images of transmigratory cups, percentage of WT and Rap1b / neutrophils encircled with ICAM-rich transmigratory cups.…”