Mitogenic and oncogenic properties of the small G protein Rap1b

Abstract

It has been widely reported that the small GTP-binding protein Rap1 has an anti-Ras and antimitogenic activity. Thus, it is generally accepted that a normal physiological role of Rap1 proteins is to antagonize Ras mitogenic signals, presumably by forming nonproductive complexes with proteins that are typically effectors or modulators of Ras. Rap1 is activated by signals that raise intracellular levels of cAMP, a molecule that has long been known to exert both inhibitory and stimulatory effects on cell growth. We have now tested the intriguing hypothesis that Rap1 could have mitogenic effects in systems in which cAMP stimulates cell proliferation. The result of experiments addressing this possibility revealed that Rap1 has full oncogenic potential. Expression of Rap1 in these cells results in a decreased doubling time, an increased saturation density, and an unusual anchorage-dependent morphological transformation. Most significantly, however, Rap1-expressing cells formed tumors when injected into nude mice. Thus, we propose that the view that holds Rap1 as an antimitogenic protein should be restricted and conclude that Rap1 is a conditional oncoprotein.

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

“…Other reports have suggested that Rap1a expression in Wistar Rat thyroid (WRT) cells promotes differentiation without affecting cell proliferation (42). Nevertheless, the present results are consis- tent with our previous observation using Swiss 3T3 cells (17) and extend the mitogenic effect of Rap1b to a differentiated system, the thyroid cell, where cAMP physiologically acts as a bona fide comitogen. In this model, Rap1b enhances the mitogenic response while maintaining differentiation.…”

“…Exogenous expression of Rap1b in Swiss 3T3 fibroblasts was sufficient to endow these cells with both mitogenic and tumorigenic properties (17). In that report, we emphasized that the mitogenic effects of Rap1b were linked to its ability to promote S phase entry and introduced the hypothesis that Rap1b can be viewed as a conditional oncogene.…”

“…Twenty-four hours later, cells were collected (cell harvester), and filters were counted by scintillation. G 1 ͞S entry was also evaluated by BrdUrd incorporation as described (17). Briefly, for the transient assays, HA-Rap1b or HA-Rap-GAP plasmids were transfected into PCCL3 cells as described.…”

“…Interestingly, the reported negative effects of Rap1b on mitogenesis are seen in systems where cAMP inhibits cell proliferation (22). We asked whether cAMP stimulation of mitogenesis could also be associated with Rap1b activity, and hypothesized that Rap's mitogenic action might depend on cell-specific signal transduction programs, such that Rap1b, like cAMP, can either stimulate or inhibit cell proliferation (17).…”

“…Because agonists that increase intracellular cAMP rapidly phosphorylate (15) and activate Rap1b (16), we hypothesized that Rap1 could mediate some of cAMP's biological action (17). It has long been known that cAMP can either stimulate or inhibit cell proliferation (18,19).…”