It is not yet clear whether signalling by any of these pathways alone is sufficient to drive partial, anchorage-independent activation of the cell cycle machinery, because the effector mutants used in this study may bind to additional proteins that could also participate in these actions. anchorage-independent growth of NIH 3T3 cells, but only the 12V,35S-12V,37G and 12V,37G-12V,40C combinations were complementary in Rat 6 cells. Each individual effector loop mutant partially relieved adhesion dependence of pRB phosphorylation, cyclin E-dependent kinase activity, and expression of cyclin A in NIH 3T3, but not Rat 6, cells. The pairwise combinations of effector loop mutants that were synergistic in producing anchorage-independent growth in Rat 6 cells also led to synergistic abrogation of the adhesion requirement for these cell cycle activities. The relationship between complementation in producing anchorage-independent growth and enhancement of cell cycle activities was not as clear in NIH 3T3 cells that expressed pairs of mutants, implying Rabbit Polyclonal to ATP1alpha1 the existence of either thresholds for these activities or additional requirements in the induction of anchorage-independent growth. Ectopic expression of cyclin D1, E, or A synergized with individual effector loop mutants to induce soft agar colony formation in NIH 3T3 cells, cyclin A being particularly effective. Taken together, these data indicate that Ras utilizes multiple pathways to signal to the cell cycle machinery and that these pathways synergize to GSK1292263 supplant the adhesion requirements of specific cell cycle events, leading to anchorage-independent growth. Ras proteins are small guanine nucleotide binding proteins that play a central role in signal transduction pathways that regulate cell proliferation (2). Wild-type Ras proteins are activated transiently, via guanine nucleotide exchange mechanisms, in response to a wide variety of extracellular signalling agents (3). When in the GTP-bound state, Ras is capable of binding to several different established and potential effector proteins, including members of the Raf, phosphatidylinositol 3 (OH)-kinase [PI(3)K], and Ral guanine nucleotide dissociation stimulator (RalGDS) families, Rin, protein kinase C, AF6, and the GTPase-activating proteins p120GAP and neurofibromin (17, 28). Binding to Ras leads, directly or indirectly, to activation of these effectors, which in turn activate downstream signalling cascades. Thus, Ras may be viewed as a hub from which multiple pathways radiate. Activating mutations in Ras result in constitutive signalling to these downstream elements, and such mutations are observed with high frequency in human tumors (4). Expression of mutated, oncogenic Ras in cultured rodent fibroblast cell lines induces a highly pleiotropic response, including alterations in cell morphology, loss of contact inhibition, changes in gene expression, decreased dependence on serum growth factors, and the ability to proliferate in the absence of adhesion to a substratum (i.e., anchorage-independent growth). Many of these phenotypes can be dissociated from one another. For example, introduction of Ras oncoprotein into quiescent Swiss 3T3 cells led to both morphological transformation and DNA synthesis, but only the induction of DNA synthesis required activation of protein kinase C (27). Furthermore, a Rat 6 fibroblast-derived mutant cell line, ER-1-2, responded to stable expression of the v-H-oncogene with alterations in morphology and gene expression that were nearly indistinguishable from those observed with a matched control cell line yet failed to form colonies in soft agar in response to (11, 22). Similarly, expression of a dominant negative form of Rac1 in Ras-transformed Rat 1 cells inhibited anchorage-independent growth of these cells but had only marginal effects GSK1292263 on their transformed morphology (34). The studies cited above raise the possibility that different aspects of the transformed phenotype might be controlled by distinct combinations of Ras-regulated pathways. Evidence for this notion has been elegantly provided through the use of Ras effector loop mutants. Certain point mutations in this region (amino acids 32 to 40 in H-Ras) render Ras defective for binding specific GSK1292263 effector proteins while remaining competent for binding and activating others, albeit at lower than wild-type efficiency (39, 46). Several individual mutants were defective in transformation assays but, when coexpressed, complemented each other (19, 39, 46). These and additional studies have implicated at least three effector proteins as potential synergistic mediators of transformation by Ras: Raf, PI(3)K, and RalGDS (15, 19, 20, GSK1292263 33, 39, 46, 47)..