A large number of human cancers display alterations in the Ink4a/cyclin D/Cdk4 genetic pathway, suggesting that activation of Cdk4 plays an important role in oncogenesis. Here we report that Cdk4- null mouse embryonic fibroblasts are resistant to transformation in response to Ras activation with dominant-negative (DN) p53 expression or in the Ink4a/Arf -null background, judged by foci formation, anchorage-independent growth, and tumorigenesis in athymic mice. Cdk4 -null fibroblasts proliferate at normal rates during early passages. Whereas Cdk4 +/+ Ink4a/Arf −/− cells are immortal in culture, Cdk4 −/− Ink4a/Arf −/− cells undergo senescence during continuous culture, as do wild-type cells. Activated Ras also induces premature senescence in Cdk4 −/− Ink4a/Arf −/− cells and Cdk4 −/− cells with DNp53 expression. Thus, Cdk4 deficiency causes senescence in a unique Arf/p53-independent manner, which accounts for the loss of transformation potential. Cdk4 -null cells express high levels of p21 Cip1/Waf1 with increased protein stability. Suppression of p21 Cip1/Waf1 by small interfering RNA (siRNA), as well as expression of HPV-E7 oncoprotein, restores immortalization and Ras-mediated transformation in Cdk4 −/− Ink4a/Arf −/− cells and Cdk4 −/− cells with DNp53 expression. Therefore, Cdk4 is essential for immortalization, and suppression of Cdk4 could be a prospective strategy to recruit cells with inactive Arf/p53 pathway to senescence.
CDC25A phosphatase promotes cell cycle progression by activating G 1 cyclin-dependent kinases and has been postulated to be an oncogene because of its ability to cooperate with RAS to transform rodent fibroblasts.In this study, we have identified apoptosis signal-regulating kinase 1 (ASK1) as a CDC25A-interacting protein by yeast two-hybrid screening.ASK1 activates the p38 mitogen-activated protein kinase (MAPK) and c-Jun NH 2 -terminal protein kinase-stress-activated protein kinase (JNK/SAPK) pathways upon various cellular stresses.Coimmunoprecipitation studies demonstrated that CDC25A physically associates with ASK1 in mammalian cells, and immunocytochemistry with confocal laser-scanning microscopy showed that these two proteins colocalize in the cytoplasm.The carboxyl terminus of CDC25A binds to a domain of ASK1 adjacent to its kinase domain and inhibits the kinase activity of ASK1, independent of and without effect on the phosphatase activity of CDC25A.This inhibitory action of CDC25A on ASK1 activity involves diminished homo-oligomerization of ASK1.Increased cellular expression of wild-type or phosphatase-inactive CDC25A from inducible transgenes suppresses oxidant-dependent activation of ASK1, p38, and JNK1 and reduces specific sensitivity to cell death triggered by oxidative stress, but not other apoptotic stimuli.Thus, increased expression of CDC25A, frequently observed in human cancers, could contribute to reduced cellular responsiveness to oxidative stress under mitogenic or oncogenic conditions, while it promotes cell cycle progression.These observations propose a mechanism of oncogenic transformation by the dual function of CDC25A on cell cycle progression and stress responses.
Ubiquitin-dependent degradation of Cdc25A is a major mechanism for damage-induced S-phase checkpoint. Two ubiquitin ligases, the Skp1-cullin-beta-TrCP (SCFbeta-TrCP) complex and the anaphase-promoting complex (APCCdh1), are involved in Cdc25A degradation. Here we demonstrate that the transforming growth factor beta (TGF-beta)-Smad3 pathway promotes SCF(beta-TrCP)-mediated Cdc25A ubiquitination. Cells treated with TGF-beta, as well as cells transfected with Smad3 or a constitutively active type I TGF-beta receptor, exhibit increased ubiquitination and markedly shortened half-lives of Cdc25A. Furthermore, Cdc25A is stabilized in cells transfected with Smad3 small interfering RNA (siRNA) and cells from Smad3-null mice. TGF-beta-induced ubiquitination is associated with Cdc25A phosphorylation at the beta-TrCP docking site (DS82G motif) and physical association of Cdc25A with Smad3 and beta-TrCP. Cdc25A mutant proteins deficient in DS82G phosphorylation are resistant to TGF-beta-Smad3-induced degradation, whereas a Cdc25A mutant protein defective in APCCdh1 recognition undergoes efficient degradation. Smad3 siRNA inhibits beta-TrCP-Cdc25A interaction and Cdc25A degradation in response to TGF-beta. beta-TrCP2 siRNA also inhibits Smad3-induced Cdc25A degradation. In contrast, Cdh1 siRNA had no effect on Cdc25A down-regulation by Smad3. These data suggest that Smad3 plays a key role in the regulation of Cdc25A ubiquitination by SCFbeta-TrCP and that Cdc25A stabilization observed in various cancers could be associated with defects in the TGF-beta-Smad3 pathway.
In a recent paper, Roychoudhury and Bhattacharyya [Phys. Fluids 30, 2582 (1987)] found solitary wave solutions in a relativistic plasma using an approximate pseudopotential. Since the integral of an approximation to a differential equation does not always approximate the integration of the differential equation, in the present paper a rigorous analytical study has been made to find the conditions for existence of soliton solutions in a relativistic plasma.
Schmutzer studied a homogeneous and isotropic dust model of the universe and gave some special exact solutions of it. In the present paper we obtain more general exact solutions of the above model.
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