Akt-dependent phosphorylation of p21(Cip1) regulates PCNA binding and proliferation of endothelial cells

Overexpression of oncoprotein 18 correlates with poor differentiation in lung adenocarcinomas

We examined the expression of oncoprotein 18 (Op18) in 93 lung adenocarcinomas and 10 uninvolved lung samples using quantitative two-dimensional PAGE analysis with confirmation by mass spectrometry and two-dimensional Western blot analysis. mRNA expression was examined using oligonucleotide microarrays, and the cellular localization of the Op18 protein was examined using immunohistochemical analysis of tissue microarrays. Three phosphorylated forms and one unphosphorylated form of the Op18 protein were identified and found to be overexpressed in lung adenocarcinomas as compared with normal lung. The percentage of phosphorylated to total Op18 protein isoforms increased from 3.2% in normal lung to 7.9% in lung tumors. Both the phosphorylated and unphosphorylated Op18 proteins were significantly increased in poorly differentiated tumors as compared with moderately or well differentiated lung adenocarcinomas (p<0.03), suggesting that up-regulated expression of Op18 reflects a poor differentiation status and higher cell proliferation rates. This was further verified in A549 and SKLU1 lung adenocarcinoma cell lines by examining Op18 levels and phosphorylation status following treatment that altered either cell proliferation or differentiation. The increased expression of Op18 protein was significantly correlated with its mRNA level indicating that increased transcription likely underlies elevated expression of Op18. The overexpression of Op18 proteins in poorly differentiated lung adenocarcinomas and the elevated expression of the phosphorylated forms of Op18 may offer a new target for drug- or gene-directed therapy and may have potential utility as a tumor marker.

Suppression of breast cancer growth and angiogenesis by an antisense oligodeoxynucleotide to p21(Waf1/Cip1)

Under some conditions, p21(Waf1/Cip1) plays an assembly factor role for the cyclins and cyclin-dependent kinases, and recent reports demonstrate that p21 can act as an anti-apoptotic protein. Thus, it is logical to exploit this function of p21 as an anti-cancer target. We have performed a pilot study showing that daily subcutaneous injection of a phosphorothioate antisense p21 oligodeoxynucleotide, which we have previously shown to attenuate p21 levels in vitro, into nude mice who have been implanted with highly metastatic breast cancer cells results in inhibition of tumor growth and angiogenesis. Inhibition of in vitro endothelial capillary formation confirms that these oligodeoxynucleotides have a direct effect upon tumor angiogenesis. The attractiveness of our novel approach to breast cancer therapy, which capitalizes on the anti-apoptotic function of p21, derives from the ease of transfection of antisense oligodeoxynucleotides as well as the observations that p21(-/-) mice do not develop spontaneous tumors, making techniques exploiting the assembly factor and anti-apoptotic role of p21 worthy of further study against breast cancer.

Overexpression of p73 causes apoptosis in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSM) cell proliferation contributes to the development of atherosclerosis and its associated disorders, including angioplasty restenosis. The tumor-suppressor protein p53 has been linked to the development of atherosclerotic lesions, and its homolog, p73, is proving to have contrasting functions in a variety of tissues. As an outgrowth of our previous finding that p73 is increased in serum-stimulated VSM cells and human atherosclerotic tissue, we examined p73 overexpression in VSM cells to elucidate causality of p73 expression with growth response. Overexpression of p73 results in decreased cell cycle transit and is accompanied by apoptosis. The apoptotic changes in p73 overexpressing VSM cells are independent of p53 and are associated with a decrease in levels of p21(waf1/cip1). In conjunction with our previous data finding that p73 is increased in serum-stimulated VSM cells, this work suggests a role for p73 in vascular proliferative diseases.

PDK1-dependent activation of atypical PKC leads to degradation of the p21 tumour modifier protein

p21(WAF1/CIP1) Contributes to positive and negative growth control on multiple levels. We previously mapped phosphorylation sites within the C-terminal domain of p21 that regulate proliferating cell nuclear antigen binding. In the current study, a kinase has been fractionated from mammalian cells that stoichiometrically phosphorylates p21 at the Ser146 site, and the enzyme has been identified as an insulin-responsive atypical protein kinase C (aPKC). Expression of PKCzeta or activation of the endogenous kinase by 3-phosphoinositide dependent protein kinase-1 (PDK1) decreased the half-life of p21. Conversely, dnPKCzeta or dnPDK1 increased p21 protein half-life, and a PDK1-dependent increase in the rate of p21 degradation was mediated by aPKC. Insulin stimulation gave a biphasic response with a rapid transient decrease in p21 protein levels during the initial signalling phase that was dependent on phosphatidylinositol 3- kinase, PKC and proteasome activity. Thus, aPKC provides a physiological signal for the degradation of p21. The rapid degradation of p21 protein during the signalling phase of insulin stimulation identifies a novel link between energy metabolism and a key modulator of cell cycle progression.

P21 functions to maintain quiescence of p27-deficient hepatocytes

Hepatocytes rarely proliferate in the healthy adult liver. We explored the roles of the cyclin kinase inhibitors p21 and p27 in maintaining hepatocyte quiescence. p27 is expressed throughout the wild-type liver, but the related protein p21 was not detected. However, p21 was detected in livers of p27-deficient mice. Increased p21 protein levels did not result from an increase in p21 mRNA expression, indicating that p21 expression is regulated post-transcriptionally. p21 protein levels increased in cultured primary hepatocytes treated with the proteasome inhibitor MG132 and cycloheximide, indicating that p21 expression is regulated at the level of protein stability in liver cells. Although increased expression of cyclin-dependent kinase (Cdk) 4, Cdk2, and proliferating cell nuclear antigen was detected in p27-deficient livers, increased hepatocyte proliferation was detected only in livers of mice deficient for both p21 and p27. In p27-deficient livers, p21 was found in complexes with Cdk2 and CdK4 and can compensate for the absence of p27. Our data indicate that cyclin kinase inhibitor activity is important for maintaining hepatocyte quiescence in the adult liver. Significant increases in p21 were detected in multiple tissues of mature p27-deficient mice compared with wild-type mice, suggesting that the ability of p21 to functionally substitute for p27 is not liver-specific.

E7 abolishes raf-induced arrest via mislocalization of p21(Cip1)

The cellular response to oncogenic Ras depends upon the presence or absence of cooperating mutations. In the absence of immortalizing oncogenes or genetic lesions, activation of the Ras/Raf pathway results in a p21(Cip1)-dependent cellular arrest. The human papillomavirus oncoprotein E7 transforms primary cells in cooperation with Ras and abolishes p21(Cip1)-mediated growth arrest in the presence of various antimitogenic signals. Here we have utilized a conditional Raf molecule to investigate the effects of E7 on p21(Cip1) function in the context of Raf-induced cellular arrest. E7 bypassed Raf-induced arrest and alleviated inhibition of cyclin E-CDK2 without suppressing Raf-specific synthesis of p21(Cip1) or derepressing p21(Cip1)-associated CDK2 complexes. Activation of Raf led to nuclear accumulation of p21(Cip1), and we provide evidence that this effect is mediated by inhibition of Akt, a regulator of p21(Cip1) localization. Loss of Akt activity appears to be an important event in the cellular arrest associated with Raf-induction, since maintenance of Akt activity was necessary and sufficient to bypass Raf-induced arrest. In agreement, expression of E7 sustained Akt activity and reduced nuclear accumulation of p21(Cip1), resulting in decreased association between p21(Cip1) and cyclin E-CDK2. Taken together, these data suggest that E7 inhibits p21(Cip1) function in the context of Raf signaling by altering Raf-Akt antagonism and preventing the proper subcellular localization of p21(Cip1). We propose that E7 elicits a proliferative response to Raf signaling by targeting p21(Cip1) function via a novel mechanism.

Molecular modeling-based analysis of interactions in the RFC-dependent clamp-loading process

Replication and related processes in eukaryotic cells require replication factor C (RFC) to load a molecular clamp for DNA polymerase in an ATP-driven process, involving multiple molecular interactions. The detailed understanding of this mechanism is hindered by the lack of data regarding structure, mutual arrangement, and dynamics of the players involved. In this study, we analyzed interactions that take place during loading onto DNA of either the PCNA clamp or the Rad9-Rad1-Hus1 checkpoint complex, using computationally derived molecular models. Combining the modeled structures for each RFC subunit with known structural, biochemical, and genetic data, we propose detailed models of how two of the RFC subunits, RFC1 and RFC3, interact with the C-terminal regions of PCNA. RFC1 is predicted to bind PCNA similarly to the p21-PCNA interaction, while the RFC3-PCNA binding is proposed to be similar to the E. coli delta-beta interaction. Additional sequence and structure analysis, supported by experimental data, suggests that RFC5 might be the third clamp loader subunit to bind the equivalent PCNA region. We discuss functional implications stemming from the proposed model of the RFC1-PCNA interaction and compare putative clamp-interacting regions in RFC1 and its paralogs, Rad17 and Ctf18. Based on the individual intermolecular interactions, we propose RFC and PCNA arrangement that places three RFC subunits in association with each of the three C-terminal regions in PCNA. The two other RFC subunits are positioned at the two PCNA interfaces, with the third PCNA interface left unobstructed. In addition, we map interactions at the level of individual subunits between the alternative clamp loader/clamp system, Rad17-RFC(2-5)/Rad9-Rad1-Hus1. The proposed models of interaction between two clamp/clamp loader pairs provide both structural framework for interpretation of existing experimental data and a number of specific findings that can be subjected to direct experimental testing.

PKB/Akt phosphorylates p27, impairs nuclear import of p27 and opposes p27-mediated G1 arrest

Mechanisms linking mitogenic and growth inhibitory cytokine signaling and the cell cycle have not been fully elucidated in either cancer or in normal cells. Here we show that activation of protein kinase B (PKB)/Akt, contributes to resistance to antiproliferative signals and breast cancer progression in part by impairing the nuclear import and action of p27. Akt transfection caused cytoplasmic p27 accumulation and resistance to cytokine-mediated G1 arrest. The nuclear localization signal of p27 contains an Akt consensus site at threonine 157, and p27 phosphorylation by Akt impaired its nuclear import in vitro. Akt phosphorylated wild-type p27 but not p27T157A. In cells transfected with constitutively active Akt(T308DS473D)(PKB(DD)), p27WT mislocalized to the cytoplasm, but p27T157A was nuclear. In cells with activated Akt, p27WT failed to cause G1 arrest, while the antiproliferative effect of p27T157A was not impaired. Cytoplasmic p27 was seen in 41% (52 of 128) of primary human breast cancers in conjunction with Akt activation and was correlated with a poor patient prognosis. Thus, we show a novel mechanism whereby Akt impairs p27 function that is associated with an aggressive phenotype in human breast cancer.

Regulation of angiogenesis by vascular endothelial growth factor and angiopoietin-1 in the rat aorta model: distinct temporal patterns of intracellular signaling correlate with induction of angiogenic sprouting

Vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang-1) promote the spontaneous angiogenic response of freshly cut rat aortic rings. When VEGF and Ang-1 were tested in cultures of 14-day-old rings, which are quiescent and unable to spontaneously produce neovessels, only VEGF was capable of inducing an angiogenic response. Ang-1 failed to initiate angiogenesis in this system, but significantly potentiated VEGF-induced neovessel sprouting. Potential differences in cell signaling triggered by VEGF and Ang-1 were evaluated in cultures of quiescent rings. VEGF induced biphasic and prolonged (15 minutes and 4 to 24 hours) phosphorylation of p44/42 MAPK and Akt, while the effect of Ang-1 was transient and monophasic (15 minutes). Both VEGF and Ang-1 induced rapid, monophasic (15 minutes) phosphorylation of p38 MAPK. When VEGF and Ang-1 were administered together, the second peak of VEGF-induced p44/42 MAPK phosphorylation was markedly reduced. The effect of the VEGF/Ang-1 combination on AKT phosphorylation was, instead, additive over time, and sustained over a 24-hour period. The VEGF/Ang-1 combination caused an additive effect also on p38 MAPK phosphorylation at 1 hour. Confocal microscopy of VEGF-, Ang-1, or VEGF/Ang-1-stimulated aortic rings double stained at time points of maximal phosphorylation for cell markers and signal transduction proteins demonstrated phosphorylated p44/42 MAPK, p38 MAPK, and Akt predominantly in endothelial cells. Experiments with specific inhibitors demonstrated that p44/42 MAPK and Akt, but not p38 MAPK, are necessary for neovessel sprouting. These results identify p44/42 MAPK and Akt as critical intracellular mediators of angiogenesis, whose transient phosphorylation is, however, not sufficient for the initiation of this process. The observation that sustained phosphorylation of these signaling pathways, particularly of Akt, correlates with induction of angiogenesis suggests that the duration of phosphorylation signals influences critical cellular events required for the induction of angiogenic sprouting.

The stress-activated protein kinases p38 alpha and JNK1 stabilize p21(Cip1) by phosphorylation

Stress signals activate the SAPK/JNK and p38 MAPK classes of protein kinases, which mediate cellular responses, including steps in apoptosis and the maturation of some cell types. We now show that stress signals initiated by transforming growth factor-beta 1 (TGF-beta 1) induce G(1) arrest through protein stabilization of the CDK inhibitor p21(Cip1). TGF-beta 1 was previously shown to increase p21 protein levels, which in turn mediated G(1) arrest through inactivation of the CDK2-cyclin E complex in HD3 cells (Yan, Z., Kim, G.-Y., Deng, X., and Friedman, E. (2002) J. Biol. Chem. 277, 9870-9879). We now demonstrate that the increase in p21 abundance is caused by a post-transcriptional, SMAD-independent mechanism. TGF-beta1 activated p38 alpha and JNK1, which initiated the phosphorylation of p21. TGF-beta1 treatment increased the half-life of p21 by 3-4-fold. The increase in p21 stability was detected following activation of p38 alpha and JNK1, and treatment of cells with the p38 inhibitor SB203580 prevented this increase in p21 stability. p38 alpha and JNK1 phosphorylated p21 in vivo, and both p38 alpha and JNK1 phosphorylated p21 at Ser(130) in vitro. Peptide mapping demonstrated that both TGF-beta 1 and p38 alpha induced phosphorylation of p21 at Ser(130) in vivo, and mutation of Ser(130) to alanine rendered p21 less stable than wild-type p21. TGF-beta 1 increased the stability of wild-type p21, but not the p21-S130A mutant. These findings demonstrate that SAPKs can mediate cell cycle arrest through post-translational modification of p21.

A serum factor after intestinal resection stimulates epidermal growth factor receptor signaling and proliferation in intestinal epithelial cells

BACKGROUND

In vivo, intestinal adaptation after massive small bowel resection (SBR) requires a functional epidermal growth factor (EGF) receptor (EGFR). In vitro studies have shown that serum from mice after SBR induces rat intestinal epithelial cells to proliferate. This study tested the hypothesis that the proliferative response to SBR serum is mediated by EGFR signaling.

METHODS

Serum was collected from male Sprague-Dawley rats 7 days after 75% SBR or sham operation. Rat intestinal epithelial cells were incubated in the presence of sham or SBR serum. Total EGFR expression and phosphorylation of several EGFR downstream pathways were determined by Western blotting. In other experiments, a specific EGFR inhibitor (ZD1839) was added and cell growth determined over 5 days.

RESULTS

SBR serum significantly increased total EGFR expression (3-fold) over sham operation and consistently activated the phosphatidylinositol 3-kinase pathway. Furthermore, SBR serum markedly augmented rat intestinal epithelial cell growth, an effect that was abolished by EGFR inhibition.

CONCLUSIONS

SBR serum contains a factor or factors that stimulates proliferation of intestinal epithelial cells by an EGFR and phosphatidylinositol 3-kinase signaling mechanism. These data recapitulate in vivo studies supporting the hypothesis that EGFR is a central mediator of postresection intestinal adaptation. This in vitro model may provide a novel means to gain insight into the pathophysiology of intestinal adaptation.

Cytoplasmic p21(Cip1/WAF1) regulates neurite remodeling by inhibiting Rho-kinase activity

p21(Cip1/WAF1) has cell cycle inhibitory activity by binding to and inhibiting both cyclin/Cdk kinases and proliferating cell nuclear antigen. Here we show that p21(Cip1/WAF1) is induced in the cytoplasm during the course of differentiation of chick retinal precursor cells and N1E-115 cells. Ectopic expression of p21(Cip1/WAF1) lacking the nuclear localization signal in N1E-115 cells and NIH3T3 cells affects the formation of actin structures, characteristic of inactivation of Rho. p21(Cip1/WAF1) forms a complex with Rho-kinase and inhibits its activity in vitro and in vivo. Neurite outgrowth and branching from the hippocampal neurons are promoted if p21(Cip1/WAF1) is expressed abundantly in the cytoplasm. These results suggest that cytoplasmic p21(Cip1/WAF1) may contribute to the developmental process of the newborn neurons that extend axons and dendrites into target regions.

Role of X-linked inhibitor of apoptosis protein in chemoresistance in ovarian cancer: possible involvement of the phosphoinositide-3 kinase/Akt pathway

Although cisplatin derivatives are first-line chemotherapeutic agents for the treatment of epithelial ovarian cancer, chemoresistance remains a major hurdle to successful therapy and the molecular mechanisms involved are poorly understood. Apoptosis is the cellular underpinning of cisplatin-induced cell death, which is associated with expression of specific "death" genes and down-regulation of "survival" counterparts. The X-linked inhibitor of apoptosis proteins (Xiap), an intracellular anti-apoptotic protein, plays a key role in cell survival by modulating death signaling pathways and is a determinant of cisplatin resistance in ovarian cancer cells in vitro. This review focuses on the role of Xiap and its interactions with the phosphoinositide-3 kinase (PI3K)/Akt cell survival pathway in conferring resistance of ovarian cancer cells to chemotherapeutic agents and discusses potential therapeutic strategies in overcoming chemoresistant ovarian cancer.

Role of Akt signaling in vascular homeostasis and angiogenesis

Akt is a serine/threonine protein kinase that is activated by a number of growth factors and cytokines in a phosphatidylinositol-3 kinase-dependent manner. Although antiapoptotic activity of Akt is well known, it also regulates other aspects of cellular functions, including migration, glucose metabolism, and protein synthesis. In this review, Akt signaling in endothelial cells and its critical roles in the regulation of vascular homeostasis and angiogenesis will be discussed.

Phenotypic spectrum caused by transgenic overexpression of activated Akt in the heart

The serine-threonine kinase, Akt, inhibits cardiomyocyte apoptosis acutely both in vitro and in vivo. However, the effects of chronic Akt activation in the heart are unknown. To address this issue, we generated transgenic mice (TG+) with cardiac-specific expression of a constitutively active mutant of Akt (myr-Akt) driven by the myosin heavy chain-alpha promoter. Three TG+ founders (9-19 weeks) died suddenly with massive cardiac dilatation. Two viable TG+ lines (TG564 and TG20) derived from independent founders demonstrated cardiac-specific transgene expression as well as activation of Akt and p70S6 kinase. TG564 (n = 19) showed cardiac hypertrophy with a heart/body weight ratio 2.3-fold greater than littermates (n = 17, p < 0.005). TG20 (n = 18) had less marked cardiac hypertrophy with a heart/body weight ratio 1.6-fold greater than littermates (n = 17, p < 0.005). Isolated TG564 myocytes were also hypertrophic with surface areas 1.7-fold greater than littermates (p < 0.000001). Echocardiograms in both lines demonstrated concentric hypertrophy and preserved systolic function. After ischemia-reperfusion, TG+ had a 50% reduction in infarct size versus TG- (17 +/- 3% versus 34 +/- 4%, p < 0.001). Thus, chronic Akt activation is sufficient to cause a spectrum of phenotypes from moderate cardiac hypertrophy with preserved systolic function and cardioprotection to massive cardiac dilatation and sudden death.

The protein kinase B/Akt signalling pathway in human malignancy

Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.

Selective delivery of nitric oxide to a cellular target: a pseudosubstrate-coupled dinitrosyl-iron complex inhibits the enteroviral protease 2A

Nitric oxide (NO) regulates multiple biological processes. To use NO as a potential therapeutic substance, a more selective modulation of individual NO targets is desirable. Here, we tested whether peptide conjugation of the dinitrosyl-iron complex (DNIC), a potent NO donor, confers targeted NO delivery. As target, we used the protease 2A of Coxsackie-B-viruses (2A(pro)), which can cause dilated cardiomyopathy. Through S-nitrosylation, NO inhibits this protease, which is essential for viral replication. The tetrapeptide Leu-Ser-Thr-Cys (LSTC) (based on the 2A(pro) substrate recognition motif) and DNIC generated LSTC-DNIC in vitro by S-nitrosylation as evidenced by reverse-phase chromatography. In vitro, LSTC-DNIC (IC(50) 510 nM) dose-dependently inhibited purified 2A(pro) 4.7-fold more effectively than DNIC (IC(50) 2.4 microM), whereas LSTC alone had no effect. In intact cells, expression of Coxsackievirus protease 2A by transient transfection led to eIF4G-I-cleavage. LSTC-DNIC (IC(50) 23 microM) dose-dependently inhibited eIF4G cleavage in 2A(pro)-transfected cells 3.8-fold more effectively than DNIC (IC(50) 88 microM). To test the specificity of the DNIC-conjugated LSTC peptide part, we investigated its influence on Caspase-3, a known target for S-nitrosylation. LSTC-DNIC and DNIC inhibited purified Caspase-3 in vitro (IC(50) 3.7 microM) and in intact cells similarly. LSTC conjugation of DNIC enhances its fidelity for inhibition of 2A(pro) in vitro and intracellularly. Peptide-DNIC may be useful to selectively modulate cellular processes by NO, i.e., to enhance its antiviral properties.

Lack of ERK activation and cell migration in FGF-2-deficient endothelial cells

The formation of blood capillaries from preexisting vessels (angiogenesis) and vascular remodeling secondary to atherosclerosis or vessel injury are characterized by endothelial cell migration and proliferation. Numerous growth factors control these cell functions. Basic fibroblast growth factor (FGF-2), a potent angiogenesis inducer, stimulates endothelial cell proliferation, migration, and proteinase production in vitro and in vivo. However, mice genetically deficient in FGF-2 have no apparent vascular defects. We have observed that endothelial cell migration in response to mechanical damage in vitro is accompanied by activation of the extracellular signal-regulated kinase (ERK) pathway, which can be blocked by neutralizing anti-FGF-2 antibodies. Endothelial cells from mice that are genetically deficient in FGF-2 neither migrate nor activate ERK in response to mechanical wounding. Addition of exogenous FGF-2 restores a normal cell response, which shows that impaired migration results from the genetic deficiency of this growth factor. Injury-induced ERK activation in endothelial cells occurs only at the edge of the wound. In addition, FGF-2-induced ERK activation mediates endothelial cell migration in response to wounding without a significant effect on proliferation. These data show that FGF-2 is a key regulator of endothelial cell migration during wound repair.

AKT/PKB phosphorylation of p21Cip/WAF1 enhances protein stability of p21Cip/WAF1 and promotes cell survival

p21(Cip1/WAF1) (p21), a p53-inducible protein, is a critical regulator of cell cycle and cell survival. p21 binds to and inhibits both the DNA synthesis regulator proliferating cell nuclear antigen and cyclin A/E-CDK2 complexes. Recently, p21 has also been shown to be a positive regulator of cell cycle progression as p21 is necessary for the assembly and activation of cyclin D1-CDK4/6 complexes. Furthermore, elevated p21 protein levels have been observed in various aggressive tumors as well as linked to chemoresistance. Here we demonstrate that p21 is directly phosphorylated by AKT/PKB, a survival kinase that is hyperactivated in many late stage tumors. Two sites (Thr(145) and Ser(146)) in the carboxyl terminus of p21 are phosphorylated by AKT/PKB in vitro and in vivo. Phosphorylation of Thr(145) inhibits PCNA binding, whereas phosphorylation of Ser(146) significantly increases p21 protein stability. Glioblastoma cell lines with activated AKT/PKB show enhanced p21 stability, and they are more resistant to taxol-mediated toxicity. Finally, AKT/PKB controls the assembly of cyclin D1-CDK4 complexes through modulation of p21 and cyclin D1 levels. These data imply that enhanced levels of p21 in tumors are due, in part, to phosphorylation by activated AKT/PKB. Furthermore, they suggest that one mechanism of AKT/PKB regulation of tumor cell survival and/or proliferation is to stabilize p21 protein.

Glycogen synthase kinase-3 couples AKT-dependent signaling to the regulation of p21Cip1 degradation

Signaling via the phosphoinositide 3-kinase (PI3K)/AKT pathway is crucial for the regulation of endothelial cell (EC) proliferation and survival, which involves the AKT-dependent phosphorylation of the DNA repair protein p21(Cip1) at Thr-145. Because p21(Cip1) is a short-lived protein with a high proteasomal degradation rate, we investigated the regulation of p21(Cip1) protein levels by PI3K/AKT-dependent signaling. The PI3K inhibitors Ly294002 and wortmannin reduced p21(Cip1) protein abundance in human umbilical vein EC. However, mutation of the AKT site Thr-145 into aspartate (T145D) did not increase its protein half-life. We therefore investigated whether a kinase downstream of AKT regulates p21(Cip1) protein levels. In various cell types, AKT phosphorylates and inhibits glycogen synthase kinase-3 (GSK-3). Upon serum stimulation of EC, GSK-3beta was phosphorylated at Ser-9. Site-directed mutagenesis revealed that GSK-3 in vitro phosphorylated p21(Cip1) specifically at Thr-57 within the Cdk binding domain. Overexpression of GSK-3beta decreased p21(Cip1) protein levels in EC, whereas the specific inhibition of GSK-3 with lithium chloride interfered with p21(Cip1) degradation and increased p21(Cip1) protein about 10-fold in EC and cardiac myocytes (30 mm, p < 0.001). These data indicate that GSK-3 triggers p21(Cip1) degradation. In contrast, stimulation of AKT increases p21(Cip1) via inhibitory phosphorylation of GSK-3.

Functional cooperation among Ras, STAT5, and phosphatidylinositol 3-kinase is required for full oncogenic activities of BCR/ABL in K562 cells

BCR/ABL tyrosine kinase generated from the chromosomal translocation t(9;22) causes chronic myelogenous leukemia and acute lymphoblastic leukemia. To examine the roles of BCR/ABL-activated individual signaling molecules and their cooperation in leukemogenesis, we inducibly expressed a dominant negative (DN) form of Ras, phosphatidylinositol 3-kinase, and STAT5 alone or in combination in p210 BCR/ABL-positive K562 cells. The inducibly expressed DN Ras (N17), STAT5 (694F), and DN phosphatidylinositol 3-kinase (Delta p85) inhibited the growth by 90, 55, and 40%, respectively. During the growth inhibition, the expression of cyclin D2 and cyclin D3 was suppressed by N17, 694F, or Delta p85; that of cyclin E by N17; and that of cyclin A by Delta p85. In addition, N17 induced apoptosis in a small proportion of K562, whereas 694F and Delta p85 were hardly effective. In contrast, coexpression of two DN mutants in any combinations induced severe apoptosis. During these cultures, the expression of Bcl-2 was suppressed by N17, 694F, or Delta p85, and that of Bcl-XL by N17. Furthermore, although K562 was resistant to interferon-alpha- and dexamethasone-induced apoptosis, disruption of one pathway by N17, 694F, or Delta p85 sensitized K562 to these reagents. These results suggested that cooperation among these molecules is required for full leukemogenic activities of BCR/ABL.

p21(Cip1) Promotes cyclin D1 nuclear accumulation via direct inhibition of nuclear export

There is increasing evidence that p21(Cip1) and p27(Kip1) are requisite positive regulators of cyclin D1.CDK4 assembly and nuclear accumulation. Both Cip and Kip proteins can promote nuclear accumulation of cyclin D1, but the underlying mechanism has not been elucidated. We now provide evidence that p21(Cip1) promotes the nuclear accumulation of cyclin D1 complexes via inhibition of cyclin D1 nuclear export. In vivo, we demonstrate that p21(Cip1) can inhibit glycogen synthase kinase 3 beta-triggered cyclin D1 nuclear export and phosphorylation-dependent nucleocytoplasmic shuttling. Furthermore, we find that cyclin D1 nuclear accumulation in p21/p27 null cells can be restored through inhibition of CRM1-dependent nuclear export. The ability of p21(Cip1) to inhibit cyclin D1 nuclear export correlates with its ability to bind to Thr-286-phosphorylated cyclin D1 and thereby prevents cyclin D1.CRM1 association.

The coupling of cell growth to the cell cycle

The development of a complex multicellular organism requires a coordination of growth and cell division under the control of patterning mechanisms. Studies in yeast have pioneered our understanding of the relationship between growth and cell division. In recent years, many of the pathways that regulate growth in multicellular eukaryotes have been identified. This work has revealed interesting and unexpected relationships between mechanisms that regulate growth and the cell cycle machinery.

PKB/AKT: functional insights from genetic models

Since its discovery 10 years ago, the potential functions of protein kinase B (PKB)/AKT have been catalogued with increasing efficiency. The physiological relevance of some of the proposed mechanisms by which PKB/AKT mediates many of its effects has been questioned, and recent work using new reagents and approaches has revealed some cracks in our understanding of this important molecule, and also hinted that these effects may involve other players.