Downregulation of praja2 restrains endocytosis and boosts tyrosine kinase receptors in kidney cancer.

Clear cell renal cell carcinoma (ccRCC) is the most common kidney cancer in the adult population. Late diagnosis, resistance to therapeutics and recurrence of metastatic lesions account for the highest mortality rate among kidney cancer patients. Identifying novel biomarkers for early cancer detection and elucidating the mechanisms underlying ccRCC will provide clues to treat this aggressive malignant tumor. Here, we report that the ubiquitin ligase praja2 forms a complex with-and ubiquitylates the AP2 adapter complex, contributing to receptor endocytosis and clearance. In human RCC tissues and cells, downregulation of praja2 by oncogenic miRNAs (oncomiRs) and the proteasome markedly impairs endocytosis and clearance of the epidermal growth factor receptor (EGFR), and amplifies downstream mitogenic and proliferative signaling. Restoring praja2 levels in RCC cells downregulates EGFR, rewires cancer cell metabolism and ultimately inhibits tumor cell growth and metastasis. Accordingly, genetic ablation of praja2 in mice upregulates RTKs (i.e. EGFR and VEGFR) and induces epithelial and vascular alterations in the kidney tissue.In summary, our findings identify a regulatory loop between oncomiRs and the ubiquitin proteasome system that finely controls RTKs endocytosis and clearance, positively impacting mitogenic signaling and kidney cancer growth.

Carbonic anhydrase IX subcellular localization in normoxic and hypoxic SH-SY5Y neuroblastoma cells is assisted by its C-terminal protein interaction domain.

The human carbonic anhydrase IX (CA IX) is a hypoxia-induced transmembrane protein belonging to the α-CA enzyme family. It has a crucial role in pH regulation in hypoxic cells and acts by buffering intracellular acidosis induced by hypoxia. Indeed, it is frequently expressed in cancer cells, where it contributes to tumor progression. CA IX is also able to localize in the nucleus, where it contributes to 47S rRNA precursor genes transcription; however, the mechanisms assisting its nuclear translocation still remain unclear. The aim of our study was to deepen the understanding of the mechanisms involved in CA IX subcellular distribution. To this purpose, we implemented a site-directed mutagenesis approach targeting the C-terminal domain of CA IX and evaluated the subcellular distribution of the wild-type and mutant proteins in the SH-SY5Y cell line. The mutant proteins showed impaired binding ability and altered subcellular distribution in both normoxic and hypoxic conditions. Our data suggest that CA IX nuclear translocation depends on its transit through the secretory and the endocytic pathways.

Targeted inhibition of ubiquitin signaling reverses metabolic reprogramming and suppresses glioblastoma growth.

Glioblastoma multiforme (GBM) is the most frequent and aggressive form of primary brain tumor in the adult population; its high recurrence rate and resistance to current therapeutics urgently demand a better therapy. Regulation of protein stability by the ubiquitin proteasome system (UPS) represents an important control mechanism of cell growth. UPS deregulation is mechanistically linked to the development and progression of a variety of human cancers, including GBM. Thus, the UPS represents a potentially valuable target for GBM treatment. Using an integrated approach that includes proteomics, transcriptomics and metabolic profiling, we identify praja2, a RING E3 ubiquitin ligase, as the key component of a signaling network that regulates GBM cell growth and metabolism. Praja2 is preferentially expressed in primary GBM lesions expressing the wild-type isocitrate dehydrogenase 1 gene (IDH1). Mechanistically, we found that praja2 ubiquitylates and degrades the kinase suppressor of Ras 2 (KSR2). As a consequence, praja2 restrains the activity of downstream AMP-dependent protein kinase in GBM cells and attenuates the oxidative metabolism. Delivery in the brain of siRNA targeting praja2 by transferrin-targeted self-assembling nanoparticles (SANPs) prevented KSR2 degradation and inhibited GBM growth, reducing the size of the tumor and prolonging the survival rate of treated mice. These data identify praja2 as an essential regulator of cancer cell metabolism, and as a potential therapeutic target to suppress GBM growth.

The TBC1D31/praja2 complex controls primary ciliogenesis through PKA-directed OFD1 ubiquitylation.

The primary cilium is a microtubule-based sensory organelle that dynamically links signalling pathways to cell differentiation, growth, and development. Genetic defects of primary cilia are responsible for genetic disorders known as ciliopathies. Orofacial digital type I syndrome (OFDI) is an X-linked congenital ciliopathy caused by mutations in the OFD1 gene and characterized by malformations of the face, oral cavity, digits and, in the majority of cases, polycystic kidney disease. OFD1 plays a key role in cilium biogenesis. However, the impact of signalling pathways and the role of the ubiquitin-proteasome system (UPS) in the control of OFD1 stability remain unknown. Here, we identify a novel complex assembled at centrosomes by TBC1D31, including the E3 ubiquitin ligase praja2, protein kinase A (PKA), and OFD1. We show that TBC1D31 is essential for ciliogenesis. Mechanistically, upon G-protein-coupled receptor (GPCR)-cAMP stimulation, PKA phosphorylates OFD1 at ser735, thus promoting OFD1 proteolysis through the praja2-UPS circuitry. This pathway is essential for ciliogenesis. In addition, a non-phosphorylatable OFD1 mutant dramatically affects cilium morphology and dynamics. Consistent with a role of the TBC1D31/praja2/OFD1 axis in ciliogenesis, alteration of this molecular network impairs ciliogenesis in vivo in Medaka fish, resulting in developmental defects. Our findings reveal a multifunctional transduction unit at the centrosome that links GPCR signalling to ubiquitylation and proteolysis of the ciliopathy protein OFD1, with important implications on cilium biology and development. Derangement of this control mechanism may underpin human genetic disorders.

Impact of Magnetic Stimulation on Periodontal Ligament Stem Cells.

The aim of this study was to evaluate the effect of a time-dependent magnetic field on the biological performance of periodontal ligament stem cells (PDLSCs). A Western blot analysis and Alamar Blue assay were performed to investigate the proliferative capacity of magnetically stimulated PDLSCs (PDLSCs MAG) through the study of the MAPK cascade (p-ERK1/2). The observation of ALP levels allowed the evaluation of the effect of the magnetic field on osteogenic differentiation. Metabolomics data, such as oxygen consumption rate (OCR), extracellular acidification rate (ECAR) and ATP production provided an overview of the PDLSCs MAG metabolic state. Moreover, the mitochondrial state was investigated through confocal laser scanning microscopy. Results showed a good viability for PDLSCs MAG. Magnetic stimulation can activate the ERK phosphorylation more than the FGF factor alone by promoting a better cell proliferation. Osteogenic differentiation was more effectively induced by magnetic stimulation. The metabolic panel indicated significant changes in the mitochondrial cellular respiration of PDLSCs MAG. The results suggested that periodontal ligament stem cells (PDLSCs) can respond to biophysical stimuli such as a time-dependent magnetic field, which is able to induce changes in cell proliferation and differentiation. Moreover, the magnetic stimulation also produced an effect on the cell metabolic profile. Therefore, the current study demonstrated that a time-dependent magnetic stimulation may improve the regenerative properties of PDLSCs.

Feedback inhibition of cAMP effector signaling by a chaperone-assisted ubiquitin system.

Activation of G-protein coupled receptors elevates cAMP levels promoting dissociation of protein kinase A (PKA) holoenzymes and release of catalytic subunits (PKAc). This results in PKAc-mediated phosphorylation of compartmentalized substrates that control central aspects of cell physiology. The mechanism of PKAc activation and signaling have been largely characterized. However, the modes of PKAc inactivation by regulated proteolysis were unknown. Here, we identify a regulatory mechanism that precisely tunes PKAc stability and downstream signaling. Following agonist stimulation, the recruitment of the chaperone-bound E3 ligase CHIP promotes ubiquitylation and proteolysis of PKAc, thus attenuating cAMP signaling. Genetic inactivation of CHIP or pharmacological inhibition of HSP70 enhances PKAc signaling and sustains hippocampal long-term potentiation. Interestingly, primary fibroblasts from autosomal recessive spinocerebellar ataxia 16 (SCAR16) patients carrying germline inactivating mutations of CHIP show a dramatic dysregulation of PKA signaling. This suggests the existence of a negative feedback mechanism for restricting hormonally controlled PKA activities.

Epigenetic Silencing of THY1 Tracks the Acquisition of the Notch1-EGFR Signaling in a Xenograft Model of CD44+/CD24low/CD90+ Myoepithelial Cells.

The surface glycoprotein THY is a marker of myoepithelial precursor cells, which are basal cells with epithelial-mesenchymal intermediate phenotype originating from the ectoderm. Myoepithelial precursor cells are lost during progression from in situ to invasive carcinoma. To define the functional role of Thy1-positive cells within the myoepithelial population, we tracked Thy1 expression in human breast cancer samples, isolated THY1-positive myoepithelial progenitor cells (CD44+/CD24low/CD90+), and established long-term cultures (parental cells). Parental cells were used to generate a xenograft model to examine Thy1 expression during tumor formation. Post-transplantation cell cultures lost THY1 expression through methylation at the THY1 locus and this is associated with an increase in EGFR and NOTCH1 transcript levels. Thy1-low cells are sensitive to the EGFR/HER2 dual inhibitor lapatinib. High THY1 expression is associated with poorer relapse-free survival in patients with breast cancer. THY1 methylation may track the shift of bipotent progenitors into differentiated cells. Thy1 is a good candidate biomarker in basal-like breast cancer. IMPLICATIONS: Our findings provide evidence that THY1 expression is lost in xenografts due to promoter methylation. Thy1-low cells with increased EGFR and Notch1 expression are responsive to target therapy. Because DNA methylation is often altered in early cancer development, candidate methylation markers may be exploited as biomarkers for basal-like breast cancer.

miR-34 modulates apoptotic gene expression in Ingenol mebutate treated keloid fibroblasts.

Keloids are benign skin tumors that develop in individuals who have a positive family history of keloid disorders. Keloids are characterized by a deregulated wound­healing process, atypical fibroblasts with extreme deposition of extracellular matrix components, particularly collagen, increased cell proliferation and associated failure of apoptosis. Recently ingenol­mebutate has been used as a novel agent with anti­proliferative activity on human keloids as an alternative treatment option in patients, once conventional therapies have failed. We hypothesized that microRNAs (miR/miRNA) may be involved in the balance between lesion formation and repair. A comprehensive understanding of the molecular mechanism underlying the Ingenol­mebutate response in keloid fibroblast following Ingenol­mebutate exposure has been established previously. Therefore, the present study analyzed changes in miRNAs and apoptotic gene regulation in Ingenol­mebutate treated keloid fibroblast, by reverse transcription­quantitative polymerase chain reaction and a DNA fragmentation assay. The range of upregulated miRNAs and downregulated genes encoding cell death appeared to be associated with the degree of the morphological alterations in Ingenol­mebutate treated keloids. In particular, the upregulation of miR­34a was detected in keloid fibroblasts during and following Ingenol­mebutate exposure. Keloid fibroblasts that overexpressed miR­34a showed differential expression of genes involved in the apoptotic signaling pathway such as p53. In conclusion, the Ingenol­mebutate treatment used here was effective in reducing keloid fibroblast growth in cell culture experiments and the expression of particular miRNAs modulated the pro­apoptotic gene expression following Ingenol-mebutate treatment.

Counterregulation of cAMP-directed kinase activities controls ciliogenesis.

The primary cilium emanates from the cell surface of growth-arrested cells and plays a central role in vertebrate development and tissue homeostasis. The mechanisms that control ciliogenesis have been extensively explored. However, the intersection between GPCR signaling and the ubiquitin pathway in the control of cilium stability are unknown. Here we observe that cAMP elevation promotes cilia resorption. At centriolar satellites, we identify a multimeric complex nucleated by PCM1 that includes two kinases, NEK10 and PKA, and the E3 ubiquitin ligase CHIP. We show that NEK10 is essential for ciliogenesis in mammals and for the development of medaka fish. PKA phosphorylation primes NEK10 for CHIP-mediated ubiquitination and proteolysis resulting in cilia resorption. Disarrangement of this control mechanism occurs in proliferative and genetic disorders. These findings unveil a pericentriolar kinase signalosome that efficiently links the cAMP cascade with the ubiquitin-proteasome system, thereby controlling essential aspects of ciliogenesis.

Akap1 Regulates Vascular Function and Endothelial Cells Behavior.

MitoAKAPs (mitochondrial A kinase anchoring proteins), encoded by the Akap1 gene, regulate multiple cellular processes governing mitochondrial homeostasis and cell viability. Although mitochondrial alterations have been associated to endothelial dysfunction, the role of mitoAKAPs in the vasculature is currently unknown. To test this, postischemic neovascularization, vascular function, and arterial blood pressure were analyzed in Akap1 knockout mice (Akap1-/- ) and their wild-type (wt) littermates. Primary cultures of aortic endothelial cells (ECs) were also obtained from Akap1-/- and wt mice, and ECs migration, proliferation, survival, and capillary-like network formation were analyzed under different experimental conditions. After femoral artery ligation, Akap1-/- mice displayed impaired blood flow and functional recovery, reduced skeletal muscle capillary density, and Akt phosphorylation compared with wt mice. In Akap1-/- ECs, a significant enhancement of hypoxia-induced mitophagy, mitochondrial dysfunction, reactive oxygen species production, and apoptosis were observed. Consistently, capillary-like network formation, migration, proliferation, and AKT phosphorylation were reduced in Akap1-/- ECs. Alterations in Akap1-/- ECs behavior were also confirmed in Akap1-/- mice, which exhibited a selective reduction in acetylcholine-induced vasorelaxation in mesenteric arteries and a mild but significant increase in arterial blood pressure levels compared with wt. Finally, overexpression of a constitutively active Akt mutant restored vascular reactivity and ECs function in Akap1-/- conditions. These results demonstrate the important role of mitoAKAPs in the modulation of multiple ECs functions in vivo and in vitro, suggesting that mitochondria-dependent regulation of ECs might represent a novel therapeutic approach in cardiovascular diseases characterized by endothelial dysfunction.

Mitochondrial AKAP1 supports mTOR pathway and tumor growth.

Mitochondria are the powerhouses of energy production and the sites where metabolic pathway and survival signals integrate and focus, promoting adaptive responses to hormone stimulation and nutrient availability. Increasing evidence suggests that mitochondrial bioenergetics, metabolism and signaling are linked to tumorigenesis. AKAP1 scaffolding protein integrates cAMP and src signaling on mitochondria, regulating organelle biogenesis, oxidative metabolism and cell survival. Here, we provide evidence that AKAP1 is a transcriptional target of Myc and supports the growth of cancer cells. We identify Sestrin2, a leucine sensor and inhibitor of the mammalian target of rapamycin (mTOR), as a novel component of the complex assembled by AKAP1 on mitochondria. Downregulation of AKAP1 impaired mTOR pathway and inhibited glioblastoma growth. Both effects were reversed by concomitant depletion of AKAP1 and sestrin2. High levels of AKAP1 were found in a wide variety of high-grade cancer tissues. In lung cancer, AKAP1 expression correlates with high levels of Myc, mTOR phosphorylation and reduced patient survival. Collectively, these data disclose a previously unrecognized role of AKAP1 in mTOR pathway regulation and cancer growth. AKAP1/mTOR signal integration on mitochondria may provide a new target for cancer therapy.

Binding of carbonic anhydrase IX to 45S rDNA genes is prevented by exportin-1 in hypoxic cells.

Carbonic anhydrase IX (CA IX) is a surrogate marker of hypoxia, involved in survival and pH regulation in hypoxic cells. We have recently characterized its interactome, describing a set of proteins interacting with CA IX, mainly in hypoxic cells, including several members of the nucleocytoplasmic shuttling apparatuses. Accordingly, we described complex subcellular localization for this enzyme in human cells, as well as the redistribution of a carbonic anhydrase IX pool to nucleoli during hypoxia. Starting from this evidence, we analyzed the possible contribution of carbonic anhydrase IX to transcription of the 45 S rDNA genes, a process occurring in nucleoli. We highlighted the binding of carbonic anhydrase IX to nucleolar chromatin, which is regulated by oxygen levels. In fact, CA IX was found on 45 S rDNA gene promoters in normoxic cells and less represented on these sites, in hypoxic cells and in cells subjected to acetazolamide-induced acidosis. Both conditions were associated with increased representation of carbonic anhydrase IX/exportin-1 complexes in nucleoli. 45 S rRNA transcript levels were accordingly downrepresented. Inhibition of nuclear export by leptomycin B suggests a model in which exportin-1 acts as a decoy, in hypoxic cells, preventing carbonic anhydrase IX association with 45 S rDNA gene promoters.

Proteolytic control of neurite outgrowth inhibitor NOGO-A by the cAMP/PKA pathway.

Protein kinase A (PKA) controls major aspects of neurite outgrowth and morphogenesis and plays an essential role in synaptic plasticity and memory. However, the molecular mechanism(s) of PKA action on neurite sprouting and activity are still unknown. Here, we report that in response to neurotrophin or cAMP stimulation the RING ligase praja2 ubiquitinates and degrades NOGO-A, a major inhibitor of neurite outgrowth in mammalian brain. Genetic silencing of praja2 severely inhibited neurite extension of differentiating neuroblastoma cells and mesencephalic neurons and axon outgrowth and sprouting of striatal terminals in developing rat brain. This phenotype was rescued when both praja2 and NOGO-A were depleted, suggesting that NOGO-A is, indeed, a biologically relevant target of praja2 in neuronal cells. Our findings unveil a novel mechanism that functionally couples cAMP signaling with the proteolytic turnover of NOGO-A, positively impacting on neurite outgrowth in mammalian brain.

T cell activation induces CuZn superoxide dismutase (SOD)-1 intracellular re-localization, production and secretion.

Reactive oxygen species (ROS) behave as second messengers in signal transduction for a series of receptor/ligand interactions. A major regulatory role is played by hydrogen peroxide (H2O2), more stable and able to freely diffuse through cell membranes. Copper-zinc superoxide dismutase (CuZn-SOD)-1 is a cytosolic enzyme involved in scavenging oxygen radicals to H2O2 and molecular oxygen, thus representing a major cytosolic source of peroxides. Previous studies suggested that superoxide anion and H2O2 generation are involved in T cell receptor (TCR)-dependent signaling. Here, we describe that antigen-dependent activation of human T lymphocytes significantly increased extracellular SOD-1 levels in lymphocyte cultures. This effect was accompanied by the synthesis of SOD-1-specific mRNA and by the induction of microvesicle SOD-1 secretion. It is of note that SOD-1 increased its concentration specifically in T cell population, while no significant changes were observed in the "non-T" cell counterpart. Moreover, confocal microscopy showed that antigen-dependent activation was able to modify SOD-1 intracellular localization in T cells. Indeed, was observed a clear SOD-1 recruitment by TCR clusters. The ROS scavenger N-acetylcysteine (NAC) inhibited this phenomenon. Further studies are needed to define whether SOD-1-dependent superoxide/peroxide balance is relevant for regulation of T cell activation, as well as in the functional cross talk between immune effectors.

Proteolysis of MOB1 by the ubiquitin ligase praja2 attenuates Hippo signalling and supports glioblastoma growth.

Human glioblastoma is the most frequent and aggressive form of brain tumour in the adult population. Proteolytic turnover of tumour suppressors by the ubiquitin-proteasome system is a mechanism that tumour cells can adopt to sustain their growth and invasiveness. However, the identity of ubiquitin-proteasome targets and regulators in glioblastoma are still unknown. Here we report that the RING ligase praja2 ubiquitylates and degrades Mob, a core component of NDR/LATS kinase and a positive regulator of the tumour-suppressor Hippo cascade. Degradation of Mob through the ubiquitin-proteasome system attenuates the Hippo cascade and sustains glioblastoma growth in vivo. Accordingly, accumulation of praja2 during the transition from low- to high-grade glioma is associated with significant downregulation of the Hippo pathway. These findings identify praja2 as a novel upstream regulator of the Hippo cascade, linking the ubiquitin proteasome system to deregulated glioblastoma growth.

Characterization of carbonic anhydrase IX interactome reveals proteins assisting its nuclear localization in hypoxic cells.

Carbonic anhydrase IX (CA IX) is a transmembrane protein affecting pH regulation, cell migration/invasion, and survival in hypoxic tumors. Although the pathways related to CA IX have begun to emerge, molecular partners mediating its functions remain largely unknown. Here we characterize the CA IX interactome in hypoxic HEK-293 cells. Most of the identified CA IX-binding partners contain the HEAT/ARM repeat domain and belong to the nuclear transport machinery. We show that the interaction with two of these proteins, namely XPO1 exportin and TNPO1 importin, occurs via the C-terminal region of CA IX and increases with protein phosphorylation. We also demonstrate that nuclear CA IX is enriched in hypoxic cells and is present in renal cell carcinoma tissues. These data place CA IX among the cell-surface signal transducers undergoing nuclear translocation. Accordingly, CA IX interactome involves also CAND1, which participates in both gene transcription and assembly of SCF ubiquitin ligase complexes. It is noteworthy that down-regulation of CAND1 leads to decreased CA IX protein levels apparently via affecting its stability. Our findings provide the first evidence that CA IX interacts with proteins involved in nuclear/cytoplasmic transport, gene transcription, and protein stability, and suggest the existence of nuclear CA IX protein subpopulations with a potential intracellular function, distinct from the crucial CA IX role at the cell surface.

MDR1-P-glycoprotein behaves as an oncofetal protein that promotes cell survival in gastric cancer cells.

P-glycoprotein (P-gp), traditionally linked to cancer poor prognosis and multidrug resistance, is undetectable in normal gastric mucosa and overexpressed in gastric cancer (GC). We propose that P-gp may be involved in Helicobacter pylori (Hp)-related gastric carcinogenesis by inhibiting apoptosis. Aim of the study was to evaluate the expression of P-gp in fetal stomach and in Hp-related gastric carcinogenesis, the epigenetic control of the multi-drug resistance-1 (MDR1) gene, the localization and interaction between P-gp and Bcl-x(L) and the effect of the selective silencing of P-gp on cell survival. P-gp and Bcl-xl expression was evaluated by immunohistochemistry on 28 spontaneously abortive human fetuses, 66 Hp-negative subjects, 138 Hp-positive chronic gastritis (CG) of whom 28 with intestinal metaplasia (IM) and 45 intestinal type GCs. P-gp/Bcl-x(L) colocalization was investigated by confocal immunofluorescence microscopy and protein-protein interaction by co-immunoprecipitation, in basal conditions and after stress-induced apoptosis, in GC cell lines AGS and MKN-28 and hepatocellular carcinoma cell line Hep-G2. The role of P-gp in controlling apoptosis was evaluated by knocking down its expression with a specific small interfering RNAs in stressed AGS and MKN-28 cell lines. P-gp is expressed in the gastric mucosa of all human fetuses while, it is undetectable in adult normal mucosa and re-expressed in 30/110 Hp-positive non-IM-CG, 28/28 IM-CG and 40/45 GCs. P-gp expression directly correlates with that of Bcl-x(L) and with the promoter hypomethylation of the MDR1 gene. In GC cell lines, P-gp is localized on the plasma membrane and mitochondria where it colocalizes with Bcl-x(L). Co-immunoprecipitation confirms the physical interaction between P-gp and Bcl-x(L) in AGS, MKN-28 and Hep-G2, at both basal level and after stress-induced apoptosis. The selective silencing of P-gp sensitizes GC cells to stress-induced apoptosis. P-gp behaves as an oncofetal protein that, by cross-talking with Bcl-x(L), acts as an anti-apoptotic agent in Hp-related gastric carcinogenesis.

Mitochondrial localization unveils a novel role for GRK2 in organelle biogenesis.

Metabolic stimuli such as insulin and insulin like growth factor cause cellular accumulation of G protein coupled receptor kinase 2 (GRK2), which in turn is able to induce insulin resistance. Here we show that in fibroblasts, GRK2 is able to increase ATP cellular content by enhancing mitochondrial biogenesis; also, it antagonizes ATP loss after hypoxia/reperfusion. Interestingly, GRK2 is able to localize in the mitochondrial outer membrane, possibly through one region within the RGS homology domain and one region within the catalytic domain. In vivo, GRK2 removal from the skeletal muscle results in reduced ATP production and impaired tolerance to ischemia. Our data show a novel sub-cellular localization of GRK2 in the mitochondria and an unexpected role in regulating mitochondrial biogenesis and ATP generation.

Tyr phosphatase-mediated P-ERK inhibition suppresses senescence in EIA + v-raf transformed cells, which, paradoxically, are apoptosis-protected in a MEK-dependent manner.

Activation of the Ras-Raf-extracellular signal-regulated kinase (ERK) pathway causes not only proliferation and suppression of apoptosis but also the antioncogenic response of senescence. How these contrasting effects are reconciled to achieve cell transformation and cancer formation is poorly understood. In a system of two-step carcinogenesis (dedifferentiated PC EIA, transformed PC EIA-polyoma-middle T [PC EIA + Py] and PC EIA-v-raf [PC EIA + raf] cells], v-raf cooperated with EIA by virtue of a strong prosurvival effect, not elicited by Py-middle T, evident toward serum-deprivation-and H(2)O(2)-induced apoptosis. Apoptosis was detected by DNA fragmentation and annexin V staining. The prosurvival function of v-raf was, in part, mitogen-activated protein kinase/ERK kinase (MEK)-dependent, as shown by pharmacological MEK inhibition. The MEK-dependent antiapoptotic effect of v-raf was exerted despite a lower level of P-ERK1/2 in EIA + raf cells with respect to EIA + Py/EIA cells, which was dependent on a high tyrosine phosphatase activity, as shown by orthovanadate blockade. An ERK1/2 tyrosine phosphatase was likely involved. The high tyrosine phosphatase activity was instrumental to the complete suppression of senescence, detected by ß-galactosidase activity, because tyrosine phosphatase blockade induced senescence in EIA + raf but not in EIA + Py cells. High tyrosine phosphatase activity and evasion from senescence were confirmed in an anaplastic thyroid cancer cell line. Therefore, besides EIA, EIA + raf cells suppress senescence through a new mechanism, namely, phosphatase-mediated P-ERK1/2 inhibition, but, paradoxically, retain the oncogenic effects of the Raf-ERK pathway. We propose that the survival effect of Raf is not a function of absolute P-ERK1/2 levels at a given time but is rather dynamically dependent on greater variations after an apoptotic stimulus.

Control of PKA stability and signalling by the RING ligase praja2.

Activation of G-protein-coupled receptors (GPCRs) mobilizes compartmentalized pulses of cyclic AMP. The main cellular effector of cAMP is protein kinase A (PKA), which is assembled as an inactive holoenzyme consisting of two regulatory (R) and two catalytic (PKAc) subunits. cAMP binding to R subunits dissociates the holoenzyme and releases the catalytic moiety, which phosphorylates a wide array of cellular proteins. Reassociation of PKAc and R components terminates the signal. Here we report that the RING ligase praja2 controls the stability of mammalian R subunits. Praja2 forms a stable complex with, and is phosphorylated by, PKA. Rising cAMP levels promote praja2-mediated ubiquitylation and subsequent proteolysis of compartmentalized R subunits, leading to sustained substrate phosphorylation by the activated kinase. Praja2 is required for efficient nuclear cAMP signalling and for PKA-mediated long-term memory. Thus, praja2 regulates the total concentration of R subunits, tuning the strength and duration of PKA signal output in response to cAMP.