The human tumor suppressor arf interacts with spinophilin/neurabin II, a type 1 protein-phosphatase-binding protein

SPINOPHILIN: a multiplayer tumor suppressor

SPINOPHILIN (SPN, PPP1R9B or NEURABIN-2) is a multifunctional protein that regulates protein-protein interactions in different cell signaling pathways. SPN is also one of the regulatory subunits of protein phosphatase 1 (PP1), implicated in the dephosphorylation of retinoblastoma protein (pRB) during cell cycle. The SPN gene has been described as a tumor suppressor in different human tumor contexts, in which low levels of SPN are correlated with a higher grade and worse prognosis. In addition, mutations of the SPN protein have been reported in human tumors. Recently, an oncogenic mutation of SPN, A566V, was described, which affects both the SPN-PP1 interaction and the phosphatase activity of the holoenzyme, and promotes p53-dependent tumorigenesis by increasing the cancer stem cell (CSC) pool in breast tumors. Thus, the loss or mutation of SPN could be late events that promotes tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Role of the Holoenzyme PP1-SPN in the Dephosphorylation of the RB Family of Tumor Suppressors During Cell Cycle

Simple Summary

Cell cycle progression is highly regulated by modulating the phosphorylation status of retinoblastoma (RB) family proteins. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Here, we describe the regulation of the phosphorylation status of RB family proteins, giving importance not only to their inactivation by phosphorylation but also to their dephosphorylation to restore the cell cycle.

Abstract

Cell cycle progression is highly regulated by modulating the phosphorylation status of the retinoblastoma protein (pRB) and the other two members of the RB family, p107 and p130. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Phosphatases are holoenzymes formed by a catalytic subunit and a regulatory protein with substrate specificity. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Moreover, SPN has been described as a tumor suppressor dependent on PP1 in lung and breast tumors, where it promotes tumorigenesis by increasing the cancer stem cell pool. Therefore, a connection between the cell cycle and stem cell biology has also been proposed via SPN/PP1/RB proteins.

Mutation of SPINOPHILIN (PPP1R9B) found in human tumors promotes the tumorigenic and stemness properties of cells

Rationale: SPINOPHILIN (SPN, PPP1R9B) is an important tumor suppressor involved in the progression and malignancy of different tumors depending on its association with protein phosphatase 1 (PP1) and the ability of the PP1-SPN holoenzyme to dephosphorylate retinoblastoma (pRB).

Methods: We performed a mutational analysis of SPN in human tumors, focusing on the region of interaction with PP1 and pRB. We explored the effect of the SPN-A566V mutation in an immortalized non-tumorigenic cell line of epithelial breast tissue, MCF10A, and in two different p53-mutated breast cancer cells lines, T47D and MDA-MB-468.

Results: We characterized an oncogenic mutation of SPN found in human tumor samples, SPN-A566V, that affects both the SPN-PP1 interaction and its phosphatase activity. The SPN-A566V mutation does not affect the interaction of the PP1-SPN holoenzyme with pocket proteins pRB, p107 and p130, but it affects its ability to dephosphorylate them during G0/G1 and G1, indicating that the PP1-SPN holoenzyme regulates cell cycle progression. SPN-A566V also promoted stemness, establishing a connection between the cell cycle and stem cell biology via pocket proteins and PP1-SPN regulation. However, only cells with both SPN-A566V and mutant p53 have increased tumorigenic and stemness properties.

Conclusions: SPN-A566V, or other equivalent mutations, could be late events that promote tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Dynamics of p14ARF and Focal Adhesion Kinase-Mediated Autophagy in Cancer

It has been widely shown that the focal adhesion kinase (FAK) is involved in nearly every aspect of cancer, from invasion to metastasis to epithelial–mesenchymal transition and maintenance of cancer stem cells. FAK has been shown to interact with p14ARF (alternative reading frame)—a well-established tumor suppressor—and functions in the negative regulation of cancer through both p53-dependent and -independent pathways. Interestingly, both FAK and ARF (human and mouse counterpart) proteins, as well as p53, are involved in autophagy—a process of “self-digestion”—whose main function is the recycling of cellular components and quality control of proteins and organelles. In the last years, an unexpected role of p14ARF in the survival of cancer cells has been underlined in different cellular contexts, suggesting a novel pro-oncogenic function of this protein. In this review, the mechanisms whereby ARF and FAK control autophagy are presented, as well as the role of autophagy in cell migration and spreading. Integrated investigation of these cell functions is extremely important to understand the mechanism of the basis of cell transformation and migration and thus cancer development.

PKC Dependent p14ARF Phosphorylation on Threonine 8 Drives Cell Proliferation

ARF role as tumor suppressor has been challenged in the last years by several findings of different groups ultimately showing that its functions can be strictly context dependent. We previously showed that ARF loss in HeLa cells induces spreading defects, evident as rounded morphology of depleted cells, accompanied by a decrease of phosphorylated Focal Adhesion Kinase (FAK) protein levels and anoikis. These data, together with previous finding that a PKC dependent signalling pathway can lead to ARF stabilization, led us to the hypothesis that ARF functions in cell proliferation might be regulated by phosphorylation. In line with this, we show here that upon spreading ARF is induced through PKC activation. A constitutive-phosphorylated ARF mutant on the conserved threonine 8 (T8D) is able to mediate both cell spreading and FAK activation. Finally, ARF-T8D expression confers growth advantage to cells thus leading to the intriguing hypothesis that ARF phosphorylation could be a mechanism through which pro-proliferative or anti proliferative signals could be transduced inside the cells in both physiological and pathological conditions.

p14ARF interacts with the focal adhesion kinase and protects cells from anoikis

The ARF protein functions as an important sensor of hyper-proliferative stimuli restricting cell proliferation through both p53-dependent and -independent pathways. Although to date the majority of studies on ARF have focused on its anti-proliferative role, few studies have addressed whether ARF may also have pro-survival functions. Here we show for the first time that during the process of adhesion and spreading ARF re-localizes to sites of active actin polymerization and to focal adhesion points where it interacts with the phosphorylated focal adhesion kinase. In line with its recruitment to focal adhesions, we observe that hampering ARF function in cancer cells leads to gross defects in cytoskeleton organization resulting in apoptosis through a mechanism dependent on the Death-Associated Protein Kinase. Our data uncover a novel function for p14ARF in protecting cells from anoikis that may reflect its role in anchorage independence, a hallmark of malignant tumor cells.

Low spinophilin expression enhances aggressive biological behavior of breast cancer

Spinophilin, a putative tumor suppressor gene, has been shown to be involved in the pathogenesis of certain types of cancer, but its role has never been systematically explored in breast cancer. In this study, we determined for the first time the expression pattern of spinophilin in human breast cancer molecular subtypes (n = 489) and correlated it with survival (n = 921). We stably reduced spinophilin expression in breast cancer cells and measured effects on cellular growth, apoptosis, anchorage-independent growth, migration, invasion and self-renewal capacity in vitro and metastases formation in vivo. Microarray profiling was used to determine the most abundantly expressed genes in spinophilin-silenced breast cancer cells. Spinophilin expression was significantly lower in basal-like breast cancer (p<0.001) and an independent poor prognostic factor in breast cancer patients (hazard ratio = 1.93, 95% confidence interval: 1.24-3.03; p = 0.004) A reduction of spinophilin levels increased cellular growth in breast cancer cells (p<0.05), without influencing activation of apoptosis. Anchorage-independent growth, migration and self-renewal capacity in vitro and metastatic potential in vivo were also significantly increased in spinophilin-silenced cells (p<0.05). Finally, we identified several differentially expressed genes in spinophilin-silenced cells. According to our data, low levels of spinophilin are associated with aggressive behavior of breast cancer.

Angiotensin IV upregulates the activity of protein phosphatase 1α in Neura-2A cells

The peptide angiotensin IV (Ang IV) is a derivative of angiotensin II. While insulin regulated amino peptidase (IRAP) has been proposed as a potential receptor for Ang IV, the signalling pathways of Ang IV through IRAP remain elusive. We applied high-resolution mass spectrometry to perform a systemic quantitative phosphoproteome of Neura-2A (N2A) cells treated with and without Ang IV using sta ble-isotope labeling by amino acids in cell culture (SILAC), and identified a reduction in the phosphorylation of a major Ser/Thr protein phosphorylase 1 (PP1) upon Ang IV treatment. In addition, spinophilin (spn), a PP1 regulatory protein that plays important functions in the neural system, was expressed at higher levels. Immunoblotting revealed decreased phosphorylation of p70S6 kinase (p70(S6K)) and the major cell cycle modulator retinoblastoma protein (pRB). These changes are consistent with an observed decrease in cell proliferation. Taken together, our study suggests that Ang IV functions via regulating the activity of PP1.

Spinophilin loss correlates with poor patient prognosis in advanced stages of colon carcinoma

PURPOSE

The genomic region 17q21 is frequently associated with microsatellite instability and LOH in cancer, including gastric and colorectal carcinomas. This region contains several putative tumor suppressor genes, including Brca1, NM23, prohibitin, and spinophilin (Spn, PPP1R9B, neurabin II). The scaffold protein Spn is one of the regulatory subunits of phosphatase-1 (PP1) that targets PP1 to distinct subcellular locations and couples PP1 to its target. Thus, Spn may alter cell-cycle progression via the regulation of the phosphorylation status of the retinoblastoma protein, a direct target of PP1. Therefore, we analyzed whether Spn levels were reduced in colorectal carcinomas and whether Spn levels correlated with prognosis or response to therapy.

EXPERIMENTAL DESIGN

By means of immunohistochemistry or quantitative PCR, we studied the levels of Spn in stages II, III, and IV colorectal carcinoma tumors and correlated to other clinicopathologic features as well as prognosis or response to therapy.

RESULTS

Spn was lost in a percentage of human gastric, small intestine, and colorectal carcinomas. In patients with colorectal carcinoma, tumoral Spn downregulation correlated with a more aggressive histologic phenotype (poorer tumor differentiation and higher proliferative Ki67 index). Consistent with this observation, lower Spn protein expression levels were associated with faster relapse and poorer survival in patients with stage III colorectal carcinoma, particularly among those receiving adjuvant fluoropyrimidine therapy. We validated this result in an independent cohort of patients with metastatic colorectal carcinoma treated with standard chemotherapy. Although patients that achieved an objective tumor response exhibited Spn levels similar to nontumoral tissue, nonresponding patients showed a significant reduction in Spn mRNA levels.

CONCLUSIONS

Our data suggest that Spn downregulation contributes to a more aggressive biologic behavior, induces chemoresistance, and is associated with a poorer survival in patients with advanced stages of colorectal carcinoma.

Low expression of the putative tumour suppressor spinophilin is associated with higher proliferative activity and poor prognosis in patients with hepatocellular carcinoma

BACKGROUND

Spinophilin, a multifunctional intracellular scaffold protein, is reduced in certain types of cancer and is regarded as a novel putative tumour suppressor protein. However, the role of spinophilin in hepatocellular carcinoma (HCC) has never been explored before.

METHODS

In this study, we determined for the first time the expression pattern of spinophilin in human HCC by immunohistochemistry and quantitative reverse transcriptase-PCR analysis. In addition, we performed immunohistochemical analysis of p53, p14(ARF) and the proliferation marker Ki-67. Kaplan-Meier curves and multivariate Cox proportional models were used to study the impact on clinical outcome. Small interfering RNA (siRNA) was used to silence spinophilin and to explore the effects of reduced spinophilin expression on cellular growth.

RESULTS

In our study, complete loss of spinophilin immunoreactivity was found in 44 of 104 HCCs (42.3%) and reduced levels were found in an additional 37 (35.6%) cases. After adjusting for other prognostic factors, multivariate Cox regression analysis identified low expression of spinophilin as an independent prognostic factor with respect to disease-free (hazard ratio (HR)=1.8; 95% confidence interval (CI)=1.04-3.40; P=0.043) and cancer-specific survival (HR=2.0; CI=1.1-3.8; P=0.025). Reduced spinophilin expression significantly correlated with higher Ki-67 index in HCC (P=0.014). Reducing spinophilin levels by siRNA induced a higher cellular growth rate and increased cyclin D2 expression in tumour cells (P<0.05).

CONCLUSION

This is the first study of the expression pattern and distribution of spinophilin in HCC. According to our data, the loss of spinophilin is associated with higher proliferation and might be useful as a prognostic marker in patients with HCC.

Mimicking p14ARF phosphorylation influences its ability to restrain cell proliferation

The INK4a/ARF locus on the short arm of chromosome 9 is one of the most frequently altered loci in human cancer. It is generally accepted that ARF is involved in oncogenic checkpoint pathways by sensitizing incipient cancer cells to undergo growth arrest or apoptosis through both p53-dependent and independent pathways. While intensive studies have been focused on ARF activation at the transcriptional level, only recently mechanisms governing ARF turnover have been identified. Here, we show for the first time that p14ARF is a PKC target. Prediction analysis showed many potential phosphorylation sites in PKC consensus sequences within ARF protein, and, among them, the threonine at position 8 was the most conserved. Substitution of this threonine influences both ARF stability and localization. Furthermore, a phosphomimetic ARF mutation reduces the ability to arrest cell growth although the ability to bind MDM2 and stabilize p53 result unaffected. Thus we propose that phosphorylation of ARF in both immortalized and tumor cell lines could be a mechanism to escape ARF surveillance following proliferative and oncogenic stress.

Down-regulation of spinophilin in lung tumours contributes to tumourigenesis

The scaffold protein spinophilin (Spn, PPP1R9B) is one of the regulatory subunits of phosphatase-1a (PP1), targeting it to distinct subcellular locations and to its target. Loss of Spn reduces PPP1CA levels, thereby maintaining higher levels of phosphorylated pRb. This effect contributes to an increase in p53 activity. However, in the absence of p53, reduced levels of Spn increase the tumourigenic properties of cells. In addition, Spn knockout mice have a reduced lifespan, an increased number of tumours and increased cellular proliferation in some tissues, such as the mammary ducts. In addition, the combined loss of Spn and p53 activity leads to an increase in mammary carcinomas, confirming the functional relationship between p53 and Spn. In this paper, we report that Spn is absent in 20% and reduced in another 37% of human lung tumours. Spn reduction correlates with malignant grade. Furthermore, the loss of Spn also correlates with p53 mutations. Analysis of miRNAs in a series of lung tumours showed that miRNA106a* targeting Spn is over-expressed in some patients, correlating with decreased Spn levels. Proof-of-concept experiments over-expressing miRNA106a* or Spn shRNA in lung tumour cells showed increased tumourigenicity. In conclusion, our data showed that miRNA106a* over-expression found in lung tumours might contribute to tumourigenesis through Spn down-regulation in the absence of p53.

p14(ARF) inhibits the functions of adenovirus E1A oncoprotein

The tumour suppressor ARF (alternative reading frame) is one of the most important oncogenic stress sensors. ARF provides an 'oncogenic checkpoint' function through both p53-dependent and p53-independent mechanisms. In the present study, we demonstrate a novel p53-independent interaction between p14(ARF) and the adenovirus oncoprotein E1A. p14(ARF) inhibits E1A transcriptional function and promotes ubiquitination-dependent degradation of E1A. p14(ARF) overexpression relocalizes E1A into the nucleolus and inhibits E1A-induced cellular DNA replication independent of p53. Knockdown of endogenous p14(ARF) increases E1A transactivation. In addition, E1A can competitively inhibit ARF-Mdm2 (murine double minute 2) complex formation. These results identify a novel binding partner of p14(ARF) and reveal a mutually inhibitory interaction between p14(ARF) and E1A. We speculate that the ARF-E1A interaction may represent an additional host defence mechanism to limit viral replication. Alternatively, the interaction may allow adenovirus to sense the functional state of p53 in host cells, and fine-tune its own replication activity to prevent the triggering of a detrimental host response.

CARF: an emerging regulator of p53 tumor suppressor and senescence pathway

Replicative senescence, a major outcome of normal cells with finite lifespan, is a widely accepted in vitro model for ageing studies. Limited repair and defense mechanisms of normal cells, in addition to DNA alterations and oncogene inductions under stress, are believed to result in senescence as a protective mechanism to prevent undesirable proliferation of cells. The ARF/p53/p21(cip1/waf1) tumor suppression pathway acts as a molecular sensor and regulator of cellular stress, senescence, and immortalization. Understanding the molecular regulation of this pathway by intrinsic and extrinsic signals is extremely important to address unsolved questions in senescence and cancer. CARF was first discovered as a binding partner of ARF and has since been shown to have both ARF-dependent and -independent functions that converge to regulate p53 pathway. CARF directly binds to p53 and HDM2, and functions in a negative feedback pathway. Whereas CARF transcriptionally represses HDM2 to increase p53 activity, HDM2 in return degrades CARF. Thus, CARF may act as a novel key regulator of the p53 pathway at multiple checkpoints. The aim of this article is to discuss the current knowledge about functions of CARF and its impact on p53 pathway in regulation of senescence and carcinogenesis.

Serine phosphorylation-dependent coregulation of topoisomerase I by the p14ARF tumor suppressor

p14ARF (ARF) and topoisomerase I play central roles in cancer and have recently been shown to interact. The interaction activates topoisomerase I, an important target for camptothecin-like chemotherapeutic drugs, but the regulation of the interaction is poorly understood. We have used the H358 and H23 lung cancer cell lines and purified recombinant human topoisomerase I to demonstrate that the ARF/topoisomerase I interaction is regulated by topoisomerase I serine phosphorylation, a modification that regulates topoisomerase I activity. Both cell lines express wild-type ARF and topoisomerase I proteins at equivalent levels, but H23 topoisomerase I, unlike that of H358 cells, is largely devoid of serine phosphorylation, has low activity, and complexes poorly with ARF. The ability of H23 topoisomerase I to complex with ARF can be restored by treatment with the serine kinase, casein kinase II. Consistent with these observations, we show that the response of H23 cells to camptothecin treatment is unaffected by changes in intracellular levels of ARF. However, in H358 and PC-3 cells, which express a serine phosphorylated topoisomerase I that complexes with ARF, ectopic overexpression of ARF causes sensitization to camptothecin, and siRNA-mediated down-regulation of endogenous ARF causes desensitization to camptothecin. These biological responses correlate with increased and decreased levels, respectively, of ARF/topoisomerase I complex and DNA-bound topoisomerase I. Thus, ARF is a serine phosphorylation-dependent coregulator of topoisomerase I in vivo, and it regulates cellular sensitivity to camptothecin by interacting with topoisomerase I. Certain cancer associated defects affecting ARF/topoisomerase I complex formation could contribute to cellular resistance to camptothecin.

A limited screen for protein interactions reveals new roles for protein phosphatase 1 in cell cycle control and apoptosis

Protein phosphatase 1 (PP1) catalytic subunits typically combine with other proteins that modulate their activity, direct them to distinct substrates, or serve as substrates for PP1. More than 50 PP1-interacting proteins (PIPs) have been identified so far. Given there are approximately 10 000 phosphoproteins in mammals, many PIPs remain to be discovered. We have used arrays containing 100 carefully selected antibodies to identify novel PIPs that are important in cell proliferation and cell survival in murine fetal lung epithelial cells and human A549 lung cancer cells. The antibody arrays identified 31 potential novel PIPs and 11 of 17 well-known PIPs included as controls, suggesting a sensitivity of at least 65%. A majority of the interactions between PP1 and putative PIPs were isoform- or cell type-specific. We confirmed by co-immunoprecipitation that 9 of these proteins associate with PP1: APAF-1, Bax, E-cadherin, HSP-70, Id2, p19Skp1, p53, PCNA, and PTEN. We examined two of these interactions in greater detail in A549 cells. Exposure to nicotine enhanced association of PP1 with Bax (and Bad), but also induced inhibitory phosphorylation of PP1. In addition to p19Skp1, PP1alpha antibodies also coprecipitated cullin 1, suggesting that PP1alpha is associated with the SCF1 complex. This interaction was only detectable during the G1/S transition and S phase. Forced loss of PP1 function decreased the levels of p27Kip1, a well-known SCF1 substrate, suggesting that PP1 may rescue proteins from ubiquitin/proteasome-mediated destruction. Both of these novel interactions are consistent with PP1 facilitating cell cycle arrest and/or apoptosis.

Ectopic doublecortin gene expression suppresses the malignant phenotype in glioblastoma cells

Doublecortin (DCX) is one of the three genes found from Affymetrix gene chip analysis related to glioma patient survival. Two other genes (e.g., osteonectin and semaphorin 3B) are well characterized as antioncogenic and tumor suppressor genes. However, there is no report about the involvement of DCX in cancer. Here, we show that gene transfer technology into DCX-deficient glioblastoma cell lines, such as A172, U87, U251N, RG2, and 9L, with DCX cDNA significantly suppressed growth of these glioma cells. U87 cells with ectopic expression of DCX exhibit a marked suppression of the transformed phenotype as growth arrested in the G(2) phase of the cell cycle progression, small colony formation in soft agar, and no tumor formation in nude rats. This transformed phenotype can be restored by knocking down DCX expression with DCX small interfering RNA. DCX was highly phosphorylated in glioma cells. Phosphorylation in the glioma cells was greater than in noncancer cells such as mouse NIH 3T3 and human embryonic kidney 293T cells. Coimmunoprecipitation of the phosphorylated DCX and spinophilin/neurabin II from DCX-synthesizing glioma cells indicated their interaction. This interaction would lead to a block of anchorage-independent growth as neurabin II is a synergistic inhibitor of anchorage-independent growth with p14ARF (ARF). Interaction between phosphorylated DCX and neurabin II may induce the association of the protein phosphatase 1 catalytic subunit (PP1) with neurabin II and inactivate PP1 and block mitosis during G(2) and M phases of the cell cycle progression. Thus, DCX seems to be a tumor suppressor of glioma.

Divorcing ARF and p53: an unsettled case

Mammalian cells that sustain oncogenic insults can invoke defensive programmes that either halt their division or trigger their apoptosis, but these countermeasures must be finely tuned to discriminate between physiological and potentially harmful growth-promoting states. By functioning specifically to oppose abnormally prolonged and sustained proliferative signals produced by activated oncogenes, the ARF tumour suppressor antagonizes functions of MDM2 to induce protective responses that depend on the p53 transcription factor and its many target genes. However, ARF has been reported to physically associate with proteins other than MDM2 and to have p53-independent activities, most of which remain controversial and poorly understood.

Spinophilin: from partners to functions

Spinophilin/neurabin 2 has been isolated independently by two laboratories as a protein interacting with protein phosphatase 1 (PP1) and F-actin. Gene analysis and biochemical approaches have contributed to define a number of distinct modular domains in spinophilin that govern protein-protein interactions such as two F-actin-, three potential Src homology 3 (SH3)-, a receptor- and a PP1-binding domains, a PSD95/DLG/zo-1 (PDZ) and three coiled-coil domains, and a potential leucine/isoleucine zipper (LIZ) motif. More than 30 partner proteins of spinophilin have been discovered, including cytoskeletal and cell adhesion molecules, enzymes, guanine nucleotide exchange factors (GEF) and regulator of G-protein signalling protein, membrane receptors, ion channels and others proteins like the tumour suppressor ARF. The physiological relevance of some of these interactions remains to be demonstrated. However, spinophilin structure suggests that the protein is a multifunctional protein scaffold that regulates both membrane and cytoskeletal functions. Spinophilin plays important functions in the nervous system where it is implicated in spine morphology and density regulation, synaptic plasticity and neuronal migration. Spinophilin regulates also seven-transmembrane receptor signalling and may provide a link between some of these receptors and intracellular mitogenic signalling events dependent on p70(S6) kinase and Rac G protein-GEF. Strikingly a role for spinophilin in cell growth was demonstrated and this effect was enhanced by its interaction with ARF. Here we review the current knowledge of the protein partners of spinophilin and present the available data that are contributing to the appreciation of spinophilin functions.

A novel ARF-binding protein (LZAP) alters ARF regulation of HDM2

The tumour suppressor ARF (alternative reading frame) is encoded by the INK4a (inhibitor of cyclin-dependent kinase 4)/ARF locus, which is frequently altered in human tumours. ARF binds MDM2 (murine double minute 2) and releases p53 from inhibition by MDM2, resulting in stabilization, accumulation and activation of p53. Recently, ARF has been found to associate with other proteins, but, to date, little is known about ARF-associated proteins that are implicated in post-translational regulation of ARF activity. Using a yeast two-hybrid screen, we have identified a novel protein, LZAP (LXXLL/leucine-zipper-containing ARF-binding protein), that interacts with endogenous ARF in mammalian cells. In the present study, we show that LZAP reversed the ability of ARF to inhibit HDM2's ubiquitin ligase activity towards p53, but simultaneously co-operated with ARF, maintaining p53 stability and increasing p53 transcriptional activity. Expression of LZAP, in addition to ARF, increased the percentage of cells in the G1 phase of the cell cycle. Expression of LZAP also caused activation of p53 and a p53-dependent G1 cell-cycle arrest in the absence of ARF. Taken together, our data suggest that LZAP can regulate ARF biochemical and biological activity. Additionally, LZAP has p53-dependent cell-cycle effects that are independent of ARF.

INK4a/ARF: a multifunctional tumor suppressor locus

The INK4a/ARF locus encodes two physically linked tumor suppressor proteins, p16(INK4a) and ARF, which regulate the RB and p53 pathways, respectively. The unusual genomic relationship of the open reading frames of these proteins initially fueled speculation that only one of the two was the true tumor suppressor, and loss of the other merely coincidental in cancer. Recent human and mouse genetic data, however, have firmly established that both proteins possess significant in vivo tumor suppressor activity, although there appear to be species- and cell-type specific differences between the two. For example, ARF plays a clear role in preventing Myc-induced lymphomagenesis in mice, whereas the role for p16(INK4a) is human carcinomas is more firmly established. In this review, I discuss the evolutionary history of the locus, the relative importance of these tumor suppressor genes in human cancer, and recent information suggesting novel biochemical and physiologic functions of these proteins in vivo.

The ARF tumor suppressor inhibits BCL6-mediated transcriptional repression

The ARF tumor suppressor gene antagonizes generation of various tumors. ARF-mediated tumor suppression occurs in a p53-independent manner as well as in a p53-dependent manner. We here demonstrate that BCL6 is a target of the ARF tumor suppressor. Either mouse p19(ARF) or human p14(ARF) binds to BCL6 and downregulates BCL6-induced transcriptional repression. ARF-mediated downregulation of the BCL6 activity may account in part for ARF-mediated tumor suppression.

Inhibition of p63 transcriptional activity by p14ARF: functional and physical link between human ARF tumor suppressor and a member of the p53 family

The ARF/MDM2/p53 pathway is a principal defense mechanism to protect the organism from uncontrolled effects of deregulated oncogenes. Oncogenes activate ARF, which interacts with and inhibits the ubiquitin ligase MDM2, resulting in p53 stabilization and activation. Once stabilized and activated, p53 can either induce or repress a wide array of different gene targets, which in turn can regulate cell cycle, DNA repair, and a number of apoptosis-related genes. Here we show that, unlike p53, p63, a member of the p53 family, directly interacts with p14(ARF). Through this interaction ARF inhibits p63-mediated transactivation and transrepression. In p63-transfected cells, ARF, which normally localizes into nucleoli, accumulates in the nucleoplasm. Based on these observations, we suggest that stimuli inducing p14(ARF) expression can, at the same time, activate p53 and impair p63 transcriptional activity, altering the pattern of p53 target gene expression. Here we show, for the first time, a physical and functional link between the p14(ARF) tumor suppressor protein and p63, a member of the p53 family.

Ink4a/Arf links senescence and aging

The mammalian INK4a/ARF locus encodes two linked tumor suppressor proteins, p16INK4a and ARF, which respectively regulate the retinoblastoma (RB) and p53 pathways. Genetic data have firmly established that both proteins possess significant in vivo tumor suppressor activity. In addition to their non-overlapping roles in preventing cancer, one or both proteins are induced under certain circumstances in most cultured murine and human cell types, and thereby are critical effectors of senescence. Likewise, data from murine models have suggested that this anti-cancer growth inhibitory activity of the locus can similarly affect permanent growth arrest in vivo. When such in vivo senescence occurs in a cell possessing self-renewal potential (e.g. a tissue stem cell), there is an attendant decline in the regenerative capabilities of the organ maintained by that stem cell. In turn, the concomitant decline of this stem cell reserve is a cardinal feature of mammalian aging. Expression of the INK4a/ARF locus, therefore, appears not only to be a major suppressor of cancer, but also an effector of mammalian aging.

Identification of neurabin II as a novel doublecortin interacting protein

The neuronal migration protein doublecortin (DCX) that associates with microtubules through a tandem DCX repeat, is required for the development of the complex architecture of the human cerebral cortex. Using a yeast two-hybrid screen with Dcx as bait, we have isolated neurabin II/spinophilin, an F-actin binding protein known to play a role in dendritic spine formation. The coiled-coil domain of neurabin II binds to a DCX region encompassing the C-terminal portion of the second DCX repeat and the N-terminal portion of the Ser/Pro-rich domain. Immunoprecipitation experiments with brain extracts show that neurabin II and Dcx interact in vivo. Several Dcx constructs that mimic human DCX mutant alleles failed to interact with neurabin II. Since Dcx and neurabin II colocalized in the developing and adult brain, a neurabin II-DCX heterodimer may be involved in neuronal migration and dendritic spine formation.

The ink4a/arf locus evolution in primates: characterization of three ARF sequences

The human ink4a/arf locus encodes two cell cycle regulatory proteins - the cyclin-dependent kinase inhibitor (p16(ink4a)), and the p53 activator (ARF) - through the use of alternative first exons. This genomic organization is unique in eukaryotes, with two different proteins obtained using different reading frames. The divergence between mouse or opossum and human ARF is very high, whereas proteins have the same nucleolar localization and function. To gain further insights into the relative importance of ARF in different settings, we characterized here the exon 1beta of ARF in 12 different species of primates. We did not find any polymorphism in studied species (monkeys, apes, and humans). These sequences are very similar, with few amino acids substitutions compared to the human sequence. It is strange to find such a high degree of conservation among primates when there is such a low degree of conservation between the human pig, rat, or mouse, chicken exon 1beta sequences. More surprisingly, we observe a threonine at position 31 in all human sequences, whereas an alanine is always present in other sequences. We suggest that when the radiation human/simian appeared or after, a selection of threonine occurred. Moreover, the modifications detected could play a role in different interactions between ARF and other proteins to stabilize or not these complexes.

The identification of a second actin-binding region in spinophilin/neurabin II

Spinophilin/neurabin II is an actin-associated scaffolding protein enriched in the dendritic spines of neurons. Previously, the actin-binding domain (ABD) of spinophilin was localized to a domain between amino acids (aa) 1 and 154. In a mass spectrometry screen for spinophilin-binding proteins, we have identified an additional actin-binding region between aa 151 and 282. F-actin co-sedimentation and GST affinity chromotography experiments further substantiate this result. Phalloidin staining of Rat2 fibroblasts transiently expressing GFP-spinophilin deletion constructs indicates co-localization with a subset of actin. Regions of spinophilin that lack the revised ABD (aa 1-230) do not co-localize with phalloidin-labeled actin, suggesting that the actin-binding domain contributes to directing the subcellular distribution of spinophilin. Targeting experiments using primary hippocampal cultures indicate that only the first actin-binding site contributes to dendritic spine localization. The second ABD targets to spines inefficiently and thus may interact with and affect actin filaments in a different manner than the first ABD.

Physical and functional interactions of the Arf tumor suppressor protein with nucleophosmin/B23

The Arf tumor suppressor inhibits cell cycle progression through both p53-dependent and p53-independent mechanisms, including interference with rRNA processing. Using tandem-affinity-tagged p19(Arf), we purified Arf-associated proteins from mouse NIH 3T3 fibroblasts undergoing cell cycle arrest. Tagged p19(Arf) associated with nucleolar and ribosomal proteins, including nucleophosmin/B23 (NPM), a protein thought to foster the maturation of preribosomal particles. NPM is an abundant protein, only a minor fraction of which binds to p19(Arf); however, a significant proportion of p19(Arf) associates with NPM. The interaction between p19(Arf) and NPM requires amino acid sequences at the Arf amino terminus, which are also required for Mdm2 binding, as well as the central acidic domain of NPM and an adjacent segment that regulates NPM oligomerization. The interaction between p19(Arf) and NPM occurs in primary mouse embryonic fibroblasts, including those lacking both Mdm2 and p53. In an NIH 3T3 derivative cell line (MT-Arf) engineered to conditionally express an Arf transgene, induced p19(Arf) associates with NPM and colocalizes with it in high-molecular-weight complexes (2 to 5 MDa). An NPM mutant lacking its carboxyl-terminal nucleic acid-binding domain oligomerizes with endogenous NPM, inhibits p19(Arf) from entering into 2- to 5-MDa particles, and overrides the ability of p19(Arf) to retard rRNA processing.

Functional and physical interaction of the human ARF tumor suppressor with Tat-binding protein-1

The p14ARF tumor suppressor is a key regulator of cellular proliferation, frequently inactivated in human cancer, whose mode of action is currently not completely understood. We report here that the so-called human immunodeficiency virus Tat-binding protein-1 (TBP-1), a component of the 19 S regulatory subunit of the proteasome 26 S, also involved in transcriptional regulation and with a supposed role in the control of cell proliferation, specifically interacts with ARF, both in yeast and mammalian cells. We present evidence that the overexpression of TBP-1 in various cell lines results in a sharp increase of both transfected and endogenous ARF protein levels. Moreover, this effect depends on the binding between the two proteins and, at least in part, is exerted at the post-translational level. We also show that the ARF increase following TBP-1 overexpression results in an increase in p53 protein levels and activity. Finally, our data underline a clear involvement of TBP-1 in the control of cell proliferation.

Regulation of p70 S6 kinase by complex formation between the Rac guanine nucleotide exchange factor (Rac-GEF) Tiam1 and the scaffold spinophilin

Tiam1 is a ubiquitous guanine nucleotide exchange factor (GEF) that activates the Rac GTPase. We have shown previously that the N terminus of Tiam1 contributes to the signaling specificity of its downstream target Rac via association with IB2, a scaffold that promotes Rac activation of a p38 kinase cascade. Here we show that the N terminus of Tiam1 can influence Rac signaling specificity in a different way by interaction with spinophilin, a scaffold that binds to p70 S6 kinase, another protein regulated by Rac. In particular, spinophilin binding promotes the plasma membrane localization of Tiam1 and enhances the ability of Tiam1 to activate p70 S6 kinase. In contrast, spinophilin binding suppresses the ability of Tiam to activate Pak1, a different Rac effector. Finally, a mutant spinophilin that cannot bind to Tiam1 suppresses serum-induced p70 S6 kinase activation in cells, suggesting that a Tiam1/spinophilin complex contributes to p70 S6 kinase regulation by extracellular signals. These findings add to a growing body of evidence supporting the concept that some Rac-GEFs not only activate Rac GTPases but also participate in the selection of Rac effector by binding to particular scaffolds that complex with components of specific Rac effector pathways.

Association of p14ARF with the p120E4F transcriptional repressor enhances cell cycle inhibition

The p14(ARF) tumor suppressor is a key regulator of cellular proliferation and is frequently inactivated in human cancer. This tumor suppressor functions in the p53 and pRb cell cycle regulatory pathways and can effectively activate both pathways to induce growth arrest or cell death. We now report that p14(ARF) forms a complex with the E1A-regulated transcriptional repressor, p120(E4F). p120(E4F) contacts p14(ARF) and p53 to form a ternary complex in vivo and enhances p14(ARF)-induced G(2) cell cycle arrest in a p53-dependent manner. We suggest that the interaction of p14(ARF) and p120(E4F) forms an important link between the p14(ARF) and p53 tumor suppressor proteins, both of which exhibit enhanced cell cycle inhibitory activity in the presence of this transcriptional repressor.

The actin-binding domain of spinophilin is necessary and sufficient for targeting to dendritic spines

Spinophilin is enriched in dendritic spines, small protrusions of the postsynaptic membrane along the length of the dendrite that contain the majority of excitatory synapses. Spinophilin binds to protein phosphatase 1 with high affinity and targets it to dendritic spines, therefore placing it in proximity to regulate glutamate receptor activity. Spinophilin also binds to and bundles f-actin, the main cytoskeletal constituent of dendritic spines, and may therefore serve to regulate the structure of the synapse. In this study, we sought to determine the structural basis for the targeting of spinophilin to dendritic spines. Our results show that the actin-binding domain of spinophilin is necessary and sufficient for targeting of spinophilin to dendrites and dendritic spines.

Nucleolar Arf tumor suppressor inhibits ribosomal RNA processing

The p19(Arf) tumor suppressor, a nucleolar protein, binds to Mdm2 to induce p53-dependent cell cycle arrest. Arf also prevents the proliferation of cells lacking Mdm2 and p53, albeit less efficiently. We show that p19(Arf) inhibits production of ribosomal RNA, retarding processing of 47/45S and 32S precursors. These effects correlate with but do not strictly depend upon inhibition of rRNA biosynthesis or cell cycle arrest, are not mimicked by p53, and require neither p53 nor Mdm2. Arf mutants lacking conserved amino acid residues 2-14 do not block rRNA synthesis and processing or inhibit cell proliferation. Evolution may have linked a primordial nucleolar Arf function to Mdm2 and p53, creating a more efficient checkpoint-signaling pathway for coordinating ribosomal biogenesis and cell cycle progression.

A major functional difference between the mouse and human ARF tumor suppressor proteins

Suppression of tumorigenesis is considerably more stringent in the human than in the much shorter lived mouse species, and the reasons for this difference are poorly understood. We investigated functional differences in the control of the ARF (alternative reading frame) protein that acts upstream of p53 and is encoded along with p16(INK4a) at a major tumor suppressor locus in both the human and mouse genomes. The mouse and human ARF proteins are substantially divergent at their carboxyl termini. We have shown that the mouse ARF protein (p19ARF) interacts with Pex19p in the cell cytoplasm leading to its nuclear exclusion and repression of its p53 activation function. The human ARF protein (p14ARF) is substantially smaller than its mouse counterpart and is not subject to this functional inactivation by Pex19p. In an identical cellular background, ribozymes directed against Pex19p enhanced p19ARF- but not p14ARF-activated p53 function. This is the first demonstration of a functional difference between the mouse and human ARF proteins. In view of the major role of ARF in tumor suppression, this distinction may contribute to the different levels of tumor proneness of these species.

p53-independent apoptosis is induced by the p19ARF tumor suppressor

p19(ARF) is a potent tumor suppressor. By inactivating Mdm2, p19(ARF) upregulates p53 activities to induce cell cycle arrest and sensitize cells to apoptosis in the presence of collateral signals. It has also been demonstrated that cell cycle arrest is induced by overexpressed p19(ARF) in p53-deficient mouse embryonic fibroblasts, only in the absence of the Mdm2 gene. Here, we show that apoptosis can be induced without additional apoptosis signals by expression of p19(ARF) using an adenovirus-mediated expression system in p53-intact cell lines as well as p53-deficient cell lines. Also, in primary mouse embryonic fibroblasts (MEFs) lacking p53/ARF, p53-independent apoptosis is induced irrespective of Mdm2 status by expression of p19(ARF). In agreement, p19(ARF)-mediated apoptosis in U2OS cells, but not in Saos2 cells, was attenuated by coexpression of Mdm2. We thus conclude that there is a p53-independent pathway for p19(ARF)-induced apoptosis that is insensitive to inhibition by Mdm2.

Adenovirus-mediated overexpression of p14(ARF) induces p53 and Bax-independent apoptosis

The human INK4a gene locus encodes two structurally unrelated tumor suppressor proteins, p16(INK4a) and p14(ARF), which are frequently inactivated in human cancer. Whereas p16(INK4a) acts through engagement of the Rb-cdk4/6-cyclin D pathway, both the pro-apoptotic and cell cycle-regulatory functions of p14(ARF) were shown to be primarily dependent on the presence of functional p53. Recent reports have also implicated p14(ARF) in p53-independent mechanisms of cell cycle regulation and apoptosis induction, respectively. To further explore the pro-apoptotic function of p14(ARF) in relation to functional cellular p53, we constructed a replication-deficient adenoviral vector for overexpression of p14(ARF) (Ad-p14(ARF)). As expected, Ad-p14(ARF) efficiently induced apoptosis in p53/Rb wild-type U-2OS osteosarcoma cells at low multiplicities of infection. Interestingly, Ad-p14(ARF) also induced apoptosis in both p53-deleted SAOS-2 osteosarcoma cells and HCT116 colon cancer cells with a bi-allelic knock-out of p53 (HCT116-p53(-/-)). Similarly, adenovirus-mediated overexpression of p14(ARF) induced apoptosis in p53/Bax-mutated DU145 prostate cancer cells as well as in HCT116 cells devoid of functional Bax (HCT116-Bax(-/-)). Restoration of Bax expression by retroviral gene transfer in DU145 cells did not further enhance p14(ARF)-triggered cell death. Infection with Ad-p14(ARF) induced activation of mitochondrial permeability shift transition, caspase activation and apoptotic DNA fragmentation irrespective of the presence or absence of either Bax or functional cellular p53. Nevertheless, overexpression of the anti-apoptotic Bcl-2 homolog Bcl-x(L) markedly inhibited p14(ARF)-induced apoptosis. This may indicate that p14(ARF) triggers a so far unknown activator of mitochondrial apoptosis which can be inhibited by Bcl-2 but which acts either independently or downstream of Bax. Taken together, this report demonstrates the participation of signaling pathways apart from the p53/Mdm-2 rheostat and Bax in p14(ARF)-mediated apoptosis.

ARF function does not require p53 stabilization or Mdm2 relocalization

It is generally accepted that the ARF tumor suppressor induces p53-dependent growth arrest by sequestering the p53 antagonist Mdm2 in the nucleolus. Previous mutagenic studies of murine ARF suggested that residues 1 through 14 and 26 through 37 were critical for Mdm2 binding, while the latter domain also governed ARF nucleolar localization. We show that mouse ARF residues 6 to 10 and 21 to 25 are required for ARF-induced growth arrest whereas residues 1 to 5 and 29 to 34 are dispensable. Deletion of the putative nucleolar localization signal (31)RRPR(34) did not prevent nucleolar localization. Surprisingly, unlike wild-type ARF, growth-inhibitory mutants D1-5 and D29-34 failed to stabilize p53 yet induced its transcriptional activation in reporter assays. This suggests that p53 stabilization is not essential for ARF-mediated activation of p53. Like wild-type ARF, both mutants also exhibited p53-independent function since they were able to arrest p53/Mdm2-null cells. Notably, other mutants lacking conserved residues 6 to 10 or 21 to 25 were unable to suppress growth in p53-positive cells despite nucleolar localization and the ability to import Mdm2. Those observations stood in apparent contrast to the ability of wild-type ARF to block growth in some cells without relocalizing endogenous Mdm2 to nucleoli. Together, these data show a lack of correlation between ARF activity and Mdm2 relocalization, suggesting that additional events other than Mdm2 import are required for ARF function.

Mutations in the INK4a/ARF melanoma susceptibility locus functionally impair p14ARF

The INK4a/ARF locus encodes two cell cycle regulatory proteins, the cyclin-dependent kinase inhibitor, p16(INK4a), and the p53 activator, p14(ARF). Germline mutations in this locus are associated with melanoma susceptibility in 20-40% of multiple case melanoma families. Many of these mutations specifically impair p16(INK4a), whereas mutations uniquely targeting p14(ARF) are rare. Nevertheless, the importance of p14(ARF) has not been excluded because more than 40% of INK4a/ARF alterations affect p16(INK4a) and p14(ARF). We now report that p14(ARF) is functionally impaired in melanoma kindreds carrying INK4a/ARF mutations. Of the seven INK4a/ARF mutations tested, three altered the subcellular distribution of p14(ARF) and diminished the ability of p14(ARF) to activate the p53 pathway. This work establishes the importance of p14(ARF) in melanoma predisposition.