Exclusion of c-Abl from the nucleus restrains the p73 tumor suppression function

The Emerging Role of Insulin Receptor Isoforms in Thyroid Cancer: Clinical Implications and New Perspectives

Thyroid cancer (TC) is the most common endocrine tumor. Although the majority of TCs show good prognoses, a minor proportion are aggressive and refractory to conventional therapies. So far, the molecular mechanisms underlying TC pathogenesis are incompletely understood. Evidence suggests that TC cells and their precursors are responsive to insulin and insulin-like growth factors (IGFs), and often overexpress receptors for insulin (IR) and IGF-1 (IGF-1R). IR exists in two isoforms, namely IR-A and IR-B. The first binds insulin and IGF-2, unlike IR-B, which only binds insulin. IR-A is preferentially expressed in prenatal life and contributes to development through IGF-2 action. Aggressive TC overexpresses IR-A, IGF-2, and IGF-1R. The over-activation of IR-A/IGF-2 loop in TC is associated with stem-like features and refractoriness to some targeted therapies. Importantly, both IR isoforms crosstalk with IGF-1R, giving rise to the formation of hybrids receptors (HR-A or HR-B). Other interactions have been demonstrated with other molecules such as the non-integrin collagen receptor, discoidin domain receptor 1 (DDR1), and the receptor for the hepatocyte growth factor (HGF), Met. These functional networks provide mechanisms for IR signaling diversification, which may also exert a role in TC stem cell biology, thereby contributing to TC initiation and progression. This review focuses on the molecular mechanisms by which deregulated IR isoforms and their crosstalk with other molecules and signaling pathways in TC cells and their precursors may contribute to thyroid carcinogenesis, progression, and resistance to conventional treatments. We also highlight how targeting these alterations starting from TC progenitors cells may represent new therapeutic strategies to improve the clinical management of advanced TCs.

Changing the subcellular location of the oncoprotein Bcr-Abl using rationally designed capture motifs

PURPOSE

Bcr-Abl, the causative agent of chronic myelogenous leukemia (CML), localizes in the cytoplasm where its oncogenic signaling leads to proliferation of cells. If forced into the nucleus Bcr-Abl causes apoptosis. To achieve nuclear translocation, binding domains for capture of Bcr-Abl were generated and attached to proteins with signals destined for the nucleus. These resulting proteins would be capable of binding and translocating endogenous Bcr-Abl to the nucleus.

METHODS

Bcr-Abl was targeted at 3 distinct domains for capture: by construction of high affinity intracellular antibody domains (iDabs) to regions of Bcr-Abl known to promote cytoplasmic retention, via its coiled coil domain (CC), and through a naturally occurring protein-protein interaction domain (RIN1). These binding domains were then tested for their ability to escort Bcr-Abl into the nucleus using a "protein switch" or attachment of 4 nuclear localization signals (NLSs).

RESULTS

Although RIN1, ABI7-iDab, and CCmut3 constructs all produced similar colocalization with Bcr-Abl, only 4NLS-CCmut3 produced efficient nuclear translocation of Bcr-Abl.

CONCLUSIONS

We demonstrate that a small binding domain can be used to control the subcellular localization of Bcr-Abl, which may have implications for CML therapy. Our ultimate future goal is to change the location of critical proteins to alter their function.

Interplay between kinase domain autophosphorylation and F-actin binding domain in regulating imatinib sensitivity and nuclear import of BCR-ABL

Background

The constitutively activated BCR-ABL tyrosine kinase of chronic myeloid leukemia (CML) is localized exclusively to the cytoplasm despite the three nuclear localization signals (NLS) in the ABL portion of this fusion protein. The NLS function of BCR-ABL is re-activated by a kinase inhibitor, imatinib, and in a kinase-defective BCR-ABL mutant. The mechanism of this kinase-dependent inhibition of the NLS function is not understood.

Methodology/Principal Findings

By examining the subcellular localization of mutant BCR-ABL proteins under conditions of imatinib and/or leptomycin B treatment to inhibit nuclear export, we have found that mutations of three specific tyrosines (Y232, Y253, Y257, according to ABL-1a numbering) in the kinase domain can inhibit the NLS function of kinase-proficient and kinase-defective BCR-ABL. Interestingly, binding of imatinib to the kinase-defective tyrosine-mutant restored the NLS function, suggesting that the kinase domain conformation induced by imatinib-binding is critical to the re-activation of the NLS function. The C-terminal region of ABL contains an F-actin binding domain (FABD). We examined the subcellular localization of several FABD-mutants and found that this domain is also required for the activated kinase to inhibit the NLS function; however, the binding to F-actin per se is not important. Furthermore, we found that some of the C-terminal deletions reduced the kinase sensitivity to imatinib.

Conclusions/Significance

Results from this study suggest that an autophosphorylation-dependent kinase conformation together with the C-terminal region including the FABD imposes a blockade of the BCR-ABL NLS function. Conversely, conformation of the C-terminal region including the FABD can influence the binding affinity of imatinib for the kinase domain. Elucidating the structural interactions among the kinase domain, the NLS region and the FABD may therefore provide insights on the design of next generation BCR-ABL inhibitors for the treatment of CML.

An evolutionarily acquired genotoxic response discriminates MyoD from Myf5, and differentially regulates hypaxial and epaxial myogenesis

Despite having distinct expression patterns and phenotypes in mutant mice, the myogenic regulatory factors Myf5 and MyoD have been considered to be functionally equivalent. Here, we report that these factors have a different response to DNA damage, due to the presence in MyoD and absence in Myf5 of a consensus site for Abl-mediated tyrosine phosphorylation that inhibits MyoD activity in response to DNA damage. Genotoxins failed to repress skeletal myogenesis in MyoD-null embryos; reintroduction of wild-type MyoD, but not mutant Abl phosphorylation-resistant MyoD, restored the DNA-damage-dependent inhibition of muscle differentiation. Conversely, introduction of the Abl-responsive phosphorylation motif converts Myf5 into a DNA-damage-sensitive transcription factor. Gene-dosage-dependent reduction of Abl kinase activity in MyoD-expressing cells attenuated the DNA-damage-dependent inhibition of myogenesis. The presence of a DNA-damage-responsive phosphorylation motif in vertebrate, but not in invertebrate MyoD suggests an evolved response to environmental stress, originated from basic helix-loop-helix gene duplication in vertebrate myogenesis.

The p53 family protein p73 provides new insights into cancer chemosensitivity and targeting

The p53 tumor suppressor is part of a small family of related proteins that includes two other members, p73 and p63. Interest in the p53 family members, their functions and their complex interactions and regulation, has steadily grown over recent years and does not show signs of waning. p73 is a major determinant of chemosensitivity in humans, and mutant p53 proteins carrying specific polymorphisms can induce drug resistance by inhibiting TAp73. Cooperation between TA (transactivating, proapoptotic, antiproliferative) and Delta N (truncated, antiapoptotic, pro-proliferative) p73 isoforms and among the three family members guarantees equilibrium between proliferation, differentiation, and cell death, thus creating a harmony that is lost in several human cancers. In this article, we review our current knowledge of the role of p73 in cancer chemosensitivity and the real prospect of therapy targeting this molecule. We also draw attention to the crucial role of specific phosphorylation and acetylation events for p73-induced apoptosis and drug chemosensitivity.

DeltaNp73alpha inhibits PTEN expression in thyroid cancer cells

DeltaNp73 is a N-terminally truncated p53 family member with a dominant negative function, which is upregulated in cancer. PTEN is a lipid phosphatase, which is involved in the attenuation of tyrosine kinase signaling. PTEN expression is increased by p53, and its function is blunted in several malignancies. Because in most of the thyroid carcinomas, DeltaNp73alpha is upregulated, whereas PTEN expression down regulated, we investigated whether DeltaNp73alpha may influence PTEN expression in this cell model. We found that DeltaNp73alpha overexpression in thyroid cancer cells reduces PTEN expression, whereas DeltaNp73alpha down-regulation by siRNA increases PTEN expression. Real-time PCR indicated that overexpression of DeltaNp73alpha is able to reduce PTEN mRNA levels. Moreover, chromatin immunoprecipitation (ChIP) and luciferase assays indicated that DeltaNp73alpha binds to -1031-779 region of the PTEN promoter, which is a different site than that for p53, thereby inhibiting promoter activity. Interestingly, also the transcriptionally active p73 isoforms (TAp73alpha and TAp73beta) bound to this DNA sequence and, at variance with DeltaNp73alpha, stimulated PTEN promoter activity to an extent similar to that of p53. In accordance with its effect on PTEN protein levels, DeltaNp73alpha increased phospho-Akt protein content and, as a consequence, Mdm2-mediated p53 degradation. This effect of DeltaNp73alpha resulted in increased thyroid cancer cell proliferation and reduced apoptosis and was reverted by the PI3-kinase inhibitor LY294002, indicating the role of Akt pathway in this effect. Taken together, these results indicate a novel p73 regulated mechanism for PTEN expression in thyroid cancer cells, and that, also through this mechanism, DeltaNp73alpha exerts its protumorigenic effect.

Role of c-Abl kinase in DNA mismatch repair-dependent G2 cell cycle checkpoint arrest responses

Current published data suggest that DNA mismatch repair (MMR) triggers prolonged G(2) cell cycle checkpoint arrest after alkylation damage from N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by activating ATR (ataxia telangiectasia-Rad3-related kinase). However, analyses of isogenic MMR-proficient and MMR-deficient human RKO colon cancer cells revealed that although ATR/Chk1 signaling controlled G(2) arrest in MMR-deficient cells, ATR/Chk1 activation was not involved in MMR-dependent G(2) arrest. Instead, we discovered that disrupting c-Abl activity using STI571 (Gleevec, a c-Abl inhibitor) or stable c-Abl knockdown abolished MMR-dependent p73alpha stabilization, induction of GADD45alpha protein expression, and G(2) arrest. In addition, inhibition of c-Abl also increased the survival of MNNG-exposed MMR-proficient cells to a level comparable with MMR-deficient cells. Furthermore, knocking down GADD45alpha (but not p73alpha) protein levels affected MMR-dependent G(2) arrest responses. Thus, MMR-dependent G(2) arrest responses triggered by MNNG are dependent on a human MLH1/c-Abl/GADD45alpha signaling pathway and activity. Furthermore, our data suggest that caution should be taken with therapies targeting c-Abl kinase because increased survival of mutator phenotypes may be an unwanted consequence.

p73 expression in medulloblastoma: TAp73/DeltaNp73 transcript detection and possible association of p73alpha/DeltaNp73 immunoreactivity with survival

The human p73 protein is essential for normal morphogenesis and maintenance of neural tissue. Recently, several TP73 transcripts have been revealed in medulloblastoma (MB), the most common malignant brain tumor in children. Here, we performed immunohistochemical analysis on 29 MB specimens using anti-p73alpha and anti-DeltaNp73 antibodies. Real-time PCR quantification was performed to assess TAp73 and DeltaNp73 transcripts in a subset of 13 MB samples. Normal cerebellar tissues and RNA were used for comparison. Pilot clinical-pathological correlations were also provided. We report significant differences for TAp73 and DeltaNp73 mRNA expression between tumor tissues and reference (P = 0.013, P = 0.028). Immunohistochemically, 52 and 29% MB samples were positive for p73alpha and DeltaNp73, respectively. p73alpha expression was found to be in both the nucleus and cytoplasm, whereas DeltaNp73 was localized predominantly in the cytoplasm. In normal cerebellum, positive staining for p73alpha and DeltaNp73 was observed in the Purkinje cells of newborns, not adult samples, which supports the developmental role of TP73 during organogenesis of the human cerebellum. Survival analysis has shown negative relationship of DeltaNp73-immunoreactivity with overall survival (OS) and event free survival (EFS) (P = 0.026 and P = 0.127, respectively). For p73alpha-positive cases, the negative trend in OS (P = 0.149) and EFS (P = 0.216) was also apparent. Our results indicate the involvement of p73 protein in MB tumorigenesis and define TP73 as a potential prognostic and therapeutic target for medulloblastoma.

Abl tyrosine kinase promotes dorsal ruffles but restrains lamellipodia extension during cell spreading on fibronectin

The nonreceptor Abl tyrosine kinase stimulates F-actin microspikes and membrane ruffles in response to adhesion and growth factor signals. We show here that induced dimerization of Abl-FKBP, but not the kinase-defective AblKD-FKBP, inhibits cell spreading on fibronectin. Conversely, knockdown of cellular Abl by shRNA stimulates cell spreading. The Abl kinase inhibitor, imatinib, also stimulates cell spreading and its effect is overridden by the imatinib-resistant AblT315I. Expression of Abl but not AbkKD in Abl/Arg-deficient cells again inhibits spreading. Furthermore, Abl inhibits spreading of cells that express the activated Rac, RacV12, correlating with RacV12 localization to dorsal membrane protrusions. Ectopic expression of CrkII, a Rac activator that is inactivated by Abl-mediated tyrosine phosphorylation, antagonizes Abl-mediated dorsal membrane localization of RacV12. Ectopic expression of a dynamin-2 mutant, previously shown to induce Rac-GTP localization to the dorsal membrane, abolishes the stimulatory effect of imatinib on cell spreading. These results suggest that Abl tyrosine kinase, through CrkII phosphorylation and in collaboration with dynamin-2 can regulate the partitioning of Rac-GTP to favor dorsal ruffles during cell spreading. The Abl-dependent dorsal membrane localization of activated Rac explains its positive role in ruffling and negative role in cell spreading and migration.

Blockade of tumor necrosis factor-induced Bid cleavage by caspase-resistant Rb

Tumor necrosis factor-alpha (TNF) activates caspase-8 to cleave effector caspases or Bid, resulting in type-1 or type-2 apoptosis, respectively. We show here that TNF also induces caspase-8-dependent C-terminal cleavage of the retinoblastoma protein (Rb). Interestingly, fibroblasts from Rb(MI/MI) mice, in which the C-terminal caspase cleavage site is mutated, exhibit a defect in Bid cleavage despite caspase-8 activation. Recent results suggest that TNF receptor endocytosis is required for the activation of caspase-8. Consistent with this notion, inhibition of V-ATPase, which plays an essential role in acidification and degradation of endosomes, specifically restores Bid cleavage in Rb(MI/MI) cells. Inhibition of V-ATPase sensitizes Rb(MI/MI) but not wild-type fibroblasts to TNF-induced apoptosis and stimulates inflammation-associated colonic apoptosis in Rb(MI/MI) but not wild-type mice. These results suggest that Rb cleavage is required for Bid cleavage in TNF-induced type-2 apoptosis, and this requirement can be supplanted by the inhibition of V-ATPase.

Delayed activation of Bax by DNA damage in embryonic stem cells with knock-in mutations of the Abl nuclear localization signals

The non-receptor tyrosine kinase Abl contains nuclear localization (NLS) and nuclear export signals that drive its nucleo-cytoplasmic shuttling. The nuclear Abl tyrosine kinase is activated by DNA damage through ataxia telangiectasia mutated (ATM). Previous studies have suggested nuclear Abl to have proapoptotic activity. To determine the requirement for Abl nuclear import in DNA damage-induced apoptosis, we took a genetic approach by mutating the three NLS (muNLS) of abl1 in mouse embryonic stem (ES) cells through homologous recombination. Exposure of ES cells to genotoxins caused an ATM-dependent nuclear accumulation of Abl but not Abl muNLS. ES cells expressing Abl muNLS exhibited delayed Bax activation, reduced cytochrome c release and decreased caspase-9 activity in response to DNA damage. These results provide a genetic proof that Abl nuclear entry contributes to DNA damage-induced activation of the intrinsic apoptotic pathway.

Reduction of apoptosis in Rb-deficient embryos via Abl knockout

The retinoblastoma protein RB regulates cell proliferation, differentiation and apoptosis. Homozygous knockout of Rb in mice causes embryonic lethality owing to placental defects that result in excessive apoptosis. RB binds to a number of cellular proteins including the nuclear Abl protein and inhibits its tyrosine kinase activity. Ex vivo experiments have shown that genotoxic or inflammatory stress can activate Abl kinase to stimulate apoptosis. Employing the Rb-null embryos as an in vivo model of apoptosis, we have shown that the genetic ablation of Abl can reduce apoptosis in the developing central nervous system and the embryonic liver. These results are consistent with the inhibitory interaction between RB and Abl, and provide in vivo evidence for the proapoptotic function of Abl.

Thymidine-phosphorothioate oligonucleotides induce activation and apoptosis of CLL cells independently of CpG motifs or BCL-2 gene interference

We compared antisense phosphorothioate oligonucleotides (PS-ODN) that target BCL-2 such as Genasense (G3139-PS), with other PS-ODN or phosphodiester-ODN (PO-ODN) in their relative capacity to induce apoptosis of chronic lymphocytic leukemia (CLL) B cells in vitro. Surprisingly, we found that thymidine-containing PS-ODN, but not PO-ODN, induced activation and apoptosis of CLL cells independent of BCL-2 antisense sequence or CpG motifs. All tested thimidine-containing PS-ODN, irrespective of their primary sequences, reduced the expression of Bcl-2 protein and increased the levels of the proapoptotic molecules p53, Bid, Bax in CLL cells. Apoptosis induced by thymidine-containing PS-ODN was preceded by cellular activation, could be blocked by the tyrosine-kinase inhibitor imatinib mesylate (Gleevec), and was dependent on ABL kinase. We conclude that thymidine-containing PS-ODN can activate CLL cells and induce apoptosis via a mechanism that is independent of BCL-2 gene interference or CpG motifs.

The course of etoposide-induced apoptosis in Jurkat cells lacking p53 and Bax

Jurkat T-lymphocytes lack p53 and Bax but contain p73 and Bid and are killed by etoposide (ETO). With ETO c-abl is phosphorylated and phosphorylated p73 increased. Translocation of full-length Bid to mitochondria follows, with induction of the mitochondrial permeability transition (MPT) and release of cytochrome c into the cytosol. Pronounced swelling of mitochondria was evident ultrastructurally, and the MPT inhibitor cyclosporin A prevented the release of cytochrome c. Overexpression of Bcl-2 prevented the translocation of Bid, the release of cytochrome c, and cell death. The pan-caspase inhibitor ZVAD-FMK prevented the cell killing, but not the initial release of cytochrome c. An accumulation of tBid occurred at later times in association with Bid degradation. A sequence is proposed that couples DNA damage to Bid translocation via activation of c-abl and p73. Bid translocation induces the MPT, the event that causes release of cytochrome c, activation of caspases, and cell death.

Tumor promotion by caspase-resistant retinoblastoma protein

The retinoblastoma (RB) protein regulates cell proliferation and cell death. RB is cleaved by caspase during apoptosis. A mutation of the caspase-cleavage site in the RB C terminus has been made in the mouse Rb-1 locus; the resulting Rb-MI mice are resistant to endotoxin-induced apoptosis in the intestine. The Rb-MI mice do not exhibit increased tumor incidence, because the MI mutation does not disrupt the Rb tumor suppressor function. In this study, we show that Rb-MI can promote the formation of colonic adenomas in the p53-null genetic background. Consistent with this tumor phenotype, Rb-MI reduces colorectal epithelial apoptosis and ulceration caused by dextran sulfate sodium. By contrast, Rb-MI does not affect the lymphoma phenotype of p53-null mice, in keeping with its inability to protect thymocytes and splenocytes from apoptosis. The Rb-MI protein is expressed and phosphorylated in the tumors, thereby inactivating its growth suppression function. These results suggest that RB tumor suppressor function, i.e., inhibition of proliferation, is inactivated by phosphorylation, whereas RB tumor promoting function, i.e., inhibition of apoptosis, is inactivated by caspase cleavage.

Transcription inhibition: a potential strategy for cancer therapeutics

Interference with transcription triggers a stress response leading to the induction of the tumour suppressor p53. If transcription is not restored within a certain time frame cells may undergo apoptosis in a p53-dependent and independent manner. The mechanisms by which blockage of transcription induces apoptosis may involve diminished levels of anti-apoptotic factors, inappropriate accumulation of proteins in the nucleus, accumulation of p53 at mitochondria or complications during replication. Many chemotherapeutic agents currently used in the clinic interfere with transcription and this interference may contribute to their anti-cancer activities. Future efforts should be directed towards exploring whether interference of transcription could be used as an anti-cancer therapeutic strategy.

Nucleo-cytoplasmic communication in apoptotic response to genotoxic and inflammatory stress

Genotoxic agents or inflammatory cytokines activate cellular stress responses and trigger programmed cell death. We have identified a signal transduction module, including three nuclear proteins that participate in the regulation of cell death induced by chemotherapeutic agents and tumor necrosis factor (TNF). In this nuclear signaling module, retinoblastoma protein (Rb) functions as an inhibitor of apoptotic signal transduction. Inactivation of Rb by phosphorylation or caspase-dependent cleavage/degradation is required for cell death to occur. Rb inhibits the Abl tyrosine kinase. Thus, Rb inactivation is a pre-requisite for Abl activation by DNA damage or TNF. Activation of nuclear Abl and its downstream effector p73 induces mitochondriadependent cell death. The involvement of these nuclear signal transducers in TNF induced apoptosis, which does not require new gene expression, indicates that nuclear events other than transcription can contribute to extrinsic apoptotic signal transduction.

Ectopic Expression of p73α, but Not p73β, Suppresses Myogenic Differentiation

The TRP73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the N terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the C terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaN-p73 interferes with p73-induced apoptosis. However, there has been no evidence for functional diversity of the C-terminal p73 variants. In this study, we found that p73alpha and p73beta exerted differential effect on the differentiation of C2C12 myoblasts. Although p73beta lacked any detectable effect on differentiation, p73alpha caused a substantial delay in the expression of muscle-specific genes. In co-transfection experiments p73alpha, but not p73beta, attenuated the transcriptional activity of MyoD. Microarray-based gene profiling confirmed the protraction of MyoD-dependent gene expression in C2C12 cells stably expressing p73alpha. Notwithstanding the differential effect on differentiation, p73alpha and p73beta showed similar activity in sensitizing C2C12 myoblasts to cisplatin-induced cell death. These results demonstrated a functional diversity between the two C-terminal variants of p73 and suggested that p73alpha can regulate cellular differentiation in addition to its role in stimulating cell death.

Tumor necrosis factor alpha-induced apoptosis requires p73 and c-ABL activation downstream of RB degradation

The retinoblastoma protein (RB) suppresses cell proliferation and apoptosis. We have previously shown that RB degradation is required for tumor necrosis factor alpha (TNF-alpha) to induce apoptosis. We show here the identification of two apoptotic effectors, i.e., c-ABL tyrosine kinase and p73, which are activated by TNF-alpha following RB degradation. In cells expressing a degradation-resistant RB protein (RB-MI), TNF-alpha does not activate c-ABL. RB-MI also inhibits TNF-alpha-mediated activation of p73. Genetic deletion and pharmacological inhibition of c-ABL or p73 diminish the apoptotic response to TNF-alpha in human cell lines and mouse fibroblasts. Thymocytes isolated from Rb(MI/MI), Abl(-/-), or p73(-/-) mice are resistant to TNF-alpha-induced apoptosis compared to their wild-type counterparts. This is in contrast to p53(-/-) thymocytes, which exhibit a wild-type level of apoptosis in response to TNF-alpha. Thus, c-ABL and p73 contribute to apoptosis induced by TNF-alpha, in addition to their role in promoting DNA damage-associated cell death.