Platelets Promote Metastasis via Binding Tumor CD97 Leading to Bidirectional Signaling that Coordinates Transendothelial Migration

Tumor cells initiate platelet activation leading to the secretion of bioactive molecules, which promote metastasis. Platelet receptors on tumors have not been well-characterized, resulting in a critical gap in knowledge concerning platelet-promoted metastasis. We identify a direct interaction between platelets and tumor CD97 that stimulates rapid bidirectional signaling. CD97, an adhesion G protein-coupled receptor (GPCR), is an overexpressed tumor antigen in several cancer types. Purified CD97 extracellular domain or tumor cell-associated CD97 stimulated platelet activation. CD97-initiated platelet activation led to granule secretion, including the release of ATP, a mediator of endothelial junction disruption. Lysophosphatidic acid (LPA) derived from platelets induced tumor invasiveness via proximal CD97-LPAR heterodimer signaling, coupling coincident tumor cell migration and vascular permeability to promote transendothelial migration. Consistent with this, CD97 was necessary for tumor cell-induced vascular permeability in vivo and metastasis formation in preclinical models. These findings support targeted blockade of tumor CD97 as an approach to ameliorate metastatic spread.

Use of Ciliogenesis to Detect Aneugens: The Role of Primary Cilia

Primary cilia arise from the centrosomes of quiescent or post-mitotic cells, and serve as sensory organelles that communicate mechanical and chemical stimuli from the environment to the interior of the cell. Cilium formation may, therefore, become a useful end point signaling exposure to genotoxins or aneugens. Here we have used the aneugen, zidovudine (AZT), an antiretroviral drug that induces DNA replication arrest and centrosomal amplification (>2 centrosomes per quiescent cell), to evaluate cilia formation in retinal epithelial (pigmented) cells. Since cilia are derived from centrosomes, and aneugens can induce centrosomal amplification, the production of multiple cilia arising from multiple centrosomes may reveal the aneugenic nature of the agents. Cells were exposed to AZT to induce centrosomal amplification, cultured without serum to allow the centrioles to develop cilia, and immunostained to visualize cilia and centrosomes. Nuclear DNA was stained with DAPI. Preliminary observations suggest that cells with multiple centrosomes are able to generate extra cilia.

Role of nucleotide excision repair and p53 in zidovudine (AZT)-induced centrosomal deregulation

The nucleoside reverse transcriptase inhibitor zidovudine (AZT) induces genotoxic damage that includes centrosomal amplification (CA > 2 centrosomes/cell) and micronucleus (MN) formation. Here we explored these end points in mice deficient in DNA repair and tumor suppressor function to evaluate their effect on AZT-induced DNA damage. We used mesenchymal-derived fibroblasts cultured from C57BL/6J mice that were null and wild type (WT) for Xpa, and WT, haploinsufficient and null for p53 (6 different genotypes). Dose-responses for CA formation, in cells exposed to 0, 10, and 100 μM AZT for 24 hr, were observed in all genotypes except the Xpa((+/+)) p53((+/-)) cells, which had very low levels of CA, and the Xpa((-/-)) p53((-/-)) cells, which had very high levels of CA. For CA there was a significant three-way interaction between Xpa, p53, and AZT concentration, and Xpa((-/-)) cells had significantly higher levels of CA than Xpa((+/+)) cells, only for p53((+/-)) cells. In contrast, the MN and MN + chromosomes (MN + C) data showed a lack of AZT dose response. The Xpa((-/-)) cells, with p53((+/+)) or ((+/-)) genotypes, had levels of MN and MN + C higher than the corresponding Xpa((+/+)) cells. The data show that CA is a major event induced by exposure to AZT in these cells, and that there is a complicated relationship between AZT and CA formation with respect to gene dosage of Xpa and p53. The loss of both genes resulted in high levels of damage, and p53 haploinsufficicency strongly protected Xpa((+/+)) cells from AZT-induced CA damage.

Abstract 2708: The TWEAK-FN14 pathway promotes prostate cancer bone metastasis through the NFκB pathway

Advanced prostate cancer is highly associated with castration resistance and the development of bone metastases. Understanding the mechanisms contributing to prostate cancer bone metastasis is needed in order to develop novel therapeutic agents. NFκB signaling has been implicated in prostate cancer progression. The Tweak-Fn14 axis, which activates both canonical and non-canonical NFκB pathways, has been shown to be up-regulated in many cancer types. However, the role of the Tweak-Fn14 signaling in prostate cancer progression has not been investigated. In this study, we show that oncogenic Ras transformation of the AR negative, DU145 cell line led to the acquisition of bone metastatic capability and associated increased expression of TWEAK and FN14. Knocking down FN14 using shRNAs, as well as blocking the NFκB pathway using an IKBα super repressor, in DU145/Rasb1 cells, significantly inhibited bone metastasis and improved survival. This inhibitory effect of FN14shRNA was fully rescued by activating the NFκB canonical pathway, but not the NFκB non-canonical pathway. FN14 expression is also high in the bone metastatic, AR negative prostate cancer PC3 cell line. Similarly, knocking-down FN14 in PC3 cells inhibited bone metastatic capacity following intracardiac inoculation in a xenograft model. TWEAK and FN14 are relatively low or undetectable in AR positive prostate cancer cell lines including Lncap and 22RV1. AR binding was detected in both TWEAK and FN14 promoters as determined by chromatin immunoprecipitation analysis in Lncap. Reporter assays demonstrated that AR binding inhibited FN14 and TWEAK transcription. The above experimental models were further supported by analyses of publically available expression data sets for clinical prostate cancer samples that showed FN14 expression was inversely correlated with AR gene expression signatures. In addition, individual FN14 RNA mean expression was higher in clinical metastasis of castrate (n=11) as compared to non-castrate (n=8) patients. We propose that down-regulation of AR activity under castrate conditions may lead to increased FN14 expression and NFκB activation, which provides a survival benefit for prostate cancer cells associated with disease progression. Therefore, targeting the TWEAK-FN14 pathway in prostate cancer patients provides potential, new preventive and therapeutic approaches.

Citation Format: Juan Juan Yin, Yen-Nien Liu, Ben Barrett, Lei Fang, Ross Lake, Orla Casey, Heather Tillman, Yvona Ward, Kathleen Kelly. The TWEAK-FN14 pathway promotes prostate cancer bone metastasis through the NFκB pathway. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2708. doi:10.1158/1538-7445.AM2013-2708

TMPRSS2- driven ERG expression in vivo increases self-renewal and maintains expression in a castration resistant subpopulation

Genomic rearrangements commonly occur in many types of cancers and often initiate or alter the progression of disease. Here we describe an in vivo mouse model that recapitulates the most frequent rearrangement in prostate cancer, the fusion of the promoter region of TMPRSS2 with the coding region of the transcription factor, ERG. A recombinant bacterial artificial chromosome including an extended TMPRSS2 promoter driving genomic ERG was constructed and used for transgenesis in mice. TMPRSS2-ERG expression was evaluated in tissue sections and FACS-fractionated prostate cell populations. In addition to the anticipated expression in luminal cells, TMPRSS2-ERG was similarly expressed in the Sca-1^hi/EpCAM⁺ basal/progenitor fraction, where expanded numbers of clonogenic self-renewing progenitors were found, as assayed by in vitro sphere formation. These clonogenic cells increased intrinsic self renewal in subsequent generations. In addition, ERG dependent self-renewal and invasion in vitro was demonstrated in prostate cell lines derived from the model. Clinical studies have suggested that the TMPRSS2-ERG translocation occurs early in prostate cancer development. In the model described here, the presence of the TMPRSS2-ERG fusion alone was not transforming but synergized with heterozygous Pten deletion to promote PIN. Taken together, these data suggest that one function of TMPRSS2-ERG is the expansion of self-renewing cells, which may serve as targets for subsequent mutations. Primary prostate epithelial cells demonstrated increased post transcriptional turnover of ERG compared to the TMPRSS2-ERG positive VCaP cell line, originally isolated from a prostate cancer metastasis. Finally, we determined that TMPRSS2-ERG expression occurred in both castration-sensitive and resistant prostate epithelial subpopulations, suggesting the existence of androgen-independent mechanisms of TMPRSS2 expression in prostate epithelium.

Histone deacetylase inhibitors influence chemotherapy transport by modulating expression and trafficking of a common polymorphic variant of the ABCG2 efflux transporter

Histone deacetylase inhibitors (HDI) have exhibited some efficacy in clinical trials, but it is clear that their most effective applications have yet to be fully determined. In this study, we show that HDIs influence the expression of a common polymorphic variant of the chemotherapy drug efflux transporter ABCG2, which contributes to normal tissue protection. As one of the most frequent variants in human ABCG2, the polymorphism Q141K impairs expression, localization, and function, thereby reducing drug clearance and increasing chemotherapy toxicity. Mechanistic investigations revealed that the ABCG2 Q141K variant was fully processed but retained in the aggresome, a perinuclear structure, where misfolded proteins aggregate. In screening for compounds that could correct its expression, localization, and function, we found that the microtubule-disrupting agent colchicine could induce relocalization of the variant from the aggresome to the cell surface. More strikingly, we found that HDIs could produce a similar effect but also restore protein expression to wild-type levels, yielding a restoration of ABCG2-mediated specific drug efflux activity. Notably, HDIs did not modify aggresome structures but instead rescued newly synthesized protein and prevented aggresome targeting, suggesting that HDIs disturbed trafficking along microtubules by eliciting changes in motor protein expression. Together, these results showed how HDIs are able to restore wild-type functions of the common Q141K polymorphic isoform of ABCG2. More broadly, our findings expand the potential uses of HDIs in the clinic.

LPA receptor heterodimerizes with CD97 to amplify LPA-initiated RHO-dependent signaling and invasion in prostate cancer cells

CD97, an adhesion-linked G-protein-coupled receptor (GPCR), is induced in multiple epithelial cancer lineages. We address here the signaling properties and the functional significance of CD97 expression in prostate cancer. Our findings show that CD97 signals through Gα12/13 to increase RHO-GTP levels. CD97 functioned to mediate invasion in prostate cancer cells, at least in part, by associating with lysophosphatidic acid receptor 1 (LPAR1), leading to enhanced LPA-dependent RHO and extracellular signal-regulated kinase activation. Consistent with its role in invasion, depletion of CD97 in PC3 cells resulted in decreased bone metastasis without affecting subcutaneous tumor growth. Furthermore, CD97 heterodimerized and functionally synergized with LPAR1, a GPCR implicated in cancer progression. We also found that CD97 and LPAR expression were significantly correlated in clinical prostate cancer specimens. Taken together, these findings support the investigation of CD97 as a potential therapeutic cancer target.

Abstract 2610: Histone deacetylase inhibitors mediate pharmacological rescue of the ABCG2 Q141K variant: Potential for therapeutics in cancer and gout

ABCG2 is an ATP-binding cassette half-transporter that has garnered interest in pharmacogenomics as a modulator of oral drug absorption, drug excretion, and drug distribution through expression at the blood-brain barrier and the maternal-fetal barrier. A single nucleotide polymorphism (SNP) C421A in ABCG2, resulting in a glutamic acid to lysine mutation at amino acid 141 (Q141K), confers impaired protein expression and function. Pharmacogenomic studies have linked the SNP to increased exposure to substrate drugs including irinotecan, topotecan, sunitinib and gefitinib. Moreover, it has been shown that 10 % of gout cases can be attributed to the Q141K variant.

We have studied the biology underlying altered expression and function of the ABCG2 variant. Q141K protein was resistant to degradation by Endo H, suggesting that it was fully processed and folded. Q141K ABCG2 expression was increased by the lysosome inhibitor bafilomycin, indicating that some of the protein reached the cell surface, although at reduced levels. But the largest fraction of the Q141K variant was retained in an intracellular compartment where a part was eliminated through the proteasomal pathway while the undegraded ABCG2 accumulated in the aggresomes, as shown by colocalization of the transporter with the centrosome marker, γ-tubulin. The ability of several agents to impact ABCG2 trafficking to the cell surface was evaluated. Mitoxantrone, an ABCG2 substrate previously noted to act as a pharmacological chaperone, induced a 2-fold increase in Q141K ABCG2 expression at the cell surface but only slightly enhanced ABCG2-related efflux. Romidepsin and other histone deacetylase inhibitors (HDIs), known to increase levels of ABCG2 mRNA, were also assayed. A significant increase in total protein, surface expression, and function was seen in Q141K ABCG2 variant when exposed to various HDIs. Immunofluorescence analysis by confocal microscopy showed a dramatic shift to the plasma membrane following exposure to the HDIs, whereas the localization of α-tubulin, HDAC6 and vimentin, involved in aggresome formation and structure, were unchanged. These results support the notion that Q141K ABCG2 variant is trapped in the aggresome and that HDIs aid localization to the surface. Some supplementary experiments showed that the Q141K ABCG2 states of phosphorylation, acetylation and dimerization were unmodified after HDIs treatment, and that the chaperones BiP, Hsp70 or Hsp90 were not involved in rescue. Investigations into the HDI-mediated mechanisms are underway.

In addition to non-oncologic indications, the restoration of ABCG2 function has potential applications in oncology, in improving normal tissue protection and drug elimination, as well as in cancer prevention since several carcinogens are substrates for ABCG2.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2610. doi:10.1158/1538-7445.AM2011-2610

Abstract 3528: Mutational analysis of threonine 402 adjacent to the GXXXG dimerization motif in TM1 of ABCG2

ABCG2 is an ATP-binding cassette half-transporter important in normal tissue protection, drug distribution and excretion. ABCG2 requires homodimerization for function, though the mechanism for dimerization has not been worked out. We carried out mutational analysis of threonine 402, three residues away from the GXXXG motif in TM1, to study its potential role in ABCG2 dimerization (TXXXGXXXG). Single mutations to leucine (T402L) or arginine (T402R) did not have significant impact on the ABCG2 protein. On the other hand, combining the T402 mutations with the GXXXG glycine to leucine mutations (T402L/G406L/G410L and T402R/G406L/G410L) resulted in substantially reduced expression, altered glycosylation, degradation by a proteosome independent pathway and partial retention in the endoplasmic reticulum as suggested by immunostaining, Endoglycosidase H sensitivity and MG132 and bafilomycin failed effect. The T402L/G406L/G410L mutant when incubated with the ABCG2-substrate mitoxantrone showed a shift on immunoblot analysis to the band representing the fully matured glycoprotein. The T402R/G406L/G410L mutant carrying the more drastic substitution failed to shift to the surface with mitoxantrone. The same set of mutations also displayed impaired dimerization in the TOXCAT assay for TM1 compared to the wild type. Homology modeling of ABCG2 places the TXXXGXXXG motif at the dimer interface. These studies are consistent with a role for the extended TXXXGXXXG motif in ABCG2 folding, processing, and/or dimerization.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3528.

Phosphorylation of paxillin at threonine 538 by PKCdelta regulates LFA1-mediated adhesion of lymphoid cells

We investigated the PKCdelta-mediated phosphorylation of paxillin within its LIM4 domain and the involvement of this phosphorylation in activation of LFA-1 integrins of the Baf3 pro-B lymphocytic cell line. Using phosphorylated-threonine-specific antibodies, phosphorylated amino acid analysis and paxillin phosphorylation mutants, we demonstrated that TPA, the pharmacological analog of the endogenous second messenger diacyl glycerol, stimulates paxillin phosphorylation at threonine 538 (T538). The TPA-responsive PKC isoform PKCdelta directly binds paxillin in a yeast two-hybrid assay and phosphorylates paxillin at T538 in vitro and also co-immunoprecipitates with paxillin and mediates phosphorylation of this residue in vivo. Recombinant wild-type paxillin, its phospho-inhibitory T538A or phospho-mimetic T538E mutants were expressed in the cells simultaneously with siRNA silencing of the endogenous paxillin. These experiments suggest that phosphorylation of paxillin T538 contributes to dissolution of the actin cytoskeleton, redistribution of LFA-1 integrins and an increase in their affinity. We also show that phosphorylation of T538 is involved in the activation of LFA-1 integrins by TPA.

Mutational analysis of threonine 402 adjacent to the GXXXG dimerization motif in transmembrane segment 1 of ABCG2

ABCG2 is an ATP-binding cassette half-transporter important in normal tissue protection, drug distribution, and excretion. ABCG2 requires homodimerization for function, though the mechanism for dimerization has not been elucidated. We conducted mutational analysis of threonine 402, three residues from the GXXXG motif in TM1, to study its potential role in ABCG2 dimerization (TXXXGXXXG). Single mutations to leucine (T402L) or arginine (T402R) did not have a significant impact on the ABCG2 protein. On the other hand, combining the T402 mutations with the GXXXG glycine to leucine mutations (T402L/G406L/G410L and T402R/G406L/G410L) resulted in a substantially reduced level of expression, altered glycosylation, degradation by a proteosome-independent pathway, and partial retention in the endoplasmic reticulum as suggested by immunostaining, Endo H sensitivity, and MG132 and bafilomycin failed effect. The T402L/G406L/G410L mutant when incubated with the ABCG2 substrate MX showed a shift on immunoblot analysis to the band representing the fully mature glycoprotein. The T402R/G406L/G410L mutant carrying the more drastic substitution was found to primarily localize intracellularly. The same set of mutations also displayed impaired dimerization in the TOXCAT assay for TM1 compared to that of the wild type. Homology modeling of ABCG2 places the TXXXGXXXG motif at the dimer interface. These studies are consistent with a role for the extended TXXXGXXXG motif in ABCG2 folding, processing, and/or dimerization.

Clinical outcome in children with recurrent neuroblastoma treated with ABT-751 and effect of ABT-751 on proliferation of neuroblastoma cell lines and on tubulin polymerization in vitro

BACKGROUND

ABT-751, an orally bioavailable sulfonamide, binds beta-tubulin to inhibit microtubule polymerization. We described response and event-free survival (EFS) in children with neuroblastoma and other solid tumors receiving ABT-751, assessed in vitro cytotoxicity of ABT-751 and evaluated the effect of ABT-751 on tubulin polymerization in peripheral blood mononuclear cells (PBMC) and pediatric tumor cell lines.

PROCEDURE

Patients with neuroblastoma (n = 50) or other solid tumors (n = 26) enrolled on the ABT-751 pediatric phase I and pilot trials were reviewed. The sulforhodamine B (SRB) and ACEA Real-Time Cell Electronic Sensing (RT-CES) assays were used to determine the in vitro cytotoxicity. Pharmacodynamic effects on tubulin polymerization/depolymerization were assessed by Western blot and confocal microscopy using antibodies specific for post-translational modifications of polymerized tubulin.

RESULTS

Forty-five patients with neuroblastoma were evaluated for anti-tumor response. No complete or partial responses were documented. The median EFS was 9.3 weeks for children with neuroblastoma and 3.3 weeks for children other solid tumors (P < 0.0001). The ABT-751 IC(50) was 0.6-2.6 mcM in neuroblastoma and 0.7-4.6 mcM in other solid tumor cell lines. Following drug exposure, polymerized tubulin decreased in a concentration- and time-dependent manner in cell lines.

CONCLUSIONS

In children treated with ABT-751, the EFS is longer in children with neuroblastoma as compared to other diagnoses. In vitro, ABT-751 was cytotoxic at concentrations tolerable in children. Effects of ABT-751 on polymerization and microtubule structure were time- and dose-dependent but not dependent on tumor type.

Centrosome amplification induced by the antiretroviral nucleoside reverse transcriptase inhibitors lamivudine, stavudine, and didanosine

In cultured cells, exposure to the nucleoside reverse transcriptase inhibitor (NRTI) zidovudine (AZT) induces genomic instability, cell cycle arrest, micronuclei, sister chromatid exchanges, and shortened telomeres. In previous studies, we demonstrated AZT-induced centrosome amplification (>2 centrosomes/cell). Here, we investigate centrosome amplification in cells exposed to other commonly used NRTIs. Experiments were performed using Chinese Hamster ovary (CHO) cells, and two normal human mammary epithelial cell (NHMEC) strains: M99005 and M98040, which are high and low incorporators of AZT into DNA, respectively. Cells were exposed for 24 hr to lamivudine (3TC), stavudine (d4T), didanosine (ddI), and thymidine, and stained with anti-pericentrin antibody. Dose response curves were performed to determine cytotoxicity and a lower concentration at near plasma levels and a 10 fold higher concentration were chosen for the experiments. In CHO cells, there was a concentration-dependent, significant (P < 0.05) increase in centrosome amplification for each of the NRTIs. In NHMEC strain M99005, an NRTI-induced increase (P < 0.05) in centrosome amplification was observed for the high concentrations of each NRTI and the low doses of 3TC and ddI. In NHMEC strain M98040, the high doses of ddI and d4T showed significant increases in centrosome amplification. Functional viability of amplified centrosomes was assessed by arresting microtubule nucleation with nocodazole. In cells with more than two centrosomes, the ability to recover microtubule nucleation was similar to that of unexposed cells. We conclude that centrosome amplification is a consequence of exposure to NRTIs and that cells with centrosome amplification are able to accomplish cell division.

Arginine 383 is a crucial residue in ABCG2 biogenesis

ABCG2 is an ATP-binding cassette half-transporter initially identified in multidrug-resistant cancer cell lines and recently suggested to play an important role in pharmacokinetics. Here we report studies of a conserved arginine predicted to localize near the cytoplasmic side of TM1. First, we determined the effect of losing charge and bulk at this position via substitutions with glycine and alanine. The R383G mutant when transfected into HEK cells was not detectable on immunoblot or by functional assay, while the R383A mutant exhibited detectable but significantly decreased levels compared to wild-type, partial retention in the ER and altered glycosylation. Efflux of the ABCG2-substrates mitoxantrone and pheophorbide a was observed. Our experiments suggested rapid degradation of the R383A mutant by the proteasome via a kifunensine-insensitive pathway. Interestingly, overnight treatment of the R383A mutant with mitoxantrone assisted in protein maturation as evidenced by a shift to the N-glycosylated form. The R383A mutant when expressed in insect cells, though detected on the surface, had no measurable ATPase activity. In addition, substitution with the positively charged lysine resulted in significantly decreased protein expression levels in HEK cells, while retaining function. In conclusion, arginine 383 is a crucial residue for ABCG2 biogenesis, where even the most conservative mutations have a large impact.

Centrosomal amplification and aneuploidy induced by the antiretroviral drug AZT in hamster and human cells

The centrosome directs chromosomal migration by a complex process of tubulin-chromatin binding. In this contribution centrosomal abnormalities, including centrosomal amplification, were explored in Chinese hamster ovary (CHO) and normal human mammary epithelial cells (NHMECs) exposed to the antiretroviral drug zidovudine (3'-azido-3'-deoxythymidine, AZT). Centrosomal amplification/fragmentation was observed in both cell types and kinetochore positive micronuclei were found in AZT-exposed CHO cells in correlation with dose. Normal human mammary epithelial cell (NMHEC) strain M99005, previously identified as a strain that incorporates high levels of AZT into DNA (high incorporator, HI), showed greater centrosomal amplification when compared with a second strain, NHMEC M98040, which did not incorporate AZT into DNA (low incorporator, LI). Additionally, an abnormal tubulin distribution was observed in AZT-exposed HI cells bearing multiple centrosomes. Immunofluorescent staining of human cells with Aurora A, a kinase involved in the maturation of the centrosome, confirmed the induction of centrosomal amplification and revealed multipolar mitotic figures. Flow cytometric studies revealed that cells bearing abnormal numbers of centrosomes and abnormal tubulin distribution had similar S-phase percentages suggesting that cells bearing unbalanced chromosomal segregation could divide. Therefore, AZT induces genomic instability and clastogenicity as well as alterations in proteins involved in centrosomal activation, all of which may contribute to the carcinogenic properties of this compound.

Melanoma antigen-11 inhibits the hypoxia-inducible factor prolyl hydroxylase 2 and activates hypoxic response

Activation of hypoxia-inducible factors (HIF), responsible for tumor angiogenesis and glycolytic switch, is regulated by reduced oxygen availability. Normally, HIF-alpha proteins are maintained at low levels, controlled by site-specific hydroxylation carried out by HIF prolyl hydroxylases (PHD) and subsequent proteasomal degradation via the von Hippel-Lindau ubiquitin ligase. Using a yeast two-hybrid screen, we identified an interaction between melanoma antigen-11 (MAGE-11) cancer-testis antigen and the major HIF-alpha hydroxylating enzyme PHD2. The interaction was confirmed by a pull-down assay, coimmunoprecipitation, and colocalization in both normoxic and hypoxic conditions. Furthermore, MAGE-9, the closest homologue of MAGE-11, was also found to interact with PHD2. MAGE-11 inhibited PHD activity without affecting protein levels. This inhibition was accompanied by stabilization of ectopic or endogenous HIF-1alpha protein. Knockdown of MAGE-11 by small interfering RNA results in decreased hypoxic induction of HIF-1alpha and its target genes. Inhibition of PHD by MAGE-11, and following activation of HIFs, is a novel tumor-associated HIF regulatory mechanism. This finding provides new insights into the significance of MAGE expression in tumors and may provide valuable tools for therapeutic intervention because of the restricted expression of the MAGE gene family in cancers, but not in normal tissues.

TDAG51 is an ERK signaling target that opposes ERK-mediated HME16C mammary epithelial cell transformation

Introduction

Signaling downstream of Ras is mediated by three major pathways, Raf/ERK, phosphatidylinositol 3 kinase (PI3K), and Ral guanine nucleotide exchange factor (RalGEF). Ras signal transduction pathways play an important role in breast cancer progression, as evidenced by the frequent over-expression of the Ras-activating epidermal growth factor receptors EGFR and ErbB2. Here we investigated which signal transduction pathways downstream of Ras contribute to EGFR-dependent transformation of telomerase-immortalized mammary epithelial cells HME16C. Furthermore, we examined whether a highly transcriptionally regulated ERK pathway target, PHLDA1 (TDAG51), suggested to be a tumor suppressor in breast cancer and melanoma, might modulate the transformation process.

Methods

Cellular transformation of human mammary epithelial cells by downstream Ras signal transduction pathways was examined using anchorage-independent growth assays in the presence and absence of EGFR inhibition. TDAG51 protein expression was down-regulated by interfering small hairpin RNA (shRNA), and the effects on cell proliferation and death were examined in Ras pathway-transformed breast epithelial cells.

Results

Activation of both the ERK and PI3K signaling pathways was sufficient to induce cellular transformation, which was accompanied by up-regulation of EGFR ligands, suggesting autocrine EGFR stimulation during the transformation process. Only activation of the ERK pathway was sufficient to transform cells in the presence of EGFR inhibition and was sufficient for tumorigenesis in xenografts. Up-regulation of the PHLDA1 gene product, TDAG51, was found to correlate with persistent ERK activation and anchorage-independent growth in the absence or presence of EGFR inhibition. Knockdown of this putative breast cancer tumor-suppressor gene resulted in increased ERK pathway activation and enhanced matrix-detached cellular proliferation of Ras/Raf transformed cells.

Conclusion

Our results suggest that multiple Ras signal transduction pathways contribute to mammary epithelial cell transformation, but that the ERK signaling pathway may be a crucial component downstream of EGFR activation during tumorigenesis. Furthermore, persistent activation of ERK signaling up-regulates TDAG51. This event serves as a negative regulator of both Erk activation as well as matrix-detached cellular proliferation and suggests that TDAG51 opposes ERK-mediated transformation in breast epithelial cells.

ASAP3 is a focal adhesion-associated Arf GAP that functions in cell migration and invasion

ASAP3, an Arf GTPase-activating protein previously called DDEFL1 and ACAP4, has been implicated in the pathogenesis of hepatocellular carcinoma. We have examined in vitro and in vivo functions of ASAP3 and compared it to the related Arf GAP ASAP1 that has also been implicated in oncogenesis. ASAP3 was biochemically similar to ASAP1: the pleckstrin homology domain affected function of the catalytic domain by more than 100-fold; catalysis was stimulated by phosphatidylinositol 4,5-bisphosphate; and Arf1, Arf5, and Arf6 were used as substrates in vitro. Like ASAP1, ASAP3 associated with focal adhesions and circular dorsal ruffles. Different than ASAP1, ASAP3 did not localize to invadopodia or podosomes. Cells, derived from a mammary carcinoma and from a glioblastoma, with reduced ASAP3 expression had fewer actin stress fiber, reduced levels of phosphomyosin, and migrated more slowly than control cells. Reducing ASAP3 expression also slowed invasion of mammary carcinoma cells. In contrast, reduction of ASAP1 expression had no effect on migration or invasion. We propose that ASAP3 functions nonredundantly with ASAP1 to control cell movement and may have a role in cancer cell invasion. In comparing ASAP1 and ASAP3, we also found that invadopodia are dispensable for the invasive behavior of cells derived from a mammary carcinoma.

Proteasome inhibitors increase tubulin polymerization and stabilization in tissue culture cells: a possible mechanism contributing to peripheral neuropathy and cellular toxicity following proteasome inhibition

Bortezomib (Velcade((R))), a proteasome inhibitor, is approved by the FDA for the treatment of multiple myeloma (MM). While effective, its use has been hampered by peripheral neurotoxicity of unexplained etiology. Since proteasome inhibitors alter protein degradation, we speculated that proteins regulating microtubule (MT) stability may be affected after treatment and examined MT polymerization in cells by comparing the distribution of tubulin between polymerized (P) and soluble (S) fractions. We observed increased MT polymerization following treatment of SY5Y and KCNR [neuroblastoma], HCN2, and 8226 [MM] cells, using five proteasome inhibitors; the baseline proportion of total alpha-tubulin in 'P' fractions ranged from approximately 41-68%, and increased to approximately 55-99% after treatment. Increased acetylated alpha-tubulin, a post-translational marker of stabilized MTs, was observed in the neural cell lines HCN1A and HCN2 and this was sustained up to 144 hours after the proteasome inhibitor was removed. Cell cycle analysis of three cell lines after treatment, showed approximately 50-75% increases in the G(2)M phase. Immunofluorescent localization studies of proteasome inhibitor treated cells did not reveal microtubule bundles in contrast to paclitaxel treated, suggesting MT stabilization via a mechanism other than direct drug binding. We examined the levels of microtubule associated proteins and observed a 1.4-3.7 fold increase in the microtubule associated protein MAP2, in HCN2 cells following treatment with proteasome inhibitors. These data provide a plausible explanation for the neurotoxicity observed clinically and raise the possibility that microtubule stabilization contributes to cytotoxicity.

Improved antibacterial host defense and altered peripheral granulocyte homeostasis in mice lacking the adhesion class G protein receptor CD97

CD97 is a member of the adhesion family of G protein-coupled receptors. Alternatively spliced forms of CD97 bind integrins alpha5beta1 and alphavbeta3, decay accelerating factor, or dermatan sulfate. CD97 is expressed on myeloid cells at high levels and a variety of other cell types at lower levels. Little is known about the physiological function of CD97. To begin dissecting the function of CD97, we evaluated the immune response of CD97 null mice to systemic infection by Listeria monocytogenes. CD97 null mice were significantly more resistant to listeriosis than matched wild-type mice. A major determinant of the difference in survival appeared to be the comparatively more robust accumulation of granulocytes in the blood and in infected livers of CD97 null mice within 18 h of inoculation, correlating with a decrease in the number of bacteria. CD97 null mice also displayed a mild granulocytosis in the nonchallenged state. Because there is a strong suggestion that CD97 functions in an adhesive capacity, we examined the migratory properties of granulocytes in CD97 null mice. In chimeric animals, CD97 null and wild-type granulocytes migrated similarly, as determined by inflammation-induced emigration from the bone marrow and accumulation in the peritoneum. Granulocyte development in the bone marrow of CD97 null mice was comparable to that of wild-type mice, and CD97 deficiency did not appear to stimulate granulocytosis secondary to peripheral inflammation and resultant granulocyte colony-stimulating factor induction, unlike various other models of adhesion deficiencies. Our results suggest that CD97 plays a role in peripheral granulocyte homeostasis.

Versican-thrombospondin-1 binding in vitro and colocalization in microfibrils induced by inflammation on vascular smooth muscle cells

We identified a specific interaction between two secreted proteins, thrombospondin-1 and versican, that is induced during a toll-like receptor-3-dependent inflammatory response in vascular smooth muscle cells. Thrombospondin-1 binding to versican is modulated by divalent cations. This interaction is mediated by interaction of the G1 domain of versican with the N-module of thrombospondin-1 but only weakly with the corresponding N-terminal region of thrombospondin-2. The G1 domain of versican contains two Link modules, which are known to mediate TNFalpha-stimulated gene-6 protein binding to thrombospondin-1, and the related G1 domain of aggrecan is also recognized by thrombospondin-1. Therefore, thrombospondin-1 interacts with three members of the Link-containing hyaladherin family. On the surface of poly-I:C-stimulated vascular smooth muscle cells, versican organizes into fibrillar structures that contain elastin but are largely distinct from those formed by hyaluronan. Endogenous and exogenously added thrombospondin-1 incorporates into these structures. Binding of exogenous thrombospondin-1 to these structures, to purified versican and to its G1 domain is potently inhibited by heparin. At higher concentrations, exogenous thrombospondin-1 delays the poly-I:C induced formation of structures containing versican and elastin, suggesting that thrombospondin-1 negatively modulates this component of a vascular smooth muscle inflammatory response.

Structure-function studies of the G-domain from human gem, a novel small G-protein

Gem, a member of the Rad,Gem/Kir subfamily of small G-proteins, has unique sequence features. We report here the crystallographic structure determination of the Gem G-domain in complex with nucleotide to 2.4 A resolution. Although the basic Ras protein fold is maintained, the Gem switch regions emphatically differ from the Ras paradigm. Our ensuing biochemical characterization indicates that Gem G-domain markedly prefers GDP over GTP. Two known functions of Gem are distinctly affected by spatially separated clusters of mutations.

Mutational studies of G553 in TM5 of ABCG2: a residue potentially involved in dimerization

ABCG2 is an ATP-binding cassette half-transporter conferring resistance to chemotherapeutic agents such as mitoxantrone, irinotecan, and flavopiridol. With its one transmembrane and one ATP-binding domain, ABCG2 is thought to homodimerize for function. One conserved region potentially involved in dimerization is a three-amino acid sequence in transmembrane segment 5 (residues 552-554). Mutations in the corresponding residues in the Drosophila white protein (an orthologue of ABCG2) are thought to disrupt heterodimerization. We substituted glycine 553 with leucine (G553L) followed by stable transfection in HEK 293 cells. The mutant was not detectable on the cell surface, and markedly reduced protein expression levels were observed by immunoblotting. A deficiency in N-linked glycosylation was suggested by a reduction in molecular mass compared to that of the 72 kDa wild-type ABCG2. Similar results were observed with the G553E mutant. Confocal microscopy demonstrated mostly ER localization of the G553L mutant in HEK 293 cells, even when coexpressed with the wild-type protein. Despite its altered localization, the G553L and G553E mutants were cross-linked using amine-reactive cross-linkers with multiple arm lengths, suggesting that the monomers are in the proximity of each other but are unable to complete normal trafficking. Interestingly, when expressed in Sf9 insect cells, G553L moves to the cell membrane but is unable to hydrolyze ATP or transport the Hoechst dye. Still, when coexpressed, the mutant interferes with the Hoechst transport activity of the wild-type protein. These data show that glycine 553 is important for protein trafficking and are consistent with, but do not yet prove, its involvement in ABCG2 homodimerization.

Expression of FasL in squamous cell carcinomas of the cervix and cervical intraepithelial neoplasia and its role in tumor escape mechanism

BACKGROUND

To date, several mechanisms have been described by which malignant cells escape from the immune system. One of these is through the expression of FasL. The authors hypothesized that the Fas/FasL interaction enables cervical carcinoma cells to induce apoptosis of the cells of the immune system and thereby escape from them.

METHODS

The authors tested the expression of FASL on the surface of cervical carcinoma tissues. Next, they stained the same cervical tissues with anti-human leukocyte common antigen and TUNEL to identify apoptotic cells. An in vitro functional assay was then done to test if the FASL expressed on the surface of cervical carcinoma cell lines was or was not responsible for inducing apoptosis in T-cells. Finally, they compared the expression of FASL on normal and dysplastic cervical tissues.

RESULTS

Ninety-four percent of the cervical carcinoma tissues the authors tested expressed FasL and the majority of the apoptotic cells in the specimens were leukocytes with very few tumor cells. In the in vitro functional assay, only the Fasl expressing cell line and not the Fasl negative cell line was able to induce apoptosis of the Fas-expressing Jurkat cells. On examining the normal cervical tissues, the authors found that the expression of Fasl was confined to the basal cell layer with loss of expression observed in the suprabasal layers, which made it an immune privileged site. Conversely, there was persistent expression of FasL in the dysplastic layers in cervical dysplasia and squamous cell carcinoma specimens.

CONCLUSIONS

The findings of the current study support the authors' hypothesis that persistent expression of FasL plays a role in the ability of cervical carcinoma cells to escape from the immune system.

Gem Protein Signaling and Regulation

Gem is a member of the RGK family of GTP-binding proteins within the Ras superfamily possessing a ras-like core and terminal extensions. We have used a variety of cell-based assays to investigate the physiological role of Gem and combined these assays with site-directed mutagenesis of Gem protein to identify the sites responsible for regulation of Gem activity. One function of Gem that has been explained is the inhibition of Rho kinase (ROK)-mediated cytoskeletal rearrangement. Transient expression of Gem in endothelial cells and stable transfection of fibroblasts resulted in decreased stress fiber formation and focal adhesion assembly. A neurite extension model using N1E-115 murine neuroblastoma showed that Gem inhibits actinomyosin-related contractility by specifically opposing ROKbeta activity. Phospho-specific antibodies were used in Western blot analysis to show that Gem prevents phosphorylation of the regulatory subunit of myosin light chain and myosin phosphatase by ROKbeta. On the contrary, LIMK, another substrate of ROKbeta, was unaffected by Gem expression as demonstrated by an in vitro kinase assay, suggesting that Gem exerts its effect by changing the substrate specificity of ROKbeta rather than by blocking its catalytic activity. Point mutations of Gem at serines 261 and 289 in the carboxyl-terminus inhibited Gem function, indicating that posttranslational phosphorylation of these serines regulates Gem's effect on cytoskeletal reorganization. Another biological role of Gem is inhibition of voltage-gated calcium channel activity. By use of a PC12 cell model combined with site-directed mutagenesis, we demonstrated that Gem inhibits growth hormone secretion stimulated by calcium influx through L-type calcium channels and that this function is dependent on GTP and calmodulin binding to Gem. The theory and method for the assays discussed previously are reviewed here.

Human CD69 associates with an N-terminal fragment of calreticulin at the cell surface

CD69 is thought to be a pluripotent signaling molecule expressed on the surface of a number of activated leukocytes including B, T, and NK cells, monocytes, neutrophils, and platelets. While some advances have been made regarding the mechanisms by which CD69 may participate in such diverse functions as cell aggregation, cellular cytotoxicity, and release of cytokines and inflammatory mediators, the most proximal links of signal initiation have not been identified. Our study has identified, by immunoprecipitation and direct protein sequencing (LC/MS/MS), binding of CD69 to an N-terminal protein fragment of calreticulin expressed on the cell surface of human PBMCs. Given the recently identified roles calreticulin plays in cell adhesion and angiogensis, the identification of CD69 binding directly to calreticulin may provide insights into mechanism(s) by which CD69 or other CD69 family members, i.e., LLT1 and AICL participates in such diverse functions.

Tissue inhibitors of metalloproteinase 2 inhibits endothelial cell migration through increased expression of RECK

The antiangiogenic function of the tissue inhibitors of metalloproteinases (TIMPs) has been attributed to their matrix metalloproteinase inhibitory activity. Here we demonstrate that TIMP-1 but not Ala+TIMP-1 inhibits both basal and vascular endothelial growth factor (VEGF)-stimulated migration of human microvascular endothelial cells (hMVECs), suggesting that this effect is dependent on direct inhibition of matrix metalloproteinase (MMP) activity. In contrast, TIMP-2 and mutant Ala+TIMP-2, which is devoid of MMP inhibitory activity, block hMVEC migration in response to VEGF-A stimulation. TIMP-2 and Ala+TIMP-2 also suppress basal hMVEC migration via a time-dependent mechanism mediated by enhanced expression of RECK, a membrane-anchored MMP inhibitor, which, in turn, inhibits cell migration. TIMP-2 treatment of hMVECs increases the association of Crk with C3G, resulting in enhanced Rap1 activation. hMVECs stably expressing Rap1 have increased RECK expression and display reduced cell migration compared with those expressing inactive Rap1(38N). RECK-null murine embryo fibroblasts fail to demonstrate TIMP-2-mediated decrease in cell migration despite activation of Rap1. TIMP-2-induced RECK decreases cell-associated MMP activity. Anti-RECK antibody increases MMP activity and reverses the TIMP-2-mediated reduction in cell migration. The effects of TIMP-2 on RECK expression and cell migration were confirmed in A2058 melanoma cells. These results suggest that TIMP-2 can inhibit cell migration via several distinct mechanisms. First, TIMP-2 can inhibit cell migration after VEGF stimulation by direct inhibition of MMP activity induced in response to VEGF stimulation. Secondly, TIMP-2 can disrupt VEGF signaling required for initiation of hMVEC migration. Third, TIMP-2 can enhance expression of RECK via Rap1 signaling resulting in an indirect, time-dependent inhibition of endothelial cell migration.

CD97, an adhesion receptor on inflammatory cells, stimulates angiogenesis through binding integrin counterreceptors on endothelial cells

CD97, a membrane protein expressed at high levels on inflammatory cells and some carcinomas, is a member of the adhesion G protein-coupled receptor family, whose members have bipartite structures consisting of an extracellular peptide containing adhesion motifs noncovalently coupled to a class B 7-transmembrane domain. CD97alpha, the extracellular domain of CD97, contains 3 to 5 fibrillin class 1 epidermal growth factor (EGF)-like repeats, an Arg-Gly-Asp (RGD) tripeptide, and a mucin stalk. We show here that CD97alpha promotes angiogenesis in vivo as demonstrated with purified protein in a directed in vivo angiogenesis assay (DIVAA) and by enhanced vascularization of developing tumors expressing CD97. These data suggest that CD97 can contribute to angiogenesis associated with inflammation and tumor progression. Strong integrin alpha5beta1 interactions with CD97 have been identified, but alpha v beta3 also contributes to cell attachment. Furthermore, soluble CD97 acts as a potent chemoattractant for migration and invasion of human umbilical vein endothelial cells (HUVECs), and this function is integrin dependent. CD97 EGF-like repeat 4 is known to bind chondroitin sulfate. It was found that coengagement of alpha5beta1 and chondroitotin sulfate proteoglycan by CD97 synergistically initiates endothelial cell invasion. Integrin alpha5beta1 is the first high-affinity cellular counterreceptor that has been identified for a member within this family of adhesion receptors.

Direct interaction with Hoxd proteins reverses Gli3-repressor function to promote digit formation downstream of Shh

Sonic hedgehog (Shh) signaling regulates both digit number and identity, but how different distinct digit types (identities) are specified remains unclear. Shh regulates digit formation largely by preventing cleavage of the Gli3 transcription factor to a repressor form that shuts off expression of Shh target genes. The functionally redundant 5'Hoxd genes regulate digit pattern downstream of Shh and Gli3, through as yet unknown targets. Enforced expression of any of several 5'Hoxd genes causes polydactyly of different distinct digit types with posterior transformations in a Gli3(+) background, whereas, in Gli3 null limbs, polydactylous digits are all similar, short and dysmorphic, even though endogenous 5'Hoxd genes are broadly misexpressed. We show that Hoxd12 interacts genetically and physically with Gli3, and can convert the Gli3 repressor into an activator of Shh target genes. Several 5'Hoxd genes, expressed differentially across the limb bud, interact physically with Gli3. We propose that a varying [Gli3]:[total Hoxd] ratio across the limb bud leads to differential activation of Gli3 target genes and contributes to the regulation of digit pattern. The resulting altered balance between 'effective' Gli3 activating and repressing functions may also serve to extend the Shh activity gradient spatially or temporally.

Phosphorylation of critical serine residues in Gem separates cytoskeletal reorganization from down-regulation of calcium channel activity

Gem is a small GTP-binding protein that has a ras-like core and extended chains at each terminus. The primary structure of Gem and other RGK family members (Rad, Rem, and Rem2) predicts a GTPase deficiency, leading to the question of how Gem functional activity is regulated. Two functions for Gem have been demonstrated, including inhibition of voltage-gated calcium channel activity and inhibition of Rho kinase-mediated cytoskeletal reorganization, such as stress fiber formation and neurite retraction. These functions for Gem have been ascribed to its interaction with the calcium channel beta subunit and Rho kinase beta, respectively. We show here that these functions are separable and regulated by distinct structural modifications to Gem. Phosphorylation of serines 261 and 289, located in the C-terminal extension, is required for Gem-mediated cytoskeletal reorganization, while GTP and possibly calmodulin binding are required for calcium channel inhibition. In addition to regulating cytoskeletal reorganization, phosphorylation of serine 289 in conjunction with serine 23 results in bidentate 14-3-3 binding, leading to increased Gem protein half-life. Evidence presented shows that phosphorylation of serine 261 is mediated via a cdc42/protein kinase Czeta-dependent pathway. These data demonstrate that phosphorylation of serines 261 and 289, outside the GTP-binding region of Gem, controls its inhibition of Rho kinase beta and associated changes in the cytoskeleton.

Tumor Expression of 4-1BB Ligand Sustains Tumor Lytic T Cells

Inadequate costimulation by solid tumors is generally believed to induce immune tolerance during primary tumor growth. We looked for tumor-specific immunity vs. tolerance in patients with Ewing's sarcoma. Circulating T cells from patients with progressively growing Ewing's tumors displayed MHC restricted tumor-induced proliferation and robust tumor lysis. Tumor-reactive T cells reside within the memory CD3+CD8+ subset and are CD28-/4-1BB+. Autologous Ewing's tumors expressed 4-1BBL, and tumor-induced T cell proliferation and activation required costimulation by 4-1BBL. Stimulation of PBL with anti-CD3/4-1BBL, but not anti-CD3/anti-CD28 induced tumor lytic effectors. Similarly, in a xenograft model, anti-CD3/4-1BBL expanded T cells controlled primary growth and prevented metastasis of autologous tumors while nonactivated and anti-CD3/anti-CD28 activated CD8+ cells did not. These results question prevailing models of tumor induced tolerance accompanying progressive tumor growth; rather, we show coexistence of progressive tumor growth and anti-tumor immunity, with costimulation provided by the tumor itself. They further demonstrate a potential new therapeutic role for 4-1BBL mediated costimulation in expanding tumor reactive CTLs for use in the adoptive immunotherapy of cancer.

Revelation of a cryptic major histocompatibility complex class II-restricted tumor epitope in a novel RNA-processing enzyme

CD4+ T-cell responses against human tumor antigens are a potentially critical component of the antitumor immune response. Molecular methods have been devised for rapidly identifying MHC class II-restricted tumor antigens and elucidating the recognized epitopes. We describe here the identification of neo-poly(A) polymerase (neo-PAP), a novel RNA processing enzyme overexpressed in a variety of human cancers, by screening a melanoma-derived invariant chain fusion cDNA library with tumor-reactive CD4+ T lymphocytes. A cryptic nonmutated HLA-DRbeta1*0701-restricted neo-PAP epitope was processed through the endogenous MHC class II pathway. A unique point mutation effected a nonconservative substitution of a leucine for a proline residue at a structurally important site in neo-PAP that was remote from the recognized peptide, revealing a normally silent epitope for immune recognition. Genetic aberrations such as the described point mutation can have unexpected immunological consequences, in this case leading to immune recognition of a distant normal self epitope.

The GTP binding proteins Gem and Rad are negative regulators of the Rho-Rho kinase pathway

The cytoskeletal changes that alter cellular morphogenesis and motility depend upon a complex interplay among molecules that regulate actin, myosin, and other cytoskeletal components. The Rho family of GTP binding proteins are important upstream mediators of cytoskeletal organization. Gem and Rad are members of another family of small GTP binding proteins (the Rad, Gem, and Kir family) for which biochemical functions have been mostly unknown. Here we show that Gem and Rad interface with the Rho pathway through association with the Rho effectors, Rho kinase (ROK) alpha and beta. Gem binds ROKbeta independently of RhoA in the ROKbeta coiled-coil region adjacent to the Rho binding domain. Expression of Gem inhibited ROKbeta-mediated phosphorylation of myosin light chain and myosin phosphatase, but not LIM kinase, suggesting that Gem acts by modifying the substrate specificity of ROKbeta. Gem or Rad expression led to cell flattening and neurite extension in N1E-115 neuroblastoma cells. In interference assays, Gem opposed ROKbeta- and Rad opposed ROKalpha-mediated cell rounding and neurite retraction. Gem did not oppose cell rounding initiated by ROKbeta containing a deletion of the Gem binding region, demonstrating that Gem binding to ROKbeta is required for the effects observed. In epithelial or fibroblastic cells, Gem or Rad expression resulted in stress fiber and focal adhesion disassembly. In addition, Gem reverted the anchorage-independent growth and invasiveness of Dbl-transformed fibroblasts. These results identify physiological roles for Gem and Rad in cytoskeletal regulation mediated by ROK.

Accompanying protein alterations in malignant cells with a microtubule-polymerizing drug-resistance phenotype and a primary resistance mechanism

Microtubules (MTs) are cytoskeletal components whose structural integrity is mandatory for the execution of many basic cell functions. Utilizing parental and drug-resistant ovarian carcinoma cell lines that have acquired point mutations in beta-tubulin and p53, we studied the level of expression and modification of proteins involved in apoptosis and MT integrity. Extending previous results, we demonstrated phosphorylation of pro-survival Bcl-x(L) in an epothilone-A resistant cell line, correlating it with drug sensitivity to tubulin-active compounds. Furthermore, Mcl-1 protein turned over more rapidly following exposure to tubulin-modifying agents, the stability of Mcl-1 protein paralleling the drug sensitivity profile of the paclitaxel or epothilone-A resistant cell lines. The observed decreases in Mcl-1 were not a consequence of G(2)M arrest, as determined by flow cytometry analysis, which showed prominent levels of Mcl-1 in the absence of any drug treatment in populations enriched in mitotic cells. We also observed that a paclitaxel-resistant cell line expressed Bax at a much lower level than the sensitive parental line [A2780(1A9)], consistent with its mutant p53 status. MT-associated protein-4 (MAP4), whose phosphorylation during specific phases of the cell cycle reduces its MT-polymerizing and -stabilizing capabilities, was phosphorylated in response to drug challenge without a change in expression. Phosphorylation of MAP4 correlated with sensitivity to tubulin-binding drugs and with a dissociation from MTs. We propose that the tubulin mutations, which result in a compromised paclitaxel:tubulin or epothilone:tubulin interaction and paclitaxel or epothilone resistance, indirectly inhibit downstream events that lead to cell death, and this, in turn, may contribute to the drug-resistance phenotype

Signal pathways which promote invasion and metastasis: critical and distinct contributions of extracellular signal-regulated kinase and Ral-specific guanine exchange factor pathways

Approximately 50% of metastatic tumors contain Ras mutations. Ras proteins can activate at least three downstream signaling cascades mediated by the Raf-MEK-extracellular signal-regulated kinase family, phosphatidylinositol-3 (PI3) kinase, and Ral-specific guanine nucleotide exchange factors (RalGEFs). Here we investigated the contribution of RalGEF and ERK activation to the development of experimental metastasis in vivo and associated invasive properties in vitro. Each pathway contributes distinct properties to the metastatic phenotype. Following lateral tail vein injection, 3T3 cells transformed by constitutively active Raf or MEK produced lung metastasis that displayed circumscribed, noninfiltrating borders. In contrast, 3T3 cells transformed by Ras(12V,37G), a Ras effector mutant that activates RalGEF but not Raf or P13 kinase, formed aggressive, infiltrative metastasis. Dominant negative RalB inhibited Ras(12V,37G)-activated invasion and metastasis, demonstrating the necessity of the RalGEF pathway for a fully transformed phenotype. Moreover, 3T3 cells constitutively expressing a membrane-associated form of RalGEF (RalGDS-CAAX) formed invasive tumors as well, demonstrating that activation of a RalGEF pathway is sufficient to initiate the invasive phenotype. Despite the fact that Ras(12V,37G) expression does not elevate ERK activity, inhibition of this kinase by a conditionally expressed ERK phosphatase demonstrated that ERK activity was necessary for Ras(12V,37G)-transformed cells to express matrix-degrading activity in vitro and tissue invasiveness in vivo. Therefore, these experiments have revealed a hitherto-unknown but essential interaction of the RalGEF and ERK pathways to produce a malignant phenotype. The generality of the role of the RalGEF pathway in metastasis is supported by the finding that Ras(12V,37G) increased the invasiveness of epithelial cells as well as fibroblasts.

Identification and functional characterization of neo-poly(A) polymerase, an RNA processing enzyme overexpressed in human tumors

Poly(A) polymerase (PAP) plays an essential role in polyadenylation of mRNA precursors, and it has long been thought that mammalian cells contain only a single PAP gene. We describe here the unexpected existence of a human PAP, which we call neo-PAP, encoded by a previously uncharacterized gene. cDNA was isolated from a tumor-derived cDNA library encoding an 82.8-kDa protein bearing 71% overall similarity to human PAP. Strikingly, the organization of the two PAP genes is nearly identical, indicating that they arose from a common ancestor. Neo-PAP and PAP were indistinguishable in in vitro assays of both specific and nonspecific polyadenylation and also endonucleolytic cleavage. Neo-PAP produced by transfection was exclusively nuclear, as demonstrated by immunofluorescence microscopy. However, notable sequence divergence between the C-terminal domains of neo-PAP and PAP suggested that the two enzymes might be differentially regulated. While PAP is phosphorylated throughout the cell cycle and hyperphosphorylated during M phase, neo-PAP did not show evidence of phosphorylation on Western blot analysis, which was unexpected in the context of a conserved cyclin recognition motif and multiple potential cyclin-dependent kinase (cdk) phosphorylation sites. Intriguingly, Northern blot analysis demonstrated that each PAP displayed distinct mRNA splice variants, and both PAP mRNAs were significantly overexpressed in human cancer cells compared to expression in normal or virally transformed cells. Neo-PAP may therefore be an important RNA processing enzyme that is regulated by a mechanism distinct from that utilized by PAP.

The Gem GTP-binding protein promotes morphological differentiation in neuroblastoma

Gem is a small GTP-binding protein within the Ras superfamily whose function has not been determined. We report here that ectopic Gem expression is sufficient to stimulate cell flattening and neurite extension in N1E-115 and SH-SY5Y neuroblastoma cells, suggesting a role for Gem in cytoskeletal rearrangement and/or morphological differentiation of neurons. Consistent with this potential function, in clinical samples of neuroblastoma, Gem protein was most highly expressed within cells which had differentiated to express ganglionic morphology. Gem was also observed in developing trigeminal nerve ganglia in 12.5 day mouse embryos, demonstrating that Gem expression is a property of normal ganglionic development. Although Gem expression is rare in epithelial and hematopoietic cancer cell lines, constitutive Gem levels were detected in several neuroblastoma cell lines and could be further induced as much as 10-fold following treatment with PMA or the acetylcholine muscarinic agonist, carbachol.

Microstructural analysis of silicon carbide monofilaments

In the development of monofilaments, a good understanding of the process/property relationships is essential. Transmission electron microscopy (TEM) is a powerful tool but too slow and expensive to be used routinely. Alternative, cheaper techniques have therefore been investigated. The microstructures of three SiC monofilaments (DERA Sigma SM1140+, Textron SCS-6 and Ultra-SCS) and some experimental samples were studied using a combination of TEM, electron microprobe analysis, Raman microprobe analysis, thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was found that the Raman technique was complementary to TEM and easily identified the presence of amorphous C and Si. These could not be seen by electron or X-ray diffraction techniques. DSC indicated the presence of free Si in the DERA Sigma SM1140+ monofilament by a distinctive peak at approximately 1400 degrees C. TGA showed the reaction of monofilament components with gaseous species. The Textron SCS-6 and Ultra species lost weight as C was oxidized to gaseous CO. By contrast, the Sigma monofilament gained weight from formation of SiO2 from the free Si. The separations of the transverse optical phonon peaks in the Raman spectra were correlated with the density of stacking faults in the SiC crystallites. This was similar in all monofilaments. Analysis of the polarization of the Raman scattering gave information on the orientation of crystallites. The crystallites in SM1140+ and SCS-6 were orientated predominantly with the <111> parallel to the radius. Preliminary interpretation of the polarized Raman scattering from Ultra-SCS indicated more than one orientation of crystallite. One possibility was a mixture of <111> and <110> directions parallel to the radius.

p53 is associated with cellular microtubules and is transported to the nucleus by dynein

Here we show that p53 protein is physically associated with tubulin in vivo and in vitro, and that it localizes to cellular microtubules. Treatment with vincristine or paclitaxel before DNA-damage or before leptomycin B treatment reduces nuclear accumulation of p53 and expression of mdm2 and p21. Overexpression of dynamitin or microinjection of anti-dynein antibody before DNA damage abrogates nuclear accumulation of p53. Our results indicate that transport of p53 along microtubules is dynein-dependent. The first 25 amino acids of p53 contain the residues that are essential for binding to microtubules. We propose that functional microtubules and the dynein motor protein participate in transport of p53 and facilitate its accumulation in the nucleus after DNA damage.

Coexpression of receptors for adrenomedullin, calcitonin gene-related peptide, and amylin in pancreatic beta-cells

Three receptors have been characterized by their ability to bind adrenomedullin (AM): L1, RDC1, and CRLR. Immunohistochemical analysis and RT-PCR showed that all three receptors are expressed by the insulin-producing cells of the islets of Langerhans. RDC1 and CRLR in the presence of particular modifying proteins can also bind calcitonin gene-related peptide (CGRP). Such data suggest that the inhibitory effect caused by both AM and CGRP on insulin secretion is mediated by a direct interaction with the beta-cell. We also identified receptors for amylin, the third member of the AM peptide family, in mouse insulin-secreting cells. The beta-cells located closer to the periphery of the islets had a stronger immunoreactivity for the AM/ CGRP receptors. This observation could be related to a paracrine mechanism, given the proximity of AM- and CGRP-secreting cells (F and delta-cells, respectively), which are located at the periphery of the islets. Interestingly, the smooth muscle cells in the pancreatic vasculature expressed only RDC1, which is in agreement with physiological data showing that AM functions in the cardiovascular system are mainly mediated through a CGRP1 receptor. These data further implicate AM and the other components of its peptide family as important regulators of insulin release.

Distinct but dispensable N-glycosylation of human CD69 proteins

Human CD69 is uniquely glycosylated at typical (Asn-X-Ser/Thr) and atypical (Asn-X-Cys) motifs, which represents the molecular basis for the formation of CD69 homodimers and heterodimers. Here we examined the importance of N-glycosylation for the assembly and intracellular transport of CD69 proteins using mutant CD69 molecules that specifically lack typical and atypical N-glycan attachment motifs. These studies verify the importance of Cys residues in atypical triplet sequences for N-glycan addition to human CD69 proteins in the endoplasmic reticulum (ER). In addition, these data demonstrate that monoglycosylated CD69 proteins (bearing N-glycans exclusively at atypical or typical sites) and aglycosylated CD69 molecules (lacking N-glycans) efficiently dimerize in the ER and have similar stability as wild-type CD69 molecules. Finally, these results show that CD69 proteins lacking atypical or typical N-glycan addition sites are transported to the plasma membrane.

Control of MAP kinase activation by the mitogen-induced threonine/tyrosine phosphatase PAC1

Intracellular signalling following mitogenic stimulation of quiescent cells involves the initiation of a phosphorylation cascade that leads to the rapid and reversible activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2. MAP kinase activation is mediated by dual phosphorylation within the motif Thr-Glu-Tyr by MAP kinase kinase (MEK). Following activation, the MAP kinases translocate into the nucleus where they phosphorylate several transduction targets, including transcription factors. We have previously identified PAC1 as an immediate-early mitogen-inducible tyrosine phosphatase in nuclei of T cells. Here we present several lines of evidence indicating that PAC1 is a physiologically relevant MAP kinase phosphatase. Recombinant PAC1 in vitro is a dual-specific Thr/Tyr phosphatase with stringent substrate specificity for MAP kinase. Constitutive expression of PAC1 in vivo leads to inhibition of MAP kinase activity normally stimulated by epidermal growth factor, phorbol myristyl acetate, or T-cell receptor crosslinking. The inactivation of MAP kinase by PAC1 results in inhibition of MAP kinase-regulated reporter gene expression.