KIAA1199 expression and hyaluronan degradation colocalize in multiple sclerosis lesions

Modification of hyaluronan (HA) accumulation has been shown to play a key role in regulating inflammatory processes linked to the progression of multiple sclerosis (MS). The aim of this study was to characterize the enzymatic activity involved in HA degradation observed within focal demyelinating lesions in the experimental autoimmune encephalomyelitis (EAE) animal model. EAE was induced in 3-month-old female C57BL/6J mice by immunization with myelin oligodendrocyte glycoprotein 33–35 (MOG33–35) peptide. The mice were monitored for 21 days. Formalin-fixed paraffin-embedded tissue from control and EAE mice were labeled with an immunoadhesin against HA, antibodies against KIAA1199 and glial fibrillary acidic protein, a marker for astrocytes. In situ hybridization was conducted using a KIAA1199 nucleic acid probe. In histologic sections of spinal cord from EAE mice, abnormal HA accumulation was observed in the close vicinity of the affected areas, whereas HA was totally degraded within the focal loci of damaged tissue. KIAA1199 immunoreactivity was exclusively associated with focal loci in damaged white columns of the spinal cord. KIAA1199 was mainly expressed by activated astrocytes that invaded damaged tissue. Similar findings were observed in tissue from an MS patient. Here, we show that KIAA1199, a protein that plays a role in a HA degradation pathway independent of the canonical hyaluronidases such as PH20, is specifically expressed in tissue lesions in which HA is degraded. KIAA1199 expression by activated astrocytes may explain the focal HA degradation observed during progression of MS and could represent a possible new therapeutic target.

Mutations in the catalytic domain of human matrix metalloproteinase-1 (MMP-1) that allow for regulated activity through the use of Ca2+

Conditionally active proteins regulated by a physiological parameter represent a potential new class of protein therapeutics. By systematically creating point mutations in the catalytic and linker domains of human MMP-1, we generated a protein library amenable to physiological parameter-based screening. Mutants screened for temperature-sensitive activity had mutations clustered at or near amino acids critical for metal binding. One mutant, GVSK (Gly(159) to Val, Ser(208) to Lys), contains mutations in regions of the catalytic domain involved in calcium and zinc binding. The in vitro activity of GVSK at 37 °C in high Ca(2+) (10 mm) was comparable with MMP-1 (wild type), but in low Ca(2+) (1 mm), there was an over 10-fold loss in activity despite having similar kinetic parameters. Activity decreased over 50% within 15 min and correlated with the degradation of the activated protein, suggesting that GVSK was unstable in low Ca(2+). Varying the concentration of Zn(2+) had no effect on GVSK activity in vitro. As compared with MMP-1, GVSK degraded soluble collagen I at the high but not the low Ca(2+) concentration. In vivo, MMP-1 and GVSK degraded collagen I when perfused in Zucker rat ventral skin and formed higher molecular weight complexes with α2-macroglobulin, an inhibitor of MMPs. In vitro and in vivo complex formation and subsequent enzyme inactivation occurred faster with GVSK, especially at the low Ca(2+) concentration. These data suggest that the activity of the human MMP-1 mutant GVSK can be regulated by Ca(2+) both in vitro and in vivo and may represent a novel approach to engineering matrix-remodeling enzymes for therapeutic applications.

Relative bioavailability of a 5 mg mosapride/10 mg rabeprazole fixed dose combination tablet versus separate single tablets in healthy volunteers: a single-dose randomized open-label crossover study

OBJECTIVE

To evaluate the relative bioavailability of a new formulation containing 5 mg mosapride and 10 mg rabeprazole (T) and compare it with the branded formulations of both drugs co-administered in separate tablets (R) to meet the regulatory requirements of bioequivalence in Argentina.

METHODS

A randomized-sequence, open-label, two-period crossover study was conducted on 24 healthy Caucasian volunteers in a fasting state. A single oral dose of either T or R formulations was followed by a 7-day washout period. Blood samples for mosapride were collected before administration (baseline) and at 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, 18, and 24 h after administration. Samples for rabeprazole were taken baseline and at 1, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8 and 10 h after dosing. Mosapride and rabeprazole concentrations were determined using a validated LC-MS/MS method. Adverse events were monitored based on clinical parameters and volunteer reports.

RESULTS

The geometric means (90% CI) C(max) for mosapride in T and R were 23.13 (20.05-39.45) and 23.09 (21.69-32.37) ng/mL, the AUC(0-)(t) were 70.80 (66.23-102.37) and 70.81 (66.35-93.26) ng h/mL and the AUC(0-∞) were 74.05 (69.29-106.11) and 74.98 (70.43-97.77) ng h/mL. For rabeprazole T and R the C(max) were 197.42 (186.12-239.91) and 195.50 (186.08-250.07) ng/mL, the AUC(0-)(t) were 294.90 (275.13-374.15) and 296.96 (280.11-387.89) ng h/mL and the AUC(0-∞) were 301.12 (280.78-380.82) and 304.07 (286.60-394.21), respectively. No differences were detected between the formulations. The T/R ratios (90% CI) for C(max), AUC(0-)(t) and AUC(0-∞) were 100.17% (82.35-121.84), 99.99% (87.58-114.16) and 98.77% (87.02-112.11) for mosapride, and 100.99% (85.14-119.77), 99.31% (84.74-116.38) and 99.03% (85.07-115.28) for rabeprazole. No subject complained of adverse events.

CONCLUSIONS

In this single-dose study, the mosapride/rabeprazole tablets (test formulation) met the criterion for bioequivalence with the reference formulations. Study limitations include single-dose, open-label design, and a small sample of healthy volunteers.

Prostate stem cell antigen as therapy target: tissue expression and in vivo efficacy of an immunoconjugate

We conducted an expression analysis of prostate stem cell antigen (PSCA)in normal urogenital tissues, benign prostatic hyperplasia (n = 21), prostatic intraepithelial neoplasia (n = 33), and primary (n = 137) and metastatic (n = 42) prostate adenocarcinoma, using isotopic in situ hybridization on tissue microarrays. In normal prostate, we observe PSCA expression in the terminally differentiated, secretory epithelium; strong expression was also seen in normal urothelium. Forty-eight percent of primary and 64% of metastatic prostatic adenocarcinomas expressed PSCA RNA. Our studies did not confirm a positive correlation between level of PSCA RNA expression and high Gleason grade. We characterized monoclonal anti-PSCA antibodies that recognize PSCA expressed on the surface of live cells, are efficiently internalized after antigen recognition, and kill tumor cells in vitro in an antigen-specific fashion upon conjugation with maytansinoid. Unconjugated anti-PSCA antibodies demonstrated efficacy against PSCA-positive tumors by delaying progressive tumor growth in vivo. Maytansinoid-conjugated antibodies caused complete regression of established tumors in a large proportion of animals. Our results strongly suggest that maytansinoid-conjugated anti-PSCA monoclonal antibodies should be evaluated as a therapeutic modality for patients with advanced prostate cancer.

Purification of brain peroxisomes and localization of 3-hydroxy-3-methylglutaryl coenzyme A reductase

At least three different subcellular compartments, including peroxisomes, are involved in cholesterol biosynthesis. Because proper CNS development depends on de novo cholesterol biosynthesis, peroxisomes must play a critical functional role in this process. Surprisingly, no information is available on the peroxisomal isoprenoid/cholesterol biosynthesis pathway in normal brain tissue or on the compartmentalization of isoprene metabolism in the CNS. This has been due mainly to the lack of a well-defined isolation procedure for brain tissue, and also to the presence of myelin in brain tissue, which results in significant contamination of subcellular fractions. As a first step in characterizing the peroxisomal isoprenoid pathway in the CNS, we have established a purification procedure to isolate peroxisomes and other cellular organelles from the brain stem, cerebellum and spinal cord of the mouse brain. We demonstrate by use of marker enzymes and immunoblotting with antibodies against organelle specific proteins that the isolated peroxisomes are highly purified and well separated from the ER and mitochondria, and are free of myelin contamination. The isolated peroxisomal fraction was purified at least 40-fold over the original homogenate. In addition, we show by analytical subcellular fractionation and immunoelectron microscopy that HMG-CoA reductase protein and activity are localized both in the ER and peroxisomes in the CNS.

Identification of an angiogenic mitogen selective for endocrine gland endothelium

The known endothelial mitogens stimulate growth of vascular endothelial cells without regard to their tissue of origin. Here we report a growth factor that is expressed largely in one type of tissue and acts selectively on one type of endothelium. This molecule, called endocrine-gland-derived vascular endothelial growth factor (EG-VEGF), induced proliferation, migration and fenestration (the formation of membrane discontinuities) in capillary endothelial cells derived from endocrine glands. However, EG-VEGF had little or no effect on a variety of other endothelial and non-endothelial cell types tested. Similar to VEGF, EG-VEGF possesses a HIF-1 binding site, and its expression is induced by hypoxia. Both EG-VEGF and VEGF resulted in extensive angiogenesis and cyst formation when delivered in the ovary. However, unlike VEGF, EG-VEGF failed to promote angiogenesis in the cornea or skeletal muscle. Expression of human EG-VEGF messenger RNA is restricted to the steroidogenic glands, ovary, testis, adrenal and placenta and is often complementary to the expression of VEGF, suggesting that these molecules function in a coordinated manner. EG-VEGF is an example of a class of highly specific mitogens that act to regulate proliferation and differentiation of the vascular endothelium in a tissue-specific manner.

Identification of peroxisomal targeting signals in cholesterol biosynthetic enzymes. AA-CoA thiolase, hmg-coa synthase, MPPD, and FPP synthase

At least three different subcellular compartments, including peroxisomes, are involved in cholesterol synthesis. The peroxisomal targeting signals for phosphomevalonate kinase and isopentenyl diphosphate isomerase have been identified. In the current study we identify the peroxisomal targeting signals required for four other enzymes of the cholesterol biosynthetic pathway: acetoacetyl-CoA (AA-CoA) thiolase, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, mevalonate diphosphate decarboxylase (MPPD), and farnesyl diphosphate (FPP) synthase. Data are presented that demonstrate that mitochondrial AA-CoA thiolase contains both a mitochondrial targeting signal at the amino terminus and a peroxisomal targeting signal (PTS-1) at the carboxy terminus. We also analyze a new variation of PTS-2 sequences required to target HMG-CoA synthase and MPPD to peroxisomes. In addition, we show that FPP synthase import into peroxisomes is dependent on the PTS-2 receptor and identify at the amino terminus of the protein a 20-amino acid region that is required for the peroxisomal localization of the enzyme. These data provide further support for the conclusion that peroxisomes play a critical role in cholesterol biosynthesis.

Application of microwave technology to the processing and immunolabeling of plastic-embedded and cryosections

We have adapted existing microwave irradiation (MWI) protocols and applied them to the processing and immunoelectron microscopy of both plastic-embedded and frozen sections. Rat livers were fixed by rapid MW irradiation in a mild fixation solution. Fixed liver tissue was either cryosectioned or dehydrated and embedded in Spurr's, Unicryl, or LR White resin. Frozen sections and sections of acrylic-embedded tissue were immunolabeled in the MW oven with an anti-catalase antibody, followed by gold labeling. Controls were processed conventionally at room temperature (RT). The use of MWI greatly shortened the fixation, processing, and immunolabeling times without compromising the quality of ultrastructural preservation and the specificity of labeling. The higher immunogold labeling intensity was achieved after a 15-min incubation of primary antibody and gold markers under discontinued MWI at 37C. Quantification of the immunolabeling for catalase indicated a density increase of up to fourfold in the sections immunolabeled in the MW oven over that of samples immunolabeled at RT. These studies define the general conditions of fixation and immunolabeling for both acrylic resin-embedded material and frozen sections.

Pex17p is required for import of both peroxisome membrane and lumenal proteins and interacts with Pex19p and the peroxisome targeting signal-receptor docking complex in Pichia pastoris

Pichia pastoris PEX17 was cloned by complementation of a peroxisome-deficient strain obtained from a novel screen for mutants disrupted in the localization of a peroxisomal membrane protein (PMP) reporter. PEX17 encodes a 267-amino-acid protein with low identity (18%) to the previously characterized Saccharomyces cerevisiae Pex17p. Like ScPex17p, PpPex17p contains a putative transmembrane domain near the amino terminus and two carboxyl-terminal coiled-coil regions. PpPex17p behaves as an integral PMP with a cytosolic carboxyl-terminal domain. pex17Delta mutants accumulate peroxisomal matrix proteins and certain integral PMPs in the cytosol, suggesting a critical role for Pex17p in their localization. Peroxisome remnants were observed in the pex17Delta mutant by morphological and biochemical means, suggesting that Pex17p is not absolutely required for remnant formation. Yeast two-hybrid analysis demonstrated that the carboxyl terminus of Pex19p was required for interaction with Pex17p lacking the carboxyl-terminal coiled-coil domains. Biochemical evidence confirmed the interaction between Pex19p and Pex17p. Additionally, Pex17p cross-linked to components of the peroxisome targeting signal-receptor docking complex, which unexpectedly contained Pex3p. Our evidence suggests the existence of distinct subcomplexes that contain separable pools of Pex3p, Pex19p, Pex17p, Pex14p, and the peroxisome targeting signal receptors. These distinct pools may serve different purposes for the import of matrix proteins or PMPs.

Pex22p of Pichia pastoris, essential for peroxisomal matrix protein import, anchors the ubiquitin-conjugating enzyme, Pex4p, on the peroxisomal membrane

We isolated a Pichia pastoris mutant that was unable to grow on the peroxisome-requiring media, methanol and oleate. Cloning the gene by complementation revealed that the encoded protein, Pex22p, is a new peroxin. A Deltapex22 strain does not grow on methanol or oleate and is unable to import peroxisomal matrix proteins. However, this strain targets peroxisomal membrane proteins to membranes, most likely peroxisomal remnants, detectable by fluorescence and electron microscopy. Pex22p, composed of 187 amino acids, is an integral peroxisomal membrane protein with its NH2 terminus in the matrix and its COOH terminus in the cytosol. It contains a 25-amino acid peroxisome membrane-targeting signal at its NH2 terminus. Pex22p interacts with the ubiquitin-conjugating enzyme Pex4p, a peripheral peroxisomal membrane protein, in vivo, and in a yeast two-hybrid experiment. Pex22p is required for the peroxisomal localization of Pex4p and in strains lacking Pex22p, the Pex4p is cytosolic and unstable. Therefore, Pex22p anchors Pex4p at the peroxisomal membrane. Strains that do not express Pex4p or Pex22p have similar phenotypes and lack Pex5p, suggesting that Pex4p and Pex22p act at the same step in peroxisome biogenesis. The Saccharomyces cerevisiae hypothetical protein, Yaf5p, is the functional homologue of P. pastoris Pex22p.

Pex19p interacts with Pex3p and Pex10p and is essential for peroxisome biogenesis in Pichia pastoris

We report the cloning and characterization of Pichia pastoris PEX19 by complementation of a peroxisome-deficient mutant strain. Import of peroxisomal targeting signal 1- and 2-containing peroxisomal matrix proteins is defective in pex19 mutants. PEX19 encodes a hydrophilic 299-amino acid protein with sequence similarity to Saccharomyces cerevisiae Pex19p and human and Chinese hamster PxF, all farnesylated proteins, as well as hypothetical proteins from Caenorhabditis elegans and Schizosaccharomyces pombe. The farnesylation consensus is conserved in PpPex19p but dispensable for function and appears unmodified under the conditions tested. Pex19p localizes predominantly to the cytosolic fraction. Biochemical and two-hybrid analyses confirmed that Pex19p interacts with Pex3p, as seen in S. cerevisiae, but unexpectedly also with Pex10p. Two-hybrid analysis demonstrated that the amino-terminal 42 amino acids of Pex19p interact with the carboxyl-terminal 335 amino acids of Pex3p. In addition, the extreme carboxyl terminus of Pex19p (67 amino acids) is required for interaction with the amino-terminal 380 amino acids of Pex10p. Biochemical and immunofluorescence microscopy analyses of pex19Delta cells identified the membrane protein Pex3p in peroxisome remnants that were not previously observed in S. cerevisiae. These small vesicular and tubular (early) remnants are morphologically distinct from other Pppex mutant (late) remnants, suggesting that Pex19p functions at an early stage of peroxisome biogenesis.

VEGF is required for growth and survival in neonatal mice

We employed two independent approaches to inactivate the angiogenic protein VEGF in newborn mice: inducible, Cre-loxP- mediated gene targeting, or administration of mFlt(1–3)-IgG, a soluble VEGF receptor chimeric protein. Partial inhibition of VEGF achieved by inducible gene targeting resulted in increased mortality, stunted body growth and impaired organ development, most notably of the liver. Administration of mFlt(1–3)-IgG, which achieves a higher degree of VEGF inhibition, resulted in nearly complete growth arrest and lethality. Ultrastructural analysis documented alterations in endothelial and other cell types. Histological and biochemical changes consistent with liver and renal failure were observed. Endothelial cells isolated from the liver of mFlt(1–3)-IgG-treated neonates demonstrated an increased apoptotic index, indicating that VEGF is required not only for proliferation but also for survival of endothelial cells. However, such treatment resulted in less significant alterations as the animal matured, and the dependence on VEGF was eventually lost some time after the fourth postnatal week. Administration of mFlt(1–3)-IgG to juvenile mice failed to induce apoptosis in liver endothelial cells. Thus, VEGF is essential for growth and survival in early postnatal life. However, in the fully developed animal, VEGF is likely to be involved primarily in active angiogenesis processes such as corpus luteum development.

Peroxisome degradation by microautophagy in Pichia pastoris: identification of specific steps and morphological intermediates

We used the dye N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenylhexatrienyl ) pyridinium dibromide (FM4-64) and a fusion protein, consisting of the green fluorescent protein appended to the peroxisomal targeting signal, Ser-Lys-Leu (SKL), to label the vacuolar membrane and the peroxisomal matrix, respectively, in living Pichia pastoris cells and followed by fluorescence microscopy the morphological and kinetic intermediates in the vacuolar degradation of peroxisomes by microautophagy and macroautophagy. Structures corresponding to the intermediates were also identified by electron microscopy. The kinetics of appearance and disappearance of these intermediates is consistent with a precursor-product relationship between intermediates, which form the basis of a model for microautophagy. Inhibitors affecting different steps of microautophagy did not impair peroxisome delivery to the vacuole via macroautophagy, although inhibition of vacuolar proteases affected the final vacuolar degradation of green fluorescent protein (S65T mutant version [GFP])-SKL via both autophagic pathways. P. pastoris mutants defective in peroxisome microautophagy (pag mutants) were isolated and characterized for the presence or absence of the intermediates. These mutants, comprising 6 complementation groups, support the model for microautophagy. Our studies indicate that the microautophagic degradation of peroxisomes proceeds via specific intermediates, whose generation and/or processing is controlled by PAG gene products, and shed light on the poorly understood phenomenon of peroxisome homeostasis.

Disruption of a putative SH3 domain and the proline-rich motifs in the 53-kDa substrate of the insulin receptor kinase does not alter its subcellular localization or ability to serve as a substrate

The recently identified 53-kDa substrate of the insulin receptor family was further characterized in several retroviral-generated stable cell lines overexpressing the wild type and various mutant forms of the protein. To facilitate the study of its subcellular localization in NIH3T3 cells overexpressing insulin receptor, a myc epitope-tag was added to the carboxy terminus of the 53-kDa protein. Like the endogenous protein in Chinese hamster ovary cells, the expressed myc-tagged 53-kDa protein was found partially in the particulate fraction and was tyrosine phosphorylated in insulin-stimulated cells. Immunofluorescence studies showed for the first time that a fraction of the 53-kDa protein was localized to the plasma membrane. Confocal microscopy of cells double-labeled with antibodies to the insulin receptor and the myc epitope showed the two proteins co-localize at the plasma membrane at the level of light microscopy. Further analyses of the protein sequence of the 53-kDa substrate revealed the presence of a putative SH3 domain and two proline-rich regions, putative binding sites for SH3 and WW domains. Disruption of these three motifs by the introduction of previously characterized point mutations did not affect the membrane localization of the 53-kDa protein, its ability to serve as substrate of the insulin receptor, or its colocalization with the insulin receptor, suggesting these domains are not important in the subcellular targeting of the protein and instead may function in the interaction with subsequent signaling proteins.

Effect of insulin-like growth factor binding proteins on the response of proximal tubular cells to insulin-like growth factor-I

The insulin-like growth factor binding proteins (IGFBP) are major modulators of insulin-like growth factor-I (IGF-I) action, but relatively little is known about their production by kidney tubular cells or about their modulating effects on the action of IGF-I on these cells. In this study we demonstrated that rabbit proximal tubular cells express the genes for IGFBP-2, -4 and -5 and secrete 24 and 32 kDa size binding proteins. The rate of IGFBP production by these cells was regulated by several growth factors including hydrocortisone, which was potently stimulatory, and EGF, which was inhibitory. The overall effect of these kidney cell-secreted IGFBPs was to inhibit the mitogenic activity of IGF-I. Similarly, recombinant IGFBP-3, the major circulating IGFBP that in kidney is produced close to the proximal tubules, also inhibited IGF-I stimulated DNA synthesis in cultured rabbit proximal tubular cells and in cultured opossum kidney (OK) cells. IGFBP-3 also inhibited basal DNA synthesis in OK cells in the absence of added IGF-I, suggesting that this IGFBP may have an IGF-I independent action. These findings highlight the important effect that IGFBPs have on the action of IGF-I on kidney cells and support the notion that the changes in IGFBPs observed in various renal diseases may contribute to the pathophysiology of these diseases.

Normal platelets and megakaryocytes are produced in vivo in the absence of thrombopoietin

Thrombopoietin (TPO) has been established as the major regulator of megakaryocyte and platelet production. In vitro and in vivo studies have demonstrated that TPO affects both megakaryocyte proliferation and maturation. In vitro, TPO has been reported to be essential for full development of megakaryocytes and platelets. These studies are in contrast to results observed in vivo in mice deficient in the TPO or c-mpl gene (TPO-/- and c-mpl-/-). Both TPO-/- and c-mpl-/- mice exhibit a 90% reduction in megakaryocyte and platelet levels. But even with this small number of circulating platelets, these mice do not have any excessive bleeding. Ultrastructural analysis indicates that platelets and megakaryocytes present in the knockout mice are morphologically normal. Characterization of platelet function shows that platelets from knockout mice are functionally identical to the wild-type platelets as measured by upregulation of 125I-fibrinogen binding to platelets in response to adenosine diphosphate (ADP) stimulation and by platelet attachment to the immobilized extracellular matrix proteins, collagen and von Willebrand factor (vWF). These results demonstrate that in vivo, TPO is required for the control of megakaryocyte and platelet number but not for their maturation. Other factors with megakaryocytopoietic activity may be able to compensate for the maturational role of TPO and lead to the formation of normal megakaryocytes and platelets in TPO-/- and c-mpl-/- mice.

Characterization of UT2 cells. The induction of peroxisomal 3-hydroxy-3-methylglutaryl-coenzyme a reductase

In the liver 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is present not only in the endoplasmic reticulum but also in the peroxisomes. However, to date no information is available regarding the function of the peroxisomal HMG-CoA reductase in cholesterol/isoprenoid metabolism, and the structure of the peroxisomal HMG-CoA reductase has yet to be determined. We have identified a mammalian cell line that expresses only one HMG-CoA reductase protein and that is localized exclusively to peroxisomes. This cell line was obtained by growing UT2 cells (which lack the endoplasmic reticulum HMG-CoA reductase) in the absence of mevalonate. The cells exhibited a marked increase in a 90-kDa HMG-CoA reductase that was localized exclusively to peroxisomes. The wild type Chinese hamster ovary cells contain two HMG-CoA reductase proteins, the well characterized 97-kDa protein, localized in the endoplasmic reticulum, and a 90-kDa protein localized in peroxisomes. The UT2 cells grown in the absence of mevalonate containing the up-regulated peroxisomal HMG-CoA reductase are designated UT2*. A detailed characterization and analysis of this cell line is presented in this study.

Regulation of the serum concentration of thrombopoietin in thrombocytopenic NF-E2 knockout mice

The mechanisms that regulate circulating levels of thrombopoietin (Tpo) are incompletely understood. According to one favored model, the rate of Tpo synthesis is constant, whereas the serum concentration of free Tpo is modulated through binding to c-Mpl receptor expressed on blood platelets. Additionally, a role for c-Mpl expressed on megakaryocytes is suggested, particularly by the observation that serum Tpo levels are not elevated in human immune thrombocytopenic purpura. Whereas direct binding of Tpo to platelets has been demonstrated in vitro and in vivo, the role of megakaryocytes in modulating serum Tpo levels has not been addressed experimentally. The profoundly thrombocytopenic mice lacking transcription factor p45 NF-E2 do not show the predicted increase in serum Tpo concentration. To evaluate the fate of the ligand in these animals, we injected 125I-Tpo intravenously into mutant and control mice. In contrast to normal littermates, NF-E2 knockout mice show negligible association of radioactivity with blood cellular components, consistent with an absence of platelets. There is no corresponding increase in plasma-associated radioactivity to suggest persistence in the circulation. However, a greater fraction of the radioligand is bound to hematopoietic tissues. In the bone marrow this is detected virtually exclusively in association with megakaryocytes, whereas in the spleen it is associated with megakaryocytes and small, abnormal, platelet-like particles or megakaryocyte fragments that are found within or in close contact with macrophages. These findings implicate the combination of megakaryocytes and the latter particles as a sink for circulating Tpo in NF-E2 knockout mice, and provide an explanation for the lack of elevated serum Tpo levels in this unique animal model of thrombocytopenia.

Insulin-degrading enzyme does not require peroxisomal localization for insulin degradation

Although considerable evidence implicates insulin-degrading enzyme (IDE) in the cellular metabolism of insulin in many cell types, its mechanism and site of action are not clear. In this study, we have examined the relationship between insulin-degrading enzyme's peroxisomal location and its ability to degrade insulin by mutation of its peroxisomal targeting signal (PTS), the carboxy terminal A/S-K-L tripeptide. Site-directed mutagenesis was used to destroy the peroxisomal targeting signal of human insulin-degrading enzyme by changing alanine to leucine (AL.pts), leucine to valine (LV.pts), or by deleting the entire tripeptide (DEL.pts). The alanine or leucine mutants, when expressed in COS cells, were indistinguishable from wild-type insulin-degrading enzyme with respect to size (110 kDa), amount of immunoreactive material, ability to bind insulin, in vitro activity, and cellular degradation of insulin. In contrast, the deletion mutant was shorter in size (approximately 0 kDa) and unable to bind the hormone. Thus, although the tripeptide at insulin-degrading enzyme's carboxy terminus appeared to confer enzyme stability, the conserved sequence was not required for insulin degradation. Finally, an immunocytofluorescence study showed that, whereas a significant amount of the wild-type protein was localized in peroxisomes, none of the peroxisomal targeting mutants could be detected in these organelles. These findings indicate that insulin-degrading enzyme does not require peroxisomal localization for insulin degradation and suggest that this enzyme has multiple cellular functions.

Metabolism of thrombopoietin (TPO) in vivo: determination of the binding dynamics for TPO in mice

Previous in vivo studies have established that plasma thrombopoietin (TPO) levels are regulated by binding to c-Mpl on platelets and that, in vitro, platelets bind and degrade TPO. To determine if the in vivo metabolism of TPO was specific and saturable, we injected normal CD-1 mice IV with trace amounts of 125I-rmTPO with or without a saturating concentration of rmTPO. The amount of radioactivity present in the spleen, blood cell fraction, platelet fraction, tibia/fibula, and femur was significantly greater in the mice receiving 125I-rmTPO alone. Conversely, the amount of radioactivity present in the plasma was significantly greater in the mice receiving both 125I-rmTPO and rmTPO, thus suggesting the uptake of rmTPO by the spleen, platelets, and bone marrow in vivo was saturable. Platelet and spleen homogenates from animals receiving 125I-rmTPO alone showed a degradation pattern of 125I-rmTPO similar to that observed in vitro using mouse platelet rich plasma. To determine the in vivo binding dynamics for rmTPO, mice were injected with 125I-rmTPO alone or with increasing concentrations of rmTPO; spleen and blood cell-associated radioactivity was determined at 2 hours postinjection. A 4-parameter curve fit of the data indicated that the "in vivo binding affinity" for rmTPO was approximately 6.4 microg/kg. These data indicate that after a dose of approximately 6.4 microg/kg, 50% of all c-Mpl receptors will be saturated with rmTPO. Electron microscopy indicated that radioactivity was present bound to and within megakaryocytes and platelets in both sternum and spleen and platelets in circulation. Together these data demonstrate that in vivo, 125I-rmTPO is mainly metabolized by platelets and to a small extent by cells of the megakaryocyte lineage, via a specific and saturable mechanism.

Management for quality: continuous quality improvement to increase access to outpatient mental health services

OBJECTIVE

A health maintenance organization (HMO) examined whether continuous quality improvement could be used to address the problem of long waiting times for outpatient mental health services from a closed panel of providers during peak periods of demand.

METHODS

A task force at a staff model HMO in Burlington, Vermont, used continuous quality improvement methods to identify and solve specific service access problems in five categories: quality, protocol, and standards; systems and processes; management and administration; clinical practice management; and public relations and marketing.

RESULTS

Over a two-year period, the task force identified 13 specific problems, for which solutions were implemented. For example, two new support positions were created to meet clinicians' needs. Triage categories were defined, and acceptable waiting times for appointments, along with goals for percent compliance, were established. A weekly training program in brief psychotherapy and an extensive group psychotherapy program were implemented. A network of community providers was formed to complement the HMO's fixed provider panel during periods of high demand. The average waiting time was reduced from 22 days to six days, and patients' satisfaction increased markedly.

CONCLUSIONS

Use of continuous quality improvement can guide clinical leaders in their central role of reinstating clinical quality as the goal of management. The author suggests that continuous quality improvement with balanced clinical and administrative leadership is the means to forge the needed synthesis of quality and cost capable of improving mental health.

Human platelets as a model for the binding and degradation of thrombopoietin

Recent studies have shown that plasma thrombopoietin (TPO) levels appear to be directly regulated by platelet mass and that removal of plasma TPO by platelets via binding to the c-Mpl receptor is involved in the clearance of TPO in rodents. To help elucidate the role of platelets in the clearance of TPO in humans, we studied the in vitro specific binding of recombinant human TPO (rhTPO) to human platelet-rich plasma (PRP), washed platelets (WP), and cloned c-Mpl. Using a four-parameter fit and/or Scatchard analysis, the approximate affinity of rhTPO for its receptor, which was calculated from multiple experiments using different PRP preparations, was between 128 and 846 pmol/L, with approximately 25 to 224 receptors per platelet. WP preparations gave an affinity of 260 to 540 pmol/L, with approximately 25 to 35 receptors per platelet, and erythropoietin failed to compete with 125I-rhTPO for binding to WP. Binding and dissociation studies conducted with a BiaCore apparatus yielded an affinity of 350 pmol/L for rhTPO binding to cloned c-Mpl receptors. The ability of PRP to bind and degrade 125I-rhTPO was both time- and temperature-dependent and was blocked by the addition of excess cold rhTPO. Analysis of platelet pellets by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that 125I-rhTPO was degraded into a major fragment of approximately 45 to 50 kD. When 125I-rhTPO was incubated with a platelet homogenate at pH = 7.4, a degradation pattern similar to intact platelets was observed. Together, these data show that human platelets specifically bind rhTPO with high affinity, internalize, and then degrade the rhTPO.

Nuclear transport of insulin-like growth factor-I and insulin-like growth factor binding protein-3 in opossum kidney cells

When added to cultured opossum kidney cells, IGF-I is internalized and transported to distinct intracellular compartments that depend on the cell location within the monolayer. In resting cells away from the periphery of the monolayer, IGF-I is internalized by a clathrin coated pit pathway and delivered to the endosomal compartment. In contrast, cells growing at the edges of a monolayer or an experimental wound internalize IGF-I by an alternative route which rapidly delivers IGF-I to the nucleus. Similarly to IGF-I, IGFBP-3 is also internalized and accumulates in the endosomal compartment in resting cells whereas it is targeted to the nucleus in proliferating cells. IGFBP-3, which contains a putative nuclear targeting signal, may act as a carrier for IGF-I nuclear transport. The transport of IGF-I and IGFBP-3 to two different compartments may influence their biological activity.

In vivo effects of cardiotrophin-1

Cardiotrophin-1 (CT-1) is a recently discovered cytokine that was isolated based on its ability to induce cardiac myocyte hypertrophy in vitro. In this study, the effects of chronic administration of CT-1 to mice (0.5 or 2 microg by intraperitoneal injection, twice a day for 14 days) were determined. A dose-dependent increase in both the heart weight and ventricular weight to body ratios was observed in the treated groups. The body weights of the animals were unaffected. These results indicate that CT-1 can induce cardiac hypertrophy in vivo. CT-1 was not specific for the heart, however. It stimulated the growth of the liver, kidney, and spleen, and caused atrophy of the thymus. CT-1 administration also increased the platelet counts by 70%, with no change in mean platelet volume. Red blood cell counts were increased in the treated animals, and there was a concomitant increase in haemoglobin concentration. Thus, CT-1 has a broad spectrum of biological activities in vivo. This observation is consistent with previous in-vitro findings showing that the mRNA for CT-1 is expressed in several tissues, and that CT-1 can function through binding to the leukaemia inhibitory factor (LIF) receptor and signalling through the gp130 pathway.

Characterization of a novel component of the peroxisomal protein import apparatus using fluorescent peroxisomal proteins

Fluorescent peroxisomal probes were developed by fusing green fluorescent protein (GFP) to the matrix peroxisomal targeting signals PTS1 and PTS2, as well as to an integral peroxisomal membrane protein (IPMP). These proteins were used to identify and characterize novel peroxisome assembly (pas) mutants in the yeast Pichia pastoris. Mutant cells lacking the PAS10 gene mislocalized both PTS1-GFP and PTS2-GFP to the cytoplasm but did incorporate IPMP-GFP into peroxisome membranes. Similar distributions were observed for endogenous peroxisomal matrix and membrane proteins. While peroxisomes from translocation-competent pas mutants sediment in sucrose gradients at the density of normal peroxisomes, >98% of peroxisomes from pas10 cells migrated to a much lower density and had an extremely low ratio of matrix:membrane protein. These data indicate that Pas10p plays an important role in protein translocation across the peroxisome membrane. Consistent with this hypothesis, we find that Pas10p is an integral protein of the peroxisome membrane. In addition, Pas10p contains a cytoplasmically-oriented C3HC4 zinc binding domain that is essential for its biological activity.

Heregulin is rapidly translocated to the nucleus and its transport is correlated with c-myc induction in breast cancer cells

Heregulins (neuregulins) are a family of proteins known to interact and activate the receptor tyrosine kinases ErbB2 in association with ErbB3 or ErbB4. Using immunofluorescence microscopy, electron microscopy autoradiography, and SDS-PAGE analysis of nuclear fractions, we show that the heregulin-beta1(1-244) isoform is rapidly internalized and translocated to the nucleus of SK-BR-3 breast cancer cells as an intact molecule. Heregulin-beta 1(1-244) treatment up-regulated expression of c-myc mRNA and protein, which was also observed to undergo its own translocation from the cytosol to the nucleus. c-myc thus appears to be a cellular target gene of HRGbeta 1(1-244), and its induction may be related to the nuclear translocation of heregulin.

Distribution of DNA and alginate in purulent cystic fibrosis sputum: implications to pulmonary targeting strategies

Cystic fibrosis (CF) patients frequently experience recurring airway infections characterized by thick, viscous sputum. The consistency and nature of these purulent secretions may produce a significant barrier to the successful delivery of drugs and gene therapy vectors designed to treat CF. We have carried out a series of in vitro studies to determine the distribution of two macromolecular components typically present in purulent sputum, bacterial alginate and neutrophil-derived DNA. Sputum samples were obtained from hospitalized CF patients. DNA and alginate were disrupted, respectively, by the in vitro additions of human recombinant deoxyribonuclease I (rhDNase) or alginate lyase prepared from a mucoid strain of Pseudomonas aeruginosa. N-acetyl-L-cysteine (acetylcysteine) was similarly used to collapse the mucin matrix of these samples for comparison. Using a centrifugation-based rheological method known as the compaction assay, a greater maximal response was observed for rhDNase compared to alginate lyase treatment. A simultaneous addition of these enzymes to purulent sputum produced an additive compaction response. Electron microscopy was used to identify alginate and DNA components within the mucin matrix of sputa and to evaluate changes following treatment with high concentrations of alginate lyase or rhDNase. DNA was more widely distributed throughout purulent samples than alginate. Differences in the distribution of DNA and alginate may explain, at least in part, the larger compaction response to rhDNase versus alginate lyase treatment. An improved understanding of DNA and alginate distribution within purulent CF sputum may lead to improvements in drug and vector delivery to airway epithelial cells.

Recognition of a 170 kD protein in mammalian Golgi complexes by an antibody against malarial intraerythrocytic lamellae

Human erythrocytes infected with the malarial parasite Plasmodium falciparum contain flattened membrane lamellae. It has been suggested that the lamellae may be involved in the sorting of malarial proteins to the cytoplasm and the cell membrane of the host erythrocyte. We have previously shown that the lamellae accumulate sphingolipids by virtue of their lipid composition in a manner similar to the trans-Golgi and the trans-Golgi network in mammalian cells. In this paper, we show by immunofluorescence microscopy that a monoclonal antibody to the lamellae labeled a perinuclear organelle that colocalized with WGA and the mannose-6-phosphate receptor in cultured mammalian cells. Immunoelectron microscopy experiments revealed that LWLI labels cisternae of the trans-face and the trans-Golgi network. Western blot analysis of subcellular fractions using LWLI detected a 170 kD protein which is associated with the luminal side of Golgi membranes of rat liver and is conserved in all cell lines studied. Our results indicate that (i) the 170 kD protein is a novel marker of the mammalian trans-Golgi and the trans-Golgi network and (ii) in addition to similarities in their morphological and lipid characteristics, the lamellae induced by P. falciparum in erythrocytes share proteinaceous determinants with the Golgi apparatus of mammalian cells.

Development of anti-p185HER2 immunoliposomes for cancer therapy

The product of the HER2 protooncogene, p185HER2, represents an attractive target for cancer immunotherapies. We have prepared anti-p185HER2 immunoliposomes in which Fab' fragments of a humanized anti-p185HER2 monoclonal antibody with antiproliferative properties (rhuMAb-HER2) were conjugated to either conventional or sterically stabilized liposomes. These immunoliposomes bind specifically to p185HER2-overexpressing breast cancer cells (SK-BR-3 and BT-474). High-affinity binding of anti-p185HER2 immunoliposomes is comparable to that of free rhuMAbHER2-Fab' or the intact antibody. Empty immunoliposomes inhibit the culture growth of p185HER2-overexpressing breast cancer cells, and this antiproliferative effect is superior to that of free rhuMAbHER2-Fab', indicating that liposomal anchoring of these anti-p185HER2 Fab' fragments enhances their biological activity. Efficient internalization of anti-p185HER2 immunoliposomes, demonstrated by light and electron microscopy, occurs by receptor-mediated endocytosis via the coated pit pathway and also possibly by membrane fusion. Doxorubicin-loaded anti-p185HER2 immunoliposomes are markedly and specifically cytotoxic against p185HER2-overexpressing tumor cells in vitro. Anti-p185HER2 immunoliposomes administered in vivo in Scid mice bearing human breast tumor (BT-474) xenografts can deliver doxorubicin to tumors. These results indicate that anti-p185HER2 immunoliposomes are a promising therapeutic vehicle for the treatment of p185HER2-overexpressing human cancers.

In vitro megakaryocytopoietic and thrombopoietic activity of c-mpl ligand (TPO) on purified murine hematopoietic stem cells

Recently, the ligand for c-mpl has been identified and cloned. Initial studies of this molecule indicate that it is the platelet regulatory factor, thrombopoietin (TPO). Previous work has indicated that c-mpl is expressed in very immature hematopoietic precursors and thus raised the possibility that TPO may act directly on the hematopoietic stem cell. Therefore, in these studies, we investigate the effects of TPO on hematopoietic stem cell populations isolated from the murine fetal liver and bone marrow. Cocultivation of stem cells with fetal liver stroma give rise to multilineage expansion of the stem cells but with little or no megakaryocytopoiesis. Addition of TPO to these cocultures gives significant megakaryocyte production. This production is enhanced in combination with Kit ligand or interleukin-3. The addition of TPO to stem cell suspension cultures produces a dynamic thrombopoietic system in which stem cells undergo differentiation to produce megakaryocytes and proplatelets. These experiments show that the megakaryocytopoietic and thrombopoietic activities of TPO are initiated at the level of an early progenitor cell or upon the hematopoietic stem cell.

Farnesyl-diphosphate synthase is localized in peroxisomes

In this study, we have investigated the subcellular localization of farnesyl-diphosphate synthase (FPP synthase). FPP synthase produces FPP, which is utilized in the synthesis of squalene, cholesterol, farnesylated and geranylgeranylated proteins, dolichols, coenzyme Q, and the isoprenoid moiety of heme a. This enzyme is found in the 100,000 x g supernatant fraction of cells or tissues and has been considered to be a cytoplasmic protein. In this study, analysis of FPP synthase activity and protein in fractionated rat liver together with immunofluorescent and immunoelectron microscopy studies demonstrated unequivocally that FPP synthase is largely localized in peroxisomes. These data, in combination with the previous observation that mevalonate kinase is predominantly localized in peroxisomes, suggest that peroxisomes are the major site of synthesis of FPP from mevalonate. We also demonstrate that in liver tissue obtained from patients with peroxisomal deficiency diseases (Zellweger syndrome and neonatal adrenoleukodystrophy), the activities of five enzymes involved in isoprenoid synthesis, namely mevalonate kinase, phosphomevalonate kinase, mevalonate-diphosphate decarboxylase, isopentenyl-diphosphate isomerase, and FPP synthase, are significantly reduced, consistent with a peroxisomal localization of these enzymes.

Retention and degradation of proteins containing an uncleaved glycosylphosphatidylinositol signal

Glycosylphosphatidylinositol (GPI) membrane anchor attachment is directed by a COOH-terminal signal that is proteolytically removed and replaced with a preformed GPI anchor in a coupled reaction. Failure to complete proteolytic cleavage and anchor addition results in the retention of an uncleaved precursor in a post-endoplasmic reticulum (ER) compartment. In this report, we address three issues: (i) the exact position of the transport block, (ii) the subsequent fate of the retained molecules, i.e. where are they degraded, and (iii) the mechanism whereby these proteins are selected for retention. Using decay accelerating factor (DAF), we provide evidence that failure to cleave the GPI signal totally prevents O-glycosylation, suggesting that the uncleaved polypeptides are not transported into the cis-Golgi complex. This implies that transport is blocked at the boundary between the ER-Golgi intermediate compartment and the Golgi stacks. The degradation of an intracellularly retained human growth hormone (hGH)-DAF fusion protein containing a nonfunctional GPI signal shows some features of ER degradation, i.e. the degradation is insensitive to leupeptin, chloroquine, and ammonium chloride, and is inhibited at 16 degrees C or after ATP depletion. However, morphological evidence points to a pathway resembling autophagy. To reconcile these observations, we suggest either that hGHDAF is degraded by two distinct pathways (ER degradation and autophagy) or that ER degradation takes place in an ER-associated vesicular compartment in a process resembling autophagy. Using as probes a soluble hGH receptor and an antibody recognizing only native hGH, we show that a significant fraction of the retained protein is correctly folded, ruling out general misfolding as the basis for retention. We also show that hGHDAF fusion proteins are present in high molecular weight, disulfide-linked aggregates in COS cells. We suggest a model for retention in which the uncleaved GPI signal drives the formation of large micelle-like aggregates that cannot be secreted.

Mevalonate kinase is predominantly localized in peroxisomes and is defective in patients with peroxisome deficiency disorders

We reported recently that mevalonate kinase (EC 2.7.1.36; ATP:mevalonate 5-phosphotransferase) that was isolated from rat liver and believed to be a cytosolic protein was localized in rat liver peroxisomes. In addition, we found that the mevalonate kinase monoclonal antibody used in the study also reacted with several other proteins present in the mitochondrial and cytosolic fractions. These findings raised the prospect of the presence of several isoenzymes of mevalonate kinase localized in different compartments of the cell. In the current study we produced four new polyclonal antibodies against different epitopes of mevalonate kinase to investigate the subcellular localization of the protein by several different approaches: (i) by analytical subcellular fractionation and immunoblotting of mevalonate kinase in the isolated subcellular fractions with the monospecific antibodies; (ii) by immunocryoelectron microscopy techniques; and (iii) by expressing the cDNA encoding mevalonate kinase in mammalian cells. The data obtained demonstrate that there is only one mevalonate kinase protein that is predominantly localized in peroxisomes. We also illustrate that the protein is targeted to and imported into peroxisomes. In addition, we show that in cells and tissues obtained from patients with peroxisomal deficiency diseases mevalonate kinase protein and its activity are severely reduced.

The vascular endothelial growth factor (VEGF) isoforms: differential deposition into the subepithelial extracellular matrix and bioactivity of extracellular matrix-bound VEGF

Vascular endothelial growth factor (VEGF)mRNA undergoes alternative splicing events that generate four different homodimeric isoforms, VEGF121, VEGF165, VEGF189, or VEGF206. VEGF121 is a nonheparin-binding acidic protein, which is freely diffusible. The longer forms, VEGF189 or VEGF206, are highly basic proteins tightly bound to extracellular heparin-containing proteoglycans. VEGF165 has intermediate properties. To determine the localization of VEGF isoforms, transfected human embryonic kidney CEN4 cells expressing VEGF165, VEGF189, or VEGF206 were stained by immunofluorescence with a specific monoclonal antibody. The staining was found in patches and streaks suggestive of extracellular matrix (ECM). VEGF165 was observed largely in Golgi apparatus-like structures. Immunogold labeling of cells expressing VEGF189 or VEGF206 revealed that the staining was localized to the subepithelial ECM. VEGF associated with the ECM was bioactive, because endothelial cells cultured on ECM derived from cells expressing VEGF189 or VEGF206 were markedly stimulated to proliferate. In addition, ECM-bound VEGF can be released into a soluble and bioactive form by heparin or plasmin. ECM-bound VEGF189 and VEGF206 have molecular masses consistent with the intact polypeptides. The ECM may represent an important source of VEGF and angiogenic potential.

Subcellular localization of squalene synthase in rat hepatic cells. Biochemical and immunochemical evidence

In the present study we investigated the subcellular localization of squalene synthase (farnesyl-diphosphate:farnesyl-diphosphate farnesyltransferase, EC 2.5.1.21). Squalene synthase catalyzes the formation of squalene from trans-farnesyl diphosphate in two distinct steps and is the first committed enzyme for the biosynthesis of cholesterol. Recently, a truncated form of the enzyme from rat hepatocytes was purified, and monospecific antibodies for squalene synthase were produced. This enabled the subcellular localization of squalene synthase by three different methods: (i) analytical subcellular fractionation and measurements of enzyme activities; (ii) immunodeterminations of squalene synthase in the isolated subcellular fractions with a monospecific antibody; and (iii) immunoelectron microscopy. All three methods gave consistent results. The data clearly illustrate that squalene synthase enzymatic activity and squalene synthase are exclusively localized in the endoplasmic reticulum. In rat hepatic peroxisomes we were not able to detect any squalene synthase. In addition, we also demonstrated that squalene synthase in the microsomal fraction is dramatically regulated by a number of hypolipidemic drugs and dietary treatments.

A truncated intracellular HER2/neu receptor produced by alternative RNA processing affects growth of human carcinoma cells

Cloned sequences encoding a truncated form of the HER2 receptor were obtained from cDNA libraries derived from two HER2-overexpressing human breast cancer cell lines, BT-474 and SK-BR-3. The 5' 2.1 kb of the encoded transcript is identical to that of full-length 4.6-kb HER2 transcript and would be expected to produce a secreted form of HER2 receptor containing only the extracellular ligand binding domain (ECD). The 3' end of the truncated transcript diverges 61 nucleotides before the receptor's transmembrane region, reads through a consensus splice donor site containing an in-frame stop codon, and contains a poly(A) addition site, suggesting that the truncated transcript arises by alternative RNA processing. S1 nuclease protection assays show a 40-fold variation in the abundance of the truncated 2.3-kb transcript relative to full-length 4.6-kb transcript in a panel of eight HER2-expressing tumor cell lines of gastric, ovarian, and breast cancer origin. Expression of this truncated transcript in COS-1 cells produces both secreted and intracellular forms of HER2 ECD; however, immunofluorescent labeling of HER2 ECD protein in MKN7 tumor cells that natively overexpress the 2.3-kb transcript suggests that transcriptionally generated HER2 ECD is concentrated within the perinuclear cytoplasm. Metabolic labeling and endoglycosidase studies suggest that this HER2 ECD (100 kDa) undergoes differential trafficking between the endoplasmic reticulum and Golgi compartments compared with full-length (185-kDa) HER2 receptor. Transfection studies indicate that excess production of HER2 ECD in human tumor cells overexpressing full-length HER2 receptor can result in resistance to the growth-inhibiting effects of anti-HER2 monoclonal antibodies such as muMAb4D5. These findings demonstrate alternative processing of the HER2 transcript and implicate a potentially important growth regulatory role for intracellularly sequestered HER2 ECD in HER2-amplified human tumors.

The C-terminal tripeptide of glycosomal phosphoglycerate kinase is both necessary and sufficient for import into the glycosomes of Trypanosoma brucei

Glycosomal phosphoglycerate kinase (gPGK) of Trypanosoma brucei differs from the cytoplasmic isozyme (cPGK) in its higher isoelectric point characterized by clusters of positive charges along the polypeptide chain, and a 20 amino acid C-terminal extension ending in serine-serine-leucine (SSL). While a C-terminal SSL tripeptide is apparently not capable of directing luciferase to the peroxisomes in mammalian cells [J. Cell Biol. 108 (1989), 1657-1664], we show here that it is sufficient for the import of luciferase as well as an unrelated protein, beta-glucuronidase, into the glycosomes of T. brucei, as determined by immunoelectron microscopy. The analysis of luciferase-gPGK fusion proteins indicates that the only targeting signal for import of gPGK into the glycosome resides in this C-terminal SSL sequence.

Expression of vascular endothelial growth factor does not promote transformation but confers a growth advantage in vivo to Chinese hamster ovary cells

Vascular endothelial growth factor (VEGF) is a mitogen with a specificity for endothelial cells in vitro and an angiogenic inducer in vivo. We tested the hypothesis that VEGF may confer on expressing cells a growth advantage in vivo. Dihydrofolatereductase--Chinese hamster ovary cells were transfected with expression vectors which direct the constitutive synthesis of VEGF. Neither the expression nor the exogenous administration of VEGF stimulated anchorage-dependent or anchorage-independent growth of Chinese hamster ovary cells in vitro. However, VEGF-expressing clones, unlike control cells, demonstrated an ability to proliferate in nude mice. Histologic examination revealed that the proliferative lesions were compact, well vascularized, and nonedematous. Ultrastructural analysis revealed that capillaries within the lesions were of the continuous type. These findings indicate that the expression of VEGF may confer on cells the ability to grow in vivo in the absence of transformation by purely paracrine mechanisms. Since VEGF is a widely distributed protein, this property may have relevance for a variety of physiological and pathological proliferative processes.

In vivo import of firefly luciferase into the glycosomes of Trypanosoma brucei and mutational analysis of the C-terminal targeting signal

The compartmentalization of glycolytic enzymes into specialized organelles, the glycosomes, allows the bloodstream form of Trypanosoma brucei to rely solely on glycolysis for its energy production. The biogenesis of glycosomes in these parasites has been studied intensively as a potential target for chemotherapy. We have adapted the recently developed methods for stable transformation of T. brucei to the in vivo analysis of glycosomal protein import. Firefly luciferase, a peroxisomal protein in the lantern of the insect, was expressed in stable transformants of the procyclic form of T. brucei, where it was found to accumulate inside the glycosomes. Mutational analysis of the peroxisomal targeting signal serine-lysine-leucine (SKL) located at the C-terminus of luciferase showed that replacement of the serine residue (Serine548) with a small neutral amino acid (A, C, G, H, N, P, T) still resulted in an import efficiency of 50-100% of the wild-type luciferase. Lysine549 could be substituted with an amino acid capable of hydrogen bonding (H, M, N, Q, R, S), whereas the C-terminal leucine550 could be replaced with a subset of hydrophobic amino acids (I, M, Y). Thus, a peroxisome-like C-terminal SKL-dependent targeting mechanism may function in T. brucei to import luciferase into the glycosomes. However, a few significant differences exist between the glycosomal targeting signals identified here and the tripeptide sequences that direct proteins to mammalian or yeast peroxisomes.

Endocytosis of glycophospholipid-anchored and transmembrane forms of CD4 by different endocytic pathways

A glycophosphatidylinositol (GPI)-anchored form of the CD4 receptor was constructed by fusing the extracellular domain of CD4 to the COOH-terminus of decay accelerating factor (DAF), containing a signal for GPI-anchor attachment. The internalization of GPI-linked CD4 (CD4DAF) was compared to that of transmembrane CD4 using both [125I]gp120 and anti-CD4 antibodies. We show that transmembrane CD4 is rapidly endocytosed in transfected CHO cells, while CD4DAF is internalized at a rate approximately 3-fold slower. Immunoelectron microscopy suggests that whereas transmembrane CD4 is endocytosed via clathrin-coated vesicles, CD4DAF enters cells by an alternative pathway involving non-coated microinvaginations of the plasma membrane. Following internalization CD4DAF recycles through a primaquine-insensitive compartment, whereas the recycling of transmembrane CD4 is inhibited by primaquine, suggesting that the two receptors may recycle from distinct populations of early endosomes. Colocalization of both CD4DAF and CD4 with an antibody against a lysosomal membrane protein suggests that the two endocytic pathways may converge.

Evolutionary conservation of a microbody targeting signal that targets proteins to peroxisomes, glyoxysomes, and glycosomes

Peroxisomes, glyoxysomes, glycosomes, and hydrogenosomes have each been classified as microbodies, i.e., subcellular organelles with an electron-dense matrix that is bound by a single membrane. We investigated whether these organelles might share a common evolutionary origin by asking if targeting signals used for translocation of proteins into these microbodies are related. A peroxisomal targeting signal (PTS) consisting of the COOH-terminal tripeptide serine-lysine-leucine-COOH has been identified in a number of peroxisomal proteins (Gould, S.J., G.-A. Keller, N. Hosken, J. Wilkinson, and S. Subramani. 1989. J. Cell Biol. 108:1657-1664). Antibodies raised to a peptide ending in this sequence (SKL-COOH) recognize a number of peroxisomal proteins. Immunocryoelectron microscopy experiments using this anti-SKL antibody revealed the presence of proteins containing the PTS within glyoxysomes of cells from Pichia pastoris, germinating castor bean seeds, and Neurospora crassa, as well as within the glycosomes of Trypanosoma brucei. Western blot analysis of purified organelle fractions revealed the presence of many proteins containing this PTS in both glyoxysomes and glycosomes. These results indicate that at least one of the signals, and therefore the mechanism, for protein translocation into peroxisomes, glyoxysomes, and glycosomes has been conserved, lending support to a common evolutionary origin for these microbodies. Hydrogenosomes, the fourth type of microbody, did not contain proteins that cross-reacted with the anti-PTS antibody, suggesting that this organelle is unrelated to microbodies.

Cu,Zn superoxide dismutase is a peroxisomal enzyme in human fibroblasts and hepatoma cells

The intracellular localization of Cu,Zn superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) has been examined by immunofluorescence using four monoclonal anti-Cu,Zn superoxide dismutase antibodies raised against a recombinant human Cu,Zn superoxide dismutase derivative produced and purified from Escherichia coli. Colocalization with catalase, a peroxisomal matrix enzyme, was used to demonstrate the peroxisomal localization of Cu,Zn superoxide dismutase in human fibroblasts and hepatoma cells. In the fibroblasts of Zellweger syndrome patients, the enzyme is not transported to the peroxisomal ghosts but, like catalase, remains in the cytoplasm. In addition, immunocryoelectron microscopy of yeast cells expressing human Cu,Zn superoxide dismutase showed that the enzyme is translocated to the peroxisomes.

Endogenous origin of defective retroviruslike particles from a recombinant Chinese hamster ovary cell line

The presence of budding C-type and intracytoplasmic A-type particles in Chinese hamster ovary (CHO) cells is well documented. However, extensive screening has failed to detect any evidence of infectivity. Continuous-flow ultracentrifugation has been used to concentrate extracellular particles from culture fluid of a recombinant CHO cell subclone for molecular characterization. Particles exhibiting reverse transcriptase activity and associated with mammalian C-type retrovirus structural proteins banded in sucrose gradients at a density characteristic of retroviruses. Examination of gradient-purified particles by electron microscopy revealed morphology and size similar to other retroviruses. Double-gradient-purified particles contained RNA which hybridized to probes for murine leukemia virus, and endogenous Chinese hamster intracisternal A-particle elements. DNA sequence analysis of a cDNA clone isolated from purified particles revealed multiple interruptions of the endonuclease reading frame, providing one possible explanation for the noninfectious nature of the observed particles. Sequences present as RNA in purified particles were also present as conserved, repetitive, provirus sequences in genomic DNA of all CHO cell lines examined and in Chinese hamster liver DNA. The observed particles are therefore likely to be the products of endogenous retroviruslike elements present in the germline of Chinese hamsters.

Antibodies directed against the peroxisomal targeting signal of firefly luciferase recognize multiple mammalian peroxisomal proteins

We have previously shown that the peroxisomal targeting signal in firefly luciferase consists of the COOH-terminal three amino acids of the protein, serine-lysine-leucine (Gould, S.J., G.A. Keller, N. Hosken, J. Wilkinson, and S. Subramani, 1989. J. Cell Biol. 108:1657-1664). Antibodies were raised against a synthetic peptide that contained this tripeptide at its COOH terminus. Immunofluorescence and immunocryoelectron microscopy revealed that the anti-peptide antibodies specifically detected peroxisomes in mammalian cells. Further characterization revealed that the antibodies were primarily directed against the COOH-terminal three amino acids of the peptide. In Western blot experiments, the antibodies recognized 15-20 rat liver peroxisomal proteins, but reacted with only a few proteins from other subcellular compartments. These results provide independent immunological evidence that the peroxisomal targeting signal identified in firefly luciferase is present in many peroxisomal proteins.

Peroxisomal protein import is conserved between yeast, plants, insects and mammals

We have previously demonstrated that firefly luciferase can be imported into peroxisomes of both insect and mammalian cells. To determine whether the process of protein transport into the peroxisome is functionally similar in more widely divergent eukaryotes, the cDNA encoding firefly luciferase was expressed in both yeast and plant cells. Luciferase was translocated into peroxisomes in each type of organism. Experiments were also performed to determine whether a yeast peroxisomal protein could be transported to peroxisomes in mammalian cells. We observed that a C-terminal segment of the yeast (Candida boidinii) peroxisomal protein PMP20 could act as a peroxisomal targeting signal in mammalian cells. These results suggest that at least one mechanism of protein translocation into peroxisomes has been conserved throughout eukaryotic evolution.

A conserved tripeptide sorts proteins to peroxisomes

The firefly luciferase protein contains a peroxisomal targeting signal at its extreme COOH terminus (Gould et al., 1987). Site-directed mutagenesis of the luciferase gene reveals that this peroxisomal targeting signal consists of the COOH-terminal three amino acids of the protein, serine-lysine-leucine. When this tripeptide is appended to the COOH terminus of a cytosolic protein (chloramphenicol acetyltransferase), it is sufficient to direct the fusion protein into peroxisomes. Additional mutagenesis experiments reveal that only a limited number of conservative changes can be made in this tripeptide targeting signal without abolishing its activity. These results indicate that peroxisomal protein import, unlike other types of transmembrane translocation, is dependent upon a conserved amino acid sequence.

Subcellular localization of sterol carrier protein-2 in rat hepatocytes: its primary localization to peroxisomes

Sterol carrier protein-2 (SCP-2) is a nonenzymatic protein of 13.5 kD which has been shown in in vitro experiments to be required for several stages in cholesterol utilization and biosynthesis. The subcellular localization of SCP-2 has not been definitively established. Using affinity-purified rabbit polyclonal antibodies against electrophoretically pure SCP-2 from rat liver, we demonstrate by immunoelectron microscopic labeling of ultrathin frozen sections of rat liver that the largest concentration of SCP-2 is inside peroxisomes. In addition the immunolabeling indicates that there are significant concentrations of SCP-2 inside mitochondria, and associated with the endoplasmic reticulum and the cytosol, but not inside the Golgi apparatus, lysosomes, or the nucleus. These results were confirmed by immunoblotting experiments with proteins from purified subcellular fractions of the rat liver cells carried out with the anti-SCP-2 antibodies. The large concentration of SCP-2 inside peroxisomes strongly supports the proposal that peroxisomes are critical sites of cholesterol utilization and biosynthesis. The presence of SCP-2 inside peroxisomes and mitochondria raises questions about the mechanisms involved in the differential targeting of SCP-2 to these organelles.

Identification of peroxisomal targeting signals located at the carboxy terminus of four peroxisomal proteins

As part of an effort to understand how proteins are imported into the peroxisome, we have sought to identify the peroxisomal targeting signals in four unrelated peroxisomal proteins: human catalase, rat hydratase:dehydrogenase, pig D-amino acid oxidase, and rat acyl-CoA oxidase. Using gene fusion experiments, we have identified a region of each protein that can direct heterologous proteins to peroxisomes. In each case, the peroxisomal targeting signal is contained at or near the carboxy terminus of the protein. For catalase, the peroxisomal targeting signal is located within the COOH-terminal 27 amino acids of the protein. For hydratase:dehydrogenase, D-amino acid oxidase, and acyl-CoA oxidase, the targeting signals are located within the carboxy-terminal 15, 14, and 15 amino acids, respectively. A tripeptide of the sequence Ser-Lys/His-Leu is present in each of these targeting signals as well as in the peroxisomal targeting signal identified in firefly luciferase (Gould, S.J., G.-A. Keller, and S. Subramani. 1987. J. Cell Biol. 105:2923-2931). When the peroxisomal targeting signal of the hydratase:dehydrogenase is mutated so that the Ser-Lys-Leu tripeptide is converted to Ser-Asn-Leu, it can no longer direct proteins to peroxisomes. We suggest that this tripeptide is an essential element of at least one class of peroxisomal targeting signals.

Temperature-sensitive Chinese hamster cell mutant with a defect in glycoprotein synthesis: accumulation of the EGF receptor in the endoplasmic reticulum and the role of the glucose-regulated protein GRP78

A temperature-sensitive mutant of Chinese hamster fibroblasts with a defect in glycoprotein synthesis is investigated after transfection and amplification of the gene for the human EGF receptor. We demonstrate that at the nonpermissive temperature a partially glycosylated species of the receptor accumulates in the endoplasmic reticulum. The oligosaccharides present are the high mannose types, since they can be removed completely by treatment with endoglycosidase H. Pulse-chase experiments show that the abnormal species of the receptor cannot be chased to a form that is either resistant to endoglycosidase H, or altered in its mobility on SDS polyacrylamide gels. The abnormal species of the receptor appears within the first hour of a shift to the nonpermissive temperature, and no further changes are observed upon prolonged incubation of cells at 40 degrees C. However, after 3-4 hours immunoprecipitations of the receptor yield another protein, which has properties very similar, if not identical, to the glucose-regulated protein GRP78. The induction of this protein at 40 degrees C can be suppressed completely with an inhibitor of RNA synthesis, without any effect on the glycosylation defect, or on the accumulation of the EGF receptor in the endoplasmic reticulum.

Membrane IgM: interactions with the cortical cytoskeleton in the human lymphoblastoid cell line WiL2

Cell-surface IgM (antigen receptor) sediments with the membrane fraction following osmotic lysis and homogenization of cells of the human lymphoblastoid cell line WiL2. In nonreducing buffers, SDS PAGE analysis of membrane pellets demonstrates that "native" membrane IgM exists as a dimer. In contrast to osmotic lysis, lysis of cells with the nonionic detergent Triton X-100 releases approximately 90% of the membrane-bound IgM into the supernatant; approximately 10% of the IgM pellets with the cytoskeletal fraction on centrifugation. Ligand challenge with either mu-chain-specific antibodies or concanavalin A induces a change in the state of membrane IgM making it refractory to detergent extraction, such that 43% of the IgM pellets during centrifugation. This ligand-induced retention of IgM is significantly diminished by the microfilament-disrupting agent cytochalasin D, whereas pretreatment of cells with sodium azide or colchicine results in no significant change in the percentage of membrane IgM retained by Triton X-100 residues. These results indicate that retention of IgM involves an association with the cortical actin-based cytoskeleton. Investigation of the structural basis for ligand-induced Triton X-100 retention of membrane IgM by using ferritin-conjugated antibodies, myosin subfragment S1, and stereo-imaging electron microscopy has revealed linkages between ligand-receptor (antigen-IgM) complexes and elements of the cortical actin-based cytoskeleton.