Determination of guidance levels of dose for diagnostic radiography in Taiwan

The International Atomic Energy Agency has recommended guidance levels of dose for diagnostic radiography for a typical adult patient. These levels were intended to act as thresholds to trigger investigations or corrective actions in ensuring optimized protection of patients and maintaining appropriate levels of good practice. Since guidance levels should be derived from wide scale surveys of exposure factors performed in individual hospitals, a national survey was conducted recently in Taiwan to collect these factors for the most frequent radiographic procedures. A total of 276 completed questionnaires were received and analyzed. In the questionnaire, respondents were asked to check those projections that were routinely performed in their department and to report machine data, patient data, output measurements, and technical factors including kVp, mAs, focus-to-film distance, table-to-film distance, aluminum filtration, and focal spot size. Based on the survey data, entrance skin exposures in air, i.e., free air exposures at the point of intersection of the x-ray central beam with the entrance surface of the patient, were estimated using the RADCOMP program. Entrance surface doses to air and tissue with backscatter were then evaluated by the application of the exposure-dose conversion factor and the backscatter factor obtained from TLD measurements and Monte Carlo simulations. Guidance levels were determined from survey results on the entrance surface dose based on optimization considerations involving the cost-effectiveness analysis. Except for chest PA and LAT and skull LAT procedures, all guidance levels derived in this work are less than those recommended by the International Atomic Energy Agency. Survey data and guidance levels were provided to the national authorities to help them develop quality control and radiation protection programs for medical exposures.

Decrease of muscarinic M2 cholinoceptor gene expression in the heart of aged rat

It is well known that the baroreflex activity decreases with aging. However, the mechanisms of this change are still not clear. Thus, we investigated one of the parameters to see whether aging alters gene expression of muscarinic receptors in the heart of Wistar rat aged between 2 months (adult) and 24 months (aged). The mRNA level determination by Northern blot analysis for muscarinic M2 cholinoceptors in aged rat was decreased as compared to that in 2-month-old rats. Quantification of receptor protein using selective antibodies indicated that the level of muscarinic M2 cholinoceptor in the heart of 24-month-old rats was lower than that in 2-month-old animals. These results indicate the decrease of muscarinic M2 cholinoceptor in heart with aging that may contribute as one of the parameters for dysfunction in baroreflex activity.

Molecular characterization of human tensin

Tensin is a focal-adhesion molecule that binds to actin filaments and interacts with phosphotyrosine-containing proteins. To analyse tensin's function in mammals, we have cloned tensin cDNAs from human and cow. The isolated approx. 7.7-kb human cDNA contains an open reading frame encoding 1735 amino acid residues. The amino acid sequence of human tensin shares 60% identity with chicken tensin, and contains all the structural features described previously in chicken tensin. This includes the actin-binding domains, the Src homology domain 2, and the region similar to a tumour suppressor, PTEN. Two major differences between human and chicken tensin are (i) the lack of the first 54 residues present in chicken tensin, and (ii) the addition of 34- and 38-residue inserts in human and bovine tensin. In addition, our interspecies sequencing data have uncovered the presence of a glutamine/CAG repeat that appears to have expanded in the course of evolution. Northern-blot analysis reveals a 10-kb message in most of the human tissues examined. An additional 9-kb message is detected in heart and skeletal muscles. The molecular mass predicted from the human cDNA is 185 kDa, although both endogenous and recombinant human tensin migrate as 220-kDa proteins on SDS/PAGE. The discrepancy is due to the unusually low electrophoretic mobility of the central region of the tensin polypeptide (residues 306-981). A survey of human prostate and breast cancer cell lines by Western-blot analysis shows a lack of tensin expression in most cancer cell lines, whereas these lines express considerable amounts of focal-adhesion molecules such as talin and focal-adhesion kinase. Finally, tensin is rapidly cleaved by a focal-adhesion protease, calpain II. Incubation of cells with a calpain inhibitor, MDL, prevented tensin cleavage and induced morphological change in these cells, suggesting that cleavage of tensin and other focal-adhesion constituents by calpain disrupts maintenance of normal cell shape.

Aortic regurgitation detected with Doppler echocardiography in apparently healthy adolescents

In this study, we sought to determine the prevalence and characteristics of aortic regurgitation (AR) in apparently healthy adolescent students. A total of 315 healthy junior high school students underwent echocardiographic examination. There were 158 boys and 157 girls, with a mean +/- standard deviation age of 13.3 +/- 0.9 years (range, 13-15 yr). AR was found in six (2%) students. Five of the six (83%) with AR had minimally thickened aortic valves: three (50%) had a thickened right coronary cusp and four (67%) had a thickened noncoronary cusp, but none had a thickened left coronary cusp. The AR was mild in five (83%) students and moderate in one (17%). Two of the six students had aortic valve prolapse, both of whom had minimal thickening. One student with AR did not have any abnormal structural changes, and one had aortic root dilatation. No cardiac chamber dilatation was noted in students with AR. These findings indicate a relatively high prevalence of AR in apparently healthy adolescents.

Treatment of Ras-induced cancers by the F-actin cappers tensin and chaetoglobosin K, in combination with the caspase-1 inhibitor N1445

For transforming normal fibroblasts to malignant cells, oncogenic Ras mutants such as v-Ha-ras require Rho family GTPases (Rho, Rac, and CDC42) that are responsible for controlling actin-cytoskeleton organization. Ras activates Rac through a PI-3 kinase-mediated pathway. Rac causes uncapping of actin filaments (F-actin) at the plus-ends, through phosphatidylinositol 4,5 bisphosphate (PIP2), and eventually induces membrane ruffling. Several distinct F-actin/PIP2-binding proteins, such as gelsolin, which severs and caps the plus-ends of actin filaments, or HS1, which cross-links actin filaments, have been shown to suppress v-Ha-Ras-induced malignant transformation when they are overexpressed. Interestingly, an F-actin cross-linking drug (photosensitizer) called MKT-077 suppresses Ras transformation. Thus, an F-actin capping/severing drug might also have an anticancer potential.

PURPOSE

This study was conducted to determine first whether Ras-induced malignant phenotype (anchorage-independent growth) is suppressed by overexpression of the gene encoding a large plus-end F-actin capping protein called tensin and second to test the anti-Ras potential of a unique fungal antibiotic (small compound) called chaetoglobosin K (CK) that also caps the plus-ends of actin filaments.

METHODS AND RESULTS

DNA transfection with a retroviral vector carrying the tensin cDNA was used to overexpress tensin in v-Ha-Ras-transformed NIH 3T3 cells. All stable tensin transfectants rarely formed colonies in soft agar, indicating that tensin suppresses the anchorage-independent growth. The anti-Ras action of CK was determined by incubating the Ras-transformants in the presence of CK in soft agar. Two microM CK almost completely inhibited their colony formation, indicating that CK also suppresses the malignant phenotype. However, unlike tensin, CK causes an apoptosis of Ras-transformed NIH 3T3 cells and, less effectively, of normal NIH 3T3 cells, indicating that CK has an F-actin capping-independent side effect(s). CK-induced apoptosis is at least in part caused by CK-induced inhibition of the kinase PKB/AKT. However, a specific ICE/caspase-1 inhibitor called N1445 completely abolished the CK-induced apoptosis by reactivating PKB, but without affecting the CK-induced suppression of Ras transformation.

CONCLUSIONS

Like the F-actin cross-linking drug MKT-077, the F-actin capping drug CK may be useful for the treatment of Ras-associated cancers if it is combined with the ICE inhibitor N1445, which abolishes the side effect of CK. Our observations that two distinct F-actin capping molecules (i.e., tensin and CK) suppress Ras-induced malignant phenotype strongly suggest, if not prove, that capping of actin filaments at the plus-ends alone is sufficient to block one of the Ras signaling pathways essential for its oncogenicity. This notion is compatible with the fact that Ras induces the uncapping of actin filaments at the plus-ends through the Rac/PIP2 pathway.

Small cell carcinoma of the cervix complicated by pregnancy

We report the results of treatment in a 26-year old patient with stage IB2 small cell carcinoma of the cervix complicated by pregnancy. A pathological complete remission was achieved following sandwich chemotherapy and radiotherapy. The patient remains in clinical remission 14 months after presentation.

Antimicrobial susceptibility of Clostridium difficile by E test

The in vitro inhibitory activity of 11 antimicrobials against 44 clinical isolates of Clostridium difficile was investigated. Minimum inhibitory concentrations (MICs) were determined using E test. Metronidazole (MIC90 0.38 microg/mL), teicoplanin (MIC90 0.75 microg/mL) and vancomycin (MIC90 1.0 microg/mL) were very active against the isolates examined, whereas, resistance to imipenem, cefoxitin, clindamycin and ciprofloxacin was found in most of the tested strains. We concluded that teicoplanin warrants clinical trials to determine its adequate dosage to treat C. difficile infection. The commonly used regimens to treat intra-abdominal and/or anaerobic infections (eg. imipenem, cefoxitin, clindamycin or ciprofloxacin) need special attention, while considering the side effects of C. difficile-associated diarrhea.

Inhibition of focal contact formation in cells transformed by p185neu

Signaling pathways mediated by adhesive molecules are tightly associated with cytoskeletal organization and cell growth regulation. Focal adhesion kinase (FAK) plays a prominent role in the adhesion signaling pathway through its tyrosine kinase activity and protein-protein interaction with other signaling molecules, including src, paxillin, and p130CAS, and other proteins. We explored the roles of these signaling molecules in the transformation of B104-1-1 cells, an NIH/3T3-derived cell line transformed by activated rat p185neu. The cytoskeletal organization of the p185neu-transformed cells was disrupted, and their morphology was dramatically altered. FAK, paxillin, and p130CAS appeared to be tyrosine phosphorylated in both NIH/3T3 and B104-1-1. However, the phosphorylation levels of paxillin and p130CAS were lower in B104-1-1 cells than in NIH/3T3 cells. Surprisingly, the association between FAK and paxillin was enhanced in B104-1-1 cells, suggesting reorganization of protein-protein interaction modulated by protein phosphorylation. Our results showed that even though cellular transformation by src and neu has similar consequences, such as focal adhesion disassembly and increased metastasis potential, the molecular events underlying the signaling pathways can be dramatically different.

Prevention of bone loss after heart transplantation with antiresorptive therapy: a pilot study

BACKGROUND

Heart transplantation, with its attendant glucocorticoid and cyclosporine therapy, has deleterious effects on the skeleton. We have previously reported rapid bone loss and high fracture rates (36% of patients) during the first year after heart transplantation. The bone loss was accompanied by declines in serum 1,25-dihydroxyvitamin D and osteocalcin levels and increased urinary excretion of markers of bone resorption (hydroxyproline, pyridinoline, and deoxypyridinoline). We therefore investigated whether bone loss could be prevented by bisphosphonates, agents that inhibit bone resorption.

METHODS

Serial measurements of bone mineral density (BMD) and biochemical indexes of mineral metabolism were compared in 18 group A patients who received a single intravenous infusion of pamidronate (60 mg) within 2 weeks of heart transplantation, followed by 4 cycles of oral etidronate (400 mg daily for 14 days every 3 months) and oral calcitriol 0.25 microg daily, to those of 52 patients who previously underwent transplantation (group B) who did not receive antiresorptive therapy. Both groups received elemental calcium 1000 mg and vitamin D 400 IU daily.

RESULTS

At 12 months after transplantation, there was virtually no lumbar spine bone loss in group A patients, whereas lumbar spine BMD had declined significantly in group B patients (0.2% +/- 0.9% vs 6.8% +/- 1.0%, respectively; P < .0001). Similarly, femoral neck BMD fell by 10.6% +/- 1.1% in group B patients and by only 2.7% +/- 1.4% in group A patients (P < .0001). Three incident vertebral fractures occurred in two group A patients, whereas 17 group B patients sustained 30 incident vertebral fractures, one hip fracture and three episodes of rib fractures (P < .02; test of proportions). With respect to markers of bone resorption, urinary deoxypyridinoline fell by 51% +/- 9% in group A patients and increased by 65% +/- 22% in group B patients by 3 months after transplantation (P < .0001).

CONCLUSION

In summary, heart transplant recipients treated with bisphosphonates and replacement doses of calcitriol sustained less bone loss and fewer fractures than those treated with calcium and vitamin D. We conclude that bisphosphonate therapy, in conjunction with calcitriol, shows promise for prevention of transplantation-related osteoporosis.

Gene synthesis by a LCR-based approach: high-level production of leptin-L54 using synthetic gene in Escherichia coli

Synthetic genes are very useful in genetic and protein engineering. Here we propose a general method for construction of synthetic genes. Short oligonucleotides are joined through ligase chain reaction (LCR) in high stringency conditions to make "unit fragments" which are then fused to form a full-length gene sequence by polymerase chain reaction. The procedure is simple and accurate and does not place constraints on sequence and length. In this report, a recombinant leptin gene was synthesized according to the codon preference of Escherichia coli. Besides, a substitution of the only Met at position 54 for Leu and an addition of a Met at the N-terminus were introduced in the synthetic gene. The gene was cloned in the pQE-31 expression vector and was expressed in E. coli. A large amount of recombinant leptin containing 6 x His tag was produced and purified by Ni-NTA affinity column. Finally, intact leptin-L54 was released after removing the tag by CNBr cleavage at the Met residue.

Varicella-zoster virus infection in children with underlying human immunodeficiency virus infection

This article describes a prospective longitudinal study of varicella-zoster virus (VZV) infections in human immunodeficiency virus (HIV)-infected children, designed to determine their natural history of VZV infection and possible effects of VZV on the progression of HIV infection. Varicella was usually not a serious acute problem, and it did not seem to precede clinical deterioration. The rate of zoster was high: 70% in children with low levels of CD4+ lymphocytes at the time of development of varicella. It is predicted that immunization with live attenuated varicella vaccine is unlikely to be deleterious to HIV-infected children. Moreover, if they are immunized when they still have relatively normal levels of CD4+ lymphocytes, they may have a lower rate of reactivation of VZV than if they were allowed to develop natural varicella when their CD4+ cell counts have fallen to low levels as a result of progressive HIV infection.

Integrators of epidermal growth and differentiation: distinct functions for beta 1 and beta 4 integrins

Mammalian epithelia are critically dependent on interactions with components in the underlying basal lamina for proper morphogenesis and function. Substratum attachment is essential for survival, proliferation, movement, and differentiation; detachment compromises the cell's ability to perform these functions, often resulting in human disease. Interactions with the extracellular matrix are mediated through transmembrane integrin receptors that transmit signals to the cytoskeleton and to signaling molecules within the proliferating cells of the epithelium. In the past year, novel insights have emerged regarding the specific role of integrins in their attachment to extracellular matrix and in their signal transduction pathways within the epidermis.

Bone loss and turnover after cardiac transplantation

Cardiac transplantation is associated with increased prevalence and incidence of fracture, and rapid bone loss has been reported during the first posttransplant year. To define further the pattern and etiology of bone loss after cardiac transplantation, we enrolled 70 patients (52 men and 18 women) in a prospective 3-yr study. Bone densitometry (BMD) and biochemical indexes of mineral metabolism were performed before and at defined times after transplantation. Despite supplementation with elemental calcium (1000 mg/day) and vitamin D (400 IU/day), the mean rate of bone loss during the first year was 7.3 +/- 0.9% (+/- SEM) at the lumbar spine and 10.5 +/- 1.1% at the femoral neck. The rate of bone loss slowed (P < 0.001 compared to year 1) at both sites (0.9 +/- 0.9% and 0.1 +/- 1.0%, respectively) during the second year. During the third year, lumbar spine BMD increased at a rate of 2.4 +/- 0.8%/yr (P < 0.02 compared to year 2), but femoral neck BMD did not change. At the radius, the rate of decline in BMD was negligible during the first year (0.9 +/- 0.5%), but was significant during the second (2.1 +/- 0.6%; P < 0.01) and third (2.9 +/- 0.8%; P < 0.03) years. Evaluation of the pattern of bone loss during the first year demonstrated that mean lumbar spine BMD decreased rapidly during the first 6 months, after which there was no further decline. In contrast, femoral neck BMD continued to fall at an annualized rate of 8.2 +/- 1.3% during the second half of the year. The pattern and rates of bone loss were similar in men and women. Biochemistries revealed decreases in serum testosterone and osteocalcin and increases in all bone resorption markers 1 and 3 months after transplantation, with a return to baseline by 6 months. Higher rates of bone loss were associated with greater exposure to prednisone, lower serum concentrations of vitamin D metabolites, greater suppression of osteocalcin, higher levels of bone resorption markers, and, in men, lower serum testosterone concentrations. We conclude that rapid bone loss is primarily confined to the initial year after transplantation. During the first 6 months, bone loss is accompanied by alterations in markers of bone turnover consistent with biochemical uncoupling of bone formation and resorption. Greater exposure to glucocorticoids, lower serum concentrations of vitamin D metabolites and testosterone, and higher bone turnover were associated with more rapid bone loss.

Progressive kidney degeneration in mice lacking tensin

Tensin is a focal adhesion phosphoprotein that binds to F-actin and contains a functional Src homology 2 domain. To explore the biological functions of tensin, we cloned the mouse tensin gene, determined its program of expression, and used gene targeting to generate mice lacking tensin. Even though tensin is expressed in many different tissues during embryogenesis, tensin null mice developed normally and appeared healthy postnatally for at least several months. Over time, -/- mice became frail because of abnormalities in their kidneys, an organ that expresses high levels of tensin. Mice with overt signs of weakness exhibited signs of renal failure and possessed multiple large cysts in the proximal kidney tubules, but even in tensin null mice with normal blood analysis, cysts were prevalent. Ultrastructurally, noncystic areas showed typical cell-matrix junctions that readily labeled with antibodies against other focal adhesion molecules. In abnormal regions, cell-matrix junctions were disrupted and tubule cells lacked polarity. Taken together, our data imply that, in the kidney, loss of tensin leads to a weakening, rather than a severing, of focal adhesion. All other tissues appeared normal, suggesting that, in most cases, tensin's diverse functions are redundant and may be compensated for by other focal adhesion proteins.

Platelet-derived growth factor-induced formation of tensin and phosphoinositide 3-kinase complexes

Tensin is an SH2 domain-containing cytoskeletal protein that binds to and caps actin filaments. Investigation of signal transduction mechanisms associated with tensin revealed the presence of phosphoinositide 3-kinase (PI 3-kinase) activity in tensin immunoprecipitates from platelet-derived growth factor-treated cells. Association of PI 3-kinase activity with tensin was transitory, and the amount of activity was approximately 1% of the total PI 3-kinase activity found in anti-phosphotyrosine (anti-pY) immunoprecipitates. In vitro, PI 3-kinase activity associated with the SH2 domain of tensin in a platelet-derived growth factor-dependent manner. The optimal phosphopeptide binding specificity of the SH2 domain of tensin was determined to be phospho-Y (E or D), N, (I, V, or F). Synthetic phosphopeptides containing the sequence YENI could specifically block the association of PI 3-kinase activity with tensin in a dose-dependent manner. These results suggest that PI 3-kinase interacts with the cytoskeleton via the SH2 domain of tensin and may play an important role in platelet-derived growth factor-induced cytoskeletal reorganization that is concomitant with cell migration and proliferation.

The protective effect of immunologic boosting against zoster: an analysis in leukemic children who were vaccinated against chickenpox

Whether reexposure of varicella-immune persons to varicella-zoster virus would protect against or predispose to development of zoster was analyzed. The rate of zoster in 511 leukemic recipients of varicella vaccine who had 1 or > 1 dose of varicella vaccine and in those who did or did not have a household exposure to varicella was determined. A Kaplan-Meier life-table analysis revealed that the incidence of zoster was lower in those given > 1 dose of vaccine (P < .05). A Cox proportional hazards analysis showed that both household exposure to varicella and receipt of > 1 dose of vaccine were highly protective (P < .01) against zoster. Thus, the risk of zoster is decreased by reexposure to varicella-zoster virus, either by vaccination or by close exposure to varicella.

Increased tyrosine phosphorylation of focal adhesion proteins in myeloid cell lines expressing p210BCR/ABL

The BCR/ABL oncogene causes chronic myelogenous leukemia (CML) in humans and induces growth factor independence of hematopoietic cell lines in tissue culture. p210BCR/ABL is localized at least in part to the cytoskeleton, and has been shown to interact directly with actin filaments through an actin binding domain located in the C-terminus of ABL. CML cells have reduced adhesion to some extracellular matrix components but the mechanism of this phenomenon is unknown. In this study we examined tyrosine phosphorylation of focal adhesion proteins in cells expressing p210BCR/ABL. An interleukin-3 (IL-3)-dependent cell line, 32Dc13, was transformed with a BCR/ABL cDNA, and the patterns of localization, expression, and tyrosine phosphorylation of focal adhesion proteins were compared among untransformed 32Dc13 cells with and without IL-3 stimulation and BCR/ABL-transformed 32Dc13 cells. Of the focal adhesion proteins examined, only paxillin exhibited tyrosine phosphorylation in response to IL-3; while in cells transformed by p210BCR/ABL, paxillin, vinculin, p125FAK, talin and tensin were constitutively tyrosine phosphorylated. IL-3 induced a transient association between paxillin and vinculin, while in BCR/ABL-transformed cells, several proteins coimmunoprecipitated with paxillin, including vinculin, p125FAK, talin and tensin. Pseudopodia enriched in focal adhesion proteins were transiently detected in 32Dc13 cells in response to IL-3, but constitutively detected in cells expressing p210BCR/ABL. p210BCR/ABL protein was also found concentrated in punctate structures adjacent to the cell membrane in myeloid cell lines, which often contained vinculin and paxillin.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of human paxillin, a focal adhesion protein phosphorylated by P210BCR/ABL

Paxillin is a 68-kDa focal adhesion protein that is phosphorylated on tyrosine residues in fibroblasts in response to transformation by v-src, treatment with platelet-derived growth factor, or cross-linking of integrins. Paxillin has been shown to have binding sites for the SH3 domain of Src and the SH2 domain of Crk in vitro and to coprecipitate with two other focal adhesion proteins, vinculin and focal adhesion kinase (p125fak). After preliminary studies showed that paxillin was a substrate for the hematopoietic oncogene p210BCR/ABL, we investigated the role of this protein in hematopoietic cell transformation and signal transduction. A full-length length cDNA encoding human paxillin was cloned, revealing multiple protein domains, including four tandem LIM domains, a proline-rich domain containing a consensus SH3 binding site, and three potential Crk-SH2 binding sites. The paxillin gene was localized to chromosome 12q24 by fluorescence in situ hybridization analysis. A chicken paxillin cDNA was also cloned and is predicted to encode a protein approximately 90% identical to human paxil-lin. Paxillin coprecipitated with p210BCR/ABL and multiple other cellular proteins in myeloid cell lines, suggesting the formation of multimeric complexes. In normal hematopoietic cells and myeloid cell lines, tyrosine phosphorylation of paxillin and coprecipitation with other cellular proteins was rapidly and transiently induced by interleukin-3 and several other hematopoietic growth factors. The predicted structure of paxillin implicates this molecule in protein-protein interactions involved in signal transduction from growth factor receptors and the BCR/ABL oncogene fusion protein to the cytoskeleton.

Tensin: a potential link between the cytoskeleton and signal transduction

Cytoskeletal proteins provide the structural foundation that allows cells to exist in a highly organized manner. Recent evidence suggests that certain cytoskeletal proteins not only maintain structural integrity, but might also be associated with signal transduction and suppression of tumorigenesis. Since the time of the discovery of tensin, a fair amount of data has been gathered which supports the notion that tensin is one such protein possessing these characteristics. In this review, we discuss recent studies that: (1) elucidate a role for tensin in maintenance of cellular structure and signal transduction; (2) implicate tensin as the anchor for actin filaments at the focal adhesion; (3) describe the phosphorylation of tensin; (4) describe potential targets for its Src homology region 2 domain; (5) describe the association between tensin and the nuclear protein p130; and (6) demonstrate that increased tensin expression in a cell line appears to reduce its transformation potential.

Molecular cloning of chick cardiac muscle tensin. Full-length cDNA sequence, expression, and characterization

Here we describe the molecular cloning of 7.1-kilobase cDNA encoding chick cardiac muscle tensin. It contains an open reading frame of 1,744 amino acid (aa) residues. Sequence analysis reveals that, in addition to the previously noted SH2 domain (Davis, S., Lu, M. L., Lo, S. H., Lin, S., Butler, J. A., Druker, B. J., Roberts, T. M., An, Q., and Chen, L. B. (1991) Science 252, 712-715), tensin contains virtually all of the known sequence (362 aa) of insertin, an actin-capping protein that allows actin monomer to be "inserted" (Schroer, E., and Wegner, A. (1985) Eur. J. Biochem. 153, 515-520). Moreover, tensin shares partial homology with actin (46.7% identity in 30 aa), beta-spectrin's actin-binding consensus (40% identity in 26 aa), BCR (40% identity in 25 aa), catenin alpha (35% identity in 45 aa), synapsin Ia (25.6% identity in 156 aa), IL-3 receptor (20.2% identity in 384 aa), and IL-2/EPO receptors (14% identity in 20 aa). Recombinant full-length tensin, tagged with an influenza-derived epitope, was over-expressed by a baculovirus system and purified to apparent homogeneity. It migrates as a 200-kDa protein in SDS-polyacrylamide gel electrophoresis, similar to the native tensin. The structure of the tensin molecule has been characterized by light scattering, electron microscopy, and gel filtration. Nine monoclonal antibodies recognizing different regions of tensin have been prepared and characterized. The epitope-tagged recombinant tensin gene was subcloned into a pRcCMV vector and transfected into NIH 3T3 cells. Immunofluorescence stainings with monoclonal antibodies specific for chick tensin (not cross-reactive with mouse tensin) showed that the expressed protein is indeed localized at focal contacts, as that of native tensin.

Interactions of tensin with actin and identification of its three distinct actin-binding domains

Tensin, a 200-kD phosphoprotein of focal contacts, contains sequence homologies to Src (SH2 domain), and several actin-binding proteins. These features suggest that tensin may link the cell membrane to the cytoskeleton and respond directly to tyrosine kinase signalling pathways. Here we identify three distinct actin-binding domains within tensin. Recombinant tensin purified after overexpression by a baculovirus system binds to actin filaments with Kd = 0.1 microM, cross-links actin filaments at a molar ratio of 1:10 (tensin/actin), and retards actin assembly by barbed end capping with Kd = 20 nM. Tensin fragments were constructed and expressed as fusion proteins to map domains having these activities. Three regions from tensin interact with actin: two regions composed of amino acids 1 to 263 and 263 to 463, cosediment with F-actin but do not alter the kinetics of actin assembly; a region composed of amino acids 888-989, with sequence homology to insertin, retards actin polymerization. A claw-shaped tensin dimer would have six potential actin-binding sites and could embrace the ends of two actin filaments at focal contacts.

Presence of an SH2 domain in the actin-binding protein tensin

The molecular cloning of the complementary DNA coding for a 90-kilodalton fragment of tensin, an actin-binding component of focal contacts and other submembraneous cytoskeletal structures, is reported. The derived amino acid sequence revealed the presence of a Src homology 2 (SH2) domain. This domain is shared by a number of signal transduction proteins including nonreceptor tyrosine kinases such as Abl, Fps, Src, and Src family members, the transforming protein Crk, phospholipase C-gamma 1, PI-3 (phosphatidylinositol) kinase, and guanosine triphosphatase-activating protein (GAP). Like the SH2 domain found in Src, Crk, and Abl, the SH2 domain of tensin bound specifically to a number of phosphotyrosine-containing proteins from v-src-transformed cells. Tensin was also found to be phosphorylated on tyrosine residues. These findings suggest that by possessing both actin-binding and phosphotyrosine-binding activities and being itself a target for tyrosine kinases, tensin may link signal transduction pathways with the cytoskeleton.