Differential dynamic properties of scleroderma fibroblasts in response to perturbation of environmental stimuli.

Diseases are believed to arise from dysregulation of biological systems (pathways) perturbed by environmental triggers. Biological systems as a whole are not just the sum of their components, rather ever-changing, complex and dynamic systems over time in response to internal and external perturbation. In the past, biologists have mainly focused on studying either functions of isolated genes or steady-states of small biological pathways. However, it is systems dynamics that play an essential role in giving rise to cellular function/dysfunction which cause diseases, such as growth, differentiation, division and apoptosis. Biological phenomena of the entire organism are not only determined by steady-state characteristics of the biological systems, but also by intrinsic dynamic properties of biological systems, including stability, transient-response, and controllability, which determine how the systems maintain their functions and performance under a broad range of random internal and external perturbations. As a proof of principle, we examine signal transduction pathways and genetic regulatory pathways as biological systems. We employ widely used state-space equations in systems science to model biological systems, and use expectation-maximization (EM) algorithms and Kalman filter to estimate the parameters in the models. We apply the developed state-space models to human fibroblasts obtained from the autoimmune fibrosing disease, scleroderma, and then perform dynamic analysis of partial TGF-beta pathway in both normal and scleroderma fibroblasts stimulated by silica. We find that TGF-beta pathway under perturbation of silica shows significant differences in dynamic properties between normal and scleroderma fibroblasts. Our findings may open a new avenue in exploring the functions of cells and mechanism operative in disease development.

Differential dynamic properties of scleroderma fibroblasts in response to perturbation of environmental stimuli.

Diseases are believed to arise from dysregulation of biological systems (pathways) perturbed by environmental triggers. Biological systems as a whole are not just the sum of their components, rather ever-changing, complex and dynamic systems over time in response to internal and external perturbation. In the past, biologists have mainly focused on studying either functions of isolated genes or steady-states of small biological pathways. However, it is systems dynamics that play an essential role in giving rise to cellular function/dysfunction which cause diseases, such as growth, differentiation, division and apoptosis. Biological phenomena of the entire organism are not only determined by steady-state characteristics of the biological systems, but also by intrinsic dynamic properties of biological systems, including stability, transient-response, and controllability, which determine how the systems maintain their functions and performance under a broad range of random internal and external perturbations. As a proof of principle, we examine signal transduction pathways and genetic regulatory pathways as biological systems. We employ widely used state-space equations in systems science to model biological systems, and use expectation-maximization (EM) algorithms and Kalman filter to estimate the parameters in the models. We apply the developed state-space models to human fibroblasts obtained from the autoimmune fibrosing disease, scleroderma, and then perform dynamic analysis of partial TGF-beta pathway in both normal and scleroderma fibroblasts stimulated by silica. We find that TGF-beta pathway under perturbation of silica shows significant differences in dynamic properties between normal and scleroderma fibroblasts. Our findings may open a new avenue in exploring the functions of cells and mechanism operative in disease development.

Expression and function of $\beta$-tubulin isotypes in Chinese hamster ovary (CHO) cells

The availability of isotype specific antisera for $\beta$-tubulin, coupled with genetic and biochemical analysis, has allowed the determination of $\beta$-tubulin isotype expression and distribution in Chinese hamster ovary (CHO) cells. Using genetic manipulations involving selection for colcemid resistance followed by reversion and reselection for drug resistance, we have succeeded in isolating cell lines that exhibit three major and one minor $\beta$-tubulin spots by two-dimensional gel electrophoresis. In concert with isotype specific antibodies, analysis of these mutants demonstrates that CHO cells express two copies of isotype I, at least one copy of isotype IV, and very small amounts of isotype V. Their stoichiometry is approximately 1:1:0.7:0.2. All three isotypes assemble into both cytoplasmic and spindle microtubules, and are similar in their responses to cold, colcemid, and calcium induced depolymerization. They have comparable turnover rates and are equally sensitive to depression of synthesis upon colchicine treatment. These results suggest that $\beta$-tubulin isotypes are used interchangeably to assemble microtubule structures in CHO cells. However, of 18 colcemid resistant mutants with a demonstrable alteration in $\beta$-tubulin, all were found to have the alteration in isotype I, thus leaving open the possibility that subtle differences in isotype properties may exist. Under various conditions of the cell growth, the relative proportion of each expressed isotype does not significantly seem to change except in the early G1 phase of the cell cycle. At this time the synthesis of isotype V increases more than two fold relative to isotype I and IV, while at the same time, total $\beta$-tubulin synthesis is decreased about 60-70%. ^

Function and formation of $\beta$1,4-galactosyltransferase-specific adhesions during growth and differentiation of F9 embryonal carcinoma cells

Galactosyltransferase (GalTase) is localized in the Golgi, where it functions in oligosaccharide synthesis, as well as on the cell surface where it serves as a cell adhesion molecule. GalTase-specific adhesions are functional in a number of important biological events, including F9 embryonal carcinoma (EC) cell adhesions. GalTase-based adhesions are formed by recognition and binding to terminal N-acetylglucosamine (GlcNAc) residues on its glycoprotein counterpart on adjacent cell surfaces. The object of this work has been to investigate the formation and function of GalTase-specific adhesions during F9 cell growth and differentiation. We initially investigated GalTase synthesis during differentiation and found that the increase in GalTase activity was specific for the Golgi compartment; surface GalTase levels remained constant during differentiation. These data indicated that the increase in cell adhesions expected with increased cell-matrix interaction in differentiated F9 cells is not the consequence of increased surface GalTase expression and, more interestingly, that the two pools of GalTase are under differential regulation. Synthesis and recognition of the consociate glycoprotein component was next investigated. Surface GalTase recognized several surface glycoproteins in a pattern that changes with differentiation. Uvomorulin, lysosome-associated membrane protein-1 (LAMP-1), and laminin were recognized by surface GalTase and are, therefore, potential components in GalTase-specific adhesions. Furthermore, these interactions were aberrant in an adhesion-defective F9 cell line that results, at least in part, from abnormal oligosaccharide synthesis. The function played by surface GalTase in growth and induction of differentiation was examined. Inhibition of surface GalTase function by a panel of reagents inhibited F9 cell growth. GalTase expression at both the transcription and protein levels were differentially regulated during the cell cycle, with surface expression greatest in the G1 phase. Disruption of GalTase adhesion by exposure to anti-GalTase antibodies during this period resulted in extension of the G2 phase, a result similar to that seen with agents known to inhibit growth and induce differentiation. Finally, other studies have suggested that a subset of cell adhesion molecules have the capability to induce differentiation in EC cells systems. We have determined in F9 cells that dissociating GalTase adhesion by galactosylation of and release of the consociate glycoproteins induces differentiation, as defined by increased laminin synthesis. The ability to induce differentiation by surface galactosylation was greatest in cells grown in cultures promoting cell-cell adhesions, relative to cultures with minimal cell-cell interactions. ^

Studies on the adhesion properties of the liver-metastatic murine RAW117 large-cell lymphoma cells: Roles of $\beta\sb3$ integrin

Metastasis is the major cause of death in cancer patients. Since many cancers show organ-preference of metastasis, elucidation of the underlying mechanisms of metastasis will benefit diagnosis or treatment of metastatic diseases. Adhesion mechanisms are thought to be involved in organ-preference of metastasis, because metastatic cells show organ preference in adhering to organ-derived microvascular endothelial cells. The adhesion molecules in this process remain largely unidentified. I have examined a series of murine RAW117 large-cell lymphoma cells variants selected in vivo for liver-colonizing properties ($\rm{H10{>>}L17>P}$). The highly liver-metastatic H10 cells were found to differentially express much higher levels of integrin $\alpha\rm\sb{v}\beta\sb3$ than L17 or P cells. H10 cells also adhered at higher rates to vitronectin and fibronectin than to fibrinogen, fibrin, laminin and type I collagen, and adhered at significantly higher rates to (GRGDS)$\sb4$ than to monomeric RGD-peptides. In contrast, P and L17 cells did not adhere well to the above substrates. H10 cells also spread well on vitronectin and migrated toward vitronectin concentration gradients. Pretreament of H10 cells with anti-$\beta\sb3$ monoclonal antibodies resulted in significant decreases in adhesion of H10 cells to vitronectin and immobilized (GRGDS)$\sb4$, and reduced the formation of experimental liver metastases in syngeneic Balb/c mice.^ Adhesion of RAW117 cells under hydrodynamic shear stresses was also studied because tumor cell adhesion occurs under fluid shear stresses in target organ microvessels. Similar to their properties found with static adhesion assays, H10 cells stabilized their hydrodynamic adhesion to vitronectin, fibronectin and (GRGDS)$\sb4$ much more quickly than P or L17 cells. Unlike their static adhesion properties, RAW117 cells showed differential adhesion stabilization to liver-sinusoidal endothelial cell-derived extracellular matrix ($\rm{H10{>>}L17>P}$). Although not supporting static adhesion of RAW117 cells, monomeric RGD-peptides mediated adhesion stabilization of H10 cells but not L17 or P cells. Integrin $\rm\alpha\sb{v}\beta\sb3$ was found to be involved in stabilizing H10 cell adhesion to vitronectin, (GRGDS)$\sb4$, monomeric RGD-peptide R1, and liver sinusoidal endothelial cell-derived extracellular matrix.^ This study is the first to provide evidence that integrin $\rm\alpha\sb{v}\beta\sb3$ is differentially expressed in liver-metastatic lymphoma cells and involved in differential adhesion of these cells. The results indicate that strong static adhesion and especially the unique hydrodynamic adhesion of RAW117 cells to the RGD-containing substrates correlate with liver-metastatic potentials. Thus, integrin $\rm\alpha\sb{v}\beta\sb3$ may play an important role in liver-preferential metastasis of RAW117 large-cell lymphoma cells. ^

Analysis of the mechanisms that maintain coordinate production of $\alpha$- and $\beta$-tubulin in mammalian cells

Alpha and beta tubulin are essential proteins in all eukaryotic cells. To study how cells maintain coordinate levels of these two interacting proteins, we have used PCR to add a 9 amino acid epitope from influenza hemagglutinin protein onto the carboxyl terminus of $\alpha$1 and $\beta$1-tubulin. The chimeric tubulin genes (HA$\alpha$1 and HA$\beta$1) were transfected into CHO cells and cell lines that stably express each gene were selected. Cells transfected with HA-tubulin do not exhibit any gross changes in growth or morphology. Immunofluorescence analysis demonstrated that HA-tubulins incorporate into both cytoplasmic and spindle microtubules. A quantitative biochemical assay was used to show that HA-tubulins incorporate into microtubules to a normal extent and do not alter the steady state distribution of endogenous tubulin between monomer and polymer pools. Two-dimensional gel analysis of pulse-labeled cells indicated that when HA$\beta$1-tubulin is expressed at high levels, it slightly represses the synthesis of the endogenous $\beta$-tubulin but produces a small increase in the synthesis of $\alpha$-tubulin. Analysis of cells labeled to steady state showed that HA$\beta$1-tubulin accumulates to a similar level as the wild-type gene product, but together these polypeptides produce only a small increase in total tubulin content consistent with the increased synthesis of $\alpha$-tubulin. It thus appears that HA$\beta$1-tubulin successfully competes with endogenous $\beta$-tubulin for heterodimer formation and that free $\beta$-tubulin subunits (endogenous and HA$\beta$1) are selectively degraded to maintain coordinate amounts of $\alpha$- and $\beta$-tubulin. In addition, the increased synthesis of $\alpha$-tubulin suggested the existence of a mechanism to ensure coordinate synthesis of $\alpha$- and $\beta$-tubulin subunits. To analyze whether reciprocal changes in endogenous tubulin synthesis occur when $\alpha$-tubulin is overexpressed, stably transfected CHO cell lines were isolated in which HA$\alpha$1-tubulin represents 50% of the total $\alpha$-tubulin, and its relative abundance can be further increased to 85-90% by treatment with sodium butyrate. In contrast with results obtained using HA$\beta$1-tubulin, transfection of HA$\alpha$1-tubulin decreased the synthesis of endogenous $\alpha$-tubulin to 60% of normal with little or no change in $\beta$-tubulin synthesis. When the transfected cells were treated with sodium butyrate to further increase HA$\beta$1-tubulin production, a larger decrease in the synthesis of endogenous $\alpha$-tubulin (to 30% of normal) was observed. The repression on the synthesis of endogenous $\alpha$-tubulin polypeptide was found to be directly proportional to the expression of HA$\alpha$1-tubulin indicating the existence of an autoregulatory loop, where $\alpha$-tubulin inhibits its own synthesis. To determine whether overproduction of HA$\alpha$1-tubulin affected the transcription, message stability or translation of endogenous $\alpha$-tubulin, the steady state levels of $\alpha$-tubulin mRNA were analyzed by ribonuclease protection assays. The results showed that the steady state level of $\alpha$-tubulin mRNA is not affected by the overexpression of HA$\alpha$1-tubulin, indicating that the repression is translational. The results are compatible with a model in which $\beta$-tubulin synthesis is largely unperturbed by overexpression of other tubulin subunits, and excess $\beta$-tubulin subunits are rapidly degraded to maintain coordinate $\alpha$- and $\beta$-tubulin levels at steady state. In contrast, free $\alpha$-tubulin represses its own synthesis at the translational level, suggesting that its level of production may be controlled by the amount of $\beta$-tubulin available for heterodimer formation. ^

A PHARMACOLOGICAL EVALUATION OF THE MECHANISM OF CYTOTOXICITY OF 9-BETA-D- XYLOFURANOSYLADENINE IN CHINESE HAMSTER OVARY CELLS

The biochemical determinants of cytotoxicity of the purine nucleoside analog, 9-(beta)-D-xylofuranosyladenine (xyl-A) were studied in wild-type Chinese hamster ovary cells and in nucleoside kinase deficient mutants. It was found that {('3)H}xyl-A was readily phosphorylated to the triphosphate level in both the wild-type and deoxycytidine kinase deficient mutant, but not by the adenosine kinase deficient cells. Values for the apparent Km and Vmax of this uptake process were 43.9 (mu)M and 118.7 nmol/min/10('9) cells, respectively. Cloning procedures indicated that the viability of CHO cells was decreased 90 per cent by a 5-hr incubation with 10 (mu)M xyl-A. However, the toxicity of xyl-A was increased 100-fold by the addition of a nontoxic concentration (10 (mu)M) of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) to the medium. High-pressure liquid chromatographic analysis indicated that after 5 hr, the concentration of 9-(beta)-D-xylofuranosyladenine 5'-triphosphate (xyl-ATP) in cells incubated with xyl-A plus EHNA was 2.0 mM, four times greater than in those cells incubated with xyl-A alone. Incubation with xyl-A plus EHNA had no significant effect on the cellular concentrations of 5-phosphoribosyl-1-pyrophosphate after 1 hr whereas, treatment with 3'-dexoyadenosine (cordycepin) decreased the concentration of this metabolite. Determinations of the cellular nucleoside triphosphates indicated that under conditions that resulted in an intracellular accumulation of 500 (mu)M xyl-ATP, the endogenous concentrations of neither the ribonucleoside triphosphates nor deoxyribonucleoside triphosphates were significantly different from those of control cells. The ID(,50) for {('3)H}thymidine incorporation into DNA, 105 (mu)M xyl-ATP, was four-fold less than the ID(,50) for {('3)H}uridine incorporation into RNA suggesting that the process of DNA synthesis is more sensitive to the presence of xyl-ATP. When removed from exogenous xyl-A, CHO cells failed to recover their ability to synthesize RNA and DNA, although the intracellular xyl-ATP concentration decreased to less than 35 (mu)M. The selective inhibition of RNA synthesis by 6-azauridine did not prevent the expression of toxicity by xyl-ATP. However, the selective inhibition of DNA synthesis by ara-C significantly spared toxicity in cells that had accumulated an otherwise lethal concentration of xyl-ATP. It is shown that in cells which had accumulated 1.27 mM {('3)H}xyl-ATP, {('3)H}xyl-A was found to terminate cellular RNA chains at a frequency of 1.42 (mu)mol of {('3)H}xyl-A 3' termini per mol of mononucleotide. These results indicate that a general mechanism for the toxicity of xyl-A to CHO cells includes the cellular accumulation of xyl-ATP, which serves as a substrate for RNA synthesizing enzymes and subsequently is incorporated into nascent RNA transcripts as a chain terminator. A specific mechanism involving the premature termination of RNA primers required for the initiation of DNA synthesis is proposed to account for the inhibitory action of xyl-ATP on DNA synthesis. ^

IL-2 REGULATION OF IL-24 EXPRESSION LEADS TO GROWTH SUPPRESSION OF MELANOMA CELLS

Melanoma is known to be highly resistant to chemotherapy. Treatment with high dose IL-2 has shown significant clinical benefit in a minority of metastatic melanoma patients and has lead to long term survival in a few cases. However, this treatment is associated with excessive multiorgan toxicities, which severely limits its use. We hypothesize that one mechanism of effective IL-2 therapy is through the direct upregulation of IL-24 production in melanoma tumors and subsequent IL-24 mediated tumor growth suppression. Five melanoma cell lines were treated with high dose recombinant hIL-2 at 1000U/ml. Three of the cell lines (A375, WM1341, WM793) showed statistically significant increases in their levels of IL-24 protein when measured by Western blotting, while the remaining two lines (WM35, MeWo) remained negative for IL-24 message and protein. This increase in IL-24 was abolished by either preincubating with an anti-IL-2 antibody or by blocking the IL-2 receptor directly with antibodies against the receptor chains. We also demonstrated by ELISA that these three cell lines secrete IL-24 protein in higher amounts when stimulated with IL-2 than do untreated cells. These cells were found to contain IL-2R beta and gamma message by RT-PCR and also expressed higher levels of IL-24 when treated with IL-15, which shares the IL-2R beta chain. Thus we propose that IL-2 is signaling through IL-2R beta on some melanoma cells to upregulate IL-24 protein expression. To address the biological function of IL-2 in melanoma cells, five cell lines were treated with IL-2 and cell viability determined. Cell growth was found to be significantly decreased by day 4 in the IL-24 positive cell lines while no effect on growth was seen in WM35 or MeWo. Incubating the cells with anti-IL-24 antibody or transfecting with IL-24 siRNA effectively negated the growth suppression seen with IL-2. These data support our hypothesis that in addition to its immunotherapeutic effects, IL-2 also acts directly on some melanoma tumors and that the IL-24 and IL-2R beta status of a tumor may be useful in predicting patient response to high dose IL-2.

Killing of human melanoma cells induced by activation of class I interferon-regulated signaling pathways via MDA-7/IL-24.

Restoration of the tumor-suppression function by gene transfer of the melanoma differentiation-associated gene 7 (MDA7)/interleukin 24 (IL-24) successfully induces apoptosis in melanoma tumors in vivo. To address the molecular mechanisms involved, we previously revealed that MDA7/IL-24 treatment of melanoma cells down-regulates interferon regulatory factor (IRF)-1 expression and concomitantly up-regulates IRF-2 expression, which competes with the activity of IRF-1 and reverses the induction of IRF-1-regulated inducible nitric oxide synthase (iNOS). Interferons (IFNs) influence melanoma cell survival by modulating apoptosis. A class I IFN (IFN-alpha) has been approved for the treatment of advanced melanoma with some limited success. A class II IFN (IFN-gamma), on the other hand, supports melanoma cell survival, possibly through constitutive activation of iNOS expression. We therefore conducted this study to explore the molecular pathways of MDA7/IL-24 regulation of apoptosis via the intracellular induction of IFNs in melanoma. We hypothesized that the restoration of the MDA7/IL-24 axis leads to upregulation of class I IFNs and induction of the apoptotic cascade. We found that MDA7/IL-24 induces the secretion of endogenous IFN-beta, another class I IFN, leading to the arrest of melanoma cell growth and apoptosis. We also identified a series of apoptotic markers that play a role in this pathway, including the regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas-FasL. In summary, we described a novel pathway of MDA7/IL-24 regulation of apoptosis in melanoma tumors via endogenous IFN-beta induction followed by IRF regulation and TRAIL/FasL system activation.

LY2109761, a novel transforming growth factor beta receptor type I and type II dual inhibitor, as a therapeutic approach to suppressing pancreatic cancer metastasis.

Most pancreatic cancer patients present with inoperable disease or develop metastases after surgery. Conventional therapies are usually ineffective in treating metastatic disease. It is evident that novel therapies remain to be developed. Transforming growth factor beta (TGF-beta) plays a key role in cancer metastasis, signaling through the TGF-beta type I/II receptors (TbetaRI/II). We hypothesized that targeting TbetaRI/II kinase activity with the novel inhibitor LY2109761 would suppress pancreatic cancer metastatic processes. The effect of LY2109761 has been evaluated on soft agar growth, migration, invasion using a fibroblast coculture model, and detachment-induced apoptosis (anoikis) by Annexin V flow cytometric analysis. The efficacy of LY2109761 on tumor growth, survival, and reduction of spontaneous metastasis have been evaluated in an orthotopic murine model of metastatic pancreatic cancer expressing both luciferase and green fluorescence proteins (L3.6pl/GLT). To determine whether pancreatic cancer cells or the cells in the liver microenvironment were involved in LY2109761-mediated reduction of liver metastasis, we used a model of experimental liver metastasis. LY2109761 significantly inhibited the L3.6pl/GLT soft agar growth, suppressed both basal and TGF-beta1-induced cell migration and invasion, and induced anoikis. In vivo, LY2109761, in combination with gemcitabine, significantly reduced the tumor burden, prolonged survival, and reduced spontaneous abdominal metastases. Results from the experimental liver metastasis models indicate an important role for targeting TbetaRI/II kinase activity on tumor and liver microenvironment cells in suppressing liver metastasis. Targeting TbetaRI/II kinase activity on pancreatic cancer cells or the cells of the liver microenvironment represents a novel therapeutic approach to prevent pancreatic cancer metastasis.

Characterization of the inflammatory cells in ascending thoracic aortic aneurysms in patients with Marfan syndrome, familial thoracic aortic aneurysms, and sporadic aneurysms.

OBJECTIVE: This study sought to characterize the inflammatory infiltrate in ascending thoracic aortic aneurysm in patients with Marfan syndrome, familial thoracic aortic aneurysm, or nonfamilial thoracic aortic aneurysm. BACKGROUND: Thoracic aortic aneurysms are associated with a pathologic lesion termed "medial degeneration," which is described as a noninflammatory lesion. Thoracic aortic aneurysms are a complication of Marfan syndrome and can be inherited in an autosomal dominant manner of familial thoracic aortic aneurysm. METHODS: Full aortic segments were collected from patients undergoing elective repair with Marfan syndrome (n = 5), familial thoracic aortic aneurysm (n = 6), and thoracic aortic aneurysms (n = 9), along with control aortas (n = 5). Immunohistochemistry staining was performed using antibodies directed against markers of lymphocytes and macrophages. Real-time polymerase chain reaction analysis was performed to quantify the expression level of the T-cell receptor beta-chain variable region gene. RESULTS: Immunohistochemistry of thoracic aortic aneurysm aortas demonstrated that the media and adventitia from Marfan syndrome, familial thoracic aortic aneurysm, and sporadic cases had increased numbers of T lymphocytes and macrophages when compared with control aortas. The number of T cells and macrophages in the aortic media of the aneurysm correlated inversely with the patient's age at the time of prophylactic surgical repair of the aorta. T-cell receptor profiling indicated a similar clonal nature of the T cells in the aortic wall in a majority of aneurysms, whether the patient had Marfan syndrome, familial thoracic aortic aneurysm, or sporadic disease. CONCLUSION: These results indicate that the infiltration of inflammatory cells contributes to the pathogenesis of thoracic aortic aneurysms. Superantigen-driven stimulation of T lymphocytes in the aortic tissues of patients with thoracic aortic aneurysms may contribute to the initial immune response.

Mutations affecting beta-tubulin folding and degradation.

Revertants of a colcemid-resistant Chinese hamster ovary cell line with an altered (D45Y) beta-tubulin have allowed the identification of four cis-acting mutations (L187R, Y398C, a 12-amino acid in-frame deletion, and a C-terminal truncation) that act by destabilizing the mutant tubulin and preventing it from incorporating into microtubules. These unstable beta-tubulins fail to form heterodimers and are predominantly found in association with the chaperonin CCT, suggesting that they cannot undergo productive folding. In agreement with these in vivo observations, we show that the defective beta-tubulins do not stably interact with cofactors involved in the tubulin folding pathway and, hence, fail to exchange with beta-tubulin in purified alphabeta heterodimers. Treatment of cells with MG132 causes an accumulation of the aberrant tubulins, indicating that improperly folded beta-tubulin is degraded by the proteasome. Rapid degradation of the mutant tubulin does not elicit compensatory changes in wild-type tubulin synthesis or assembly. Instead, loss of beta-tubulin from the mutant allele causes a 30-40% decrease in cellular tubulin content with no obvious effect on cell growth or survival.

14-3-3 proteins interact with the beta-thymosin repeat protein Csp24.

Conditioned stimulus pathway protein 24 (Csp24) is a beta-thymosin-like protein that is homologous to other members of the family of beta-thymosin repeat proteins that contain multiple actin binding domains. Actin co-precipitates with Csp24 and co-localizes with it in the cytosol of type-B photoreceptor cell bodies. Several signal transduction pathways have been shown to regulate the phosphorylation of Csp24 and contribute to cellular plasticity. Here, we report the identification of the adapter protein 14-3-3 in lysates of the Hermissenda circumesophageal nervous system and its interaction with Csp24. Immunoprecipitation experiments using an antibody that is broadly reactive with several isoforms of the 14-3-3 family of proteins showed that Csp24 co-precipitates with 14-3-3 protein, and nervous systems stimulated with 5-HT exhibited a significant increase in co-precipitated Csp24 probed with a phosphospecific antibody as compared with controls. These results indicate that post-translational modifications of Csp24 regulate its interaction with 14-3-3 protein, and suggest that this mechanism may contribute to the control of intrinsic enhanced excitability.

Molecular basis for class V beta-tubulin effects on microtubule assembly and paclitaxel resistance.

Vertebrates produce at least seven distinct beta-tubulin isotypes that coassemble into all cellular microtubules. The functional differences among these tubulin isoforms are largely unknown, but recent studies indicate that tubulin composition can affect microtubule properties and cellular microtubule-dependent behavior. One of the isotypes whose incorporation causes the largest change in microtubule assembly is beta5-tubulin. Overexpression of this isotype can almost completely destroy the microtubule network, yet it appears to be required in smaller amounts for normal mitotic progression. Moderate levels of overexpression can also confer paclitaxel resistance. Experiments using chimeric constructs and site-directed mutagenesis now indicate that the hypervariable C-terminal region of beta5 plays no role in these phenotypes. Instead, we demonstrate that two residues found in beta5 (Ser-239 and Ser-365) are each sufficient to inhibit microtubule assembly and confer paclitaxel resistance when introduced into beta1-tubulin; yet the single mutation of residue Ser-239 in beta5 eliminates its ability to confer these phenotypes. Despite the high degree of conservation among beta-tubulin isotypes, mutations affecting residue 365 demonstrate that amino acid substitutions can be context sensitive; i.e. an amino acid change in one isotype will not necessarily produce the same phenotype when introduced into a different isotype. Modeling studies indicate that residue Cys-239 of beta1-tubulin is close to a highly conserved Cys-354 residue suggesting the possibility that disulfide formation could play a significant role in the stability of microtubules formed with beta1- but not with beta5-tubulin.

Effects of gossypol on DNA replication in mammalian cells

Gossypol, a binaphthalene compound, possesses male infertility effects. However, its mechanism of action and effects on somatic cells are not yet understood. The purpose of this study was to examine the effects of gossypol on mammalian cell growth and DNA replication, using tissue culture cells (HeLa) as an in vivo model.^ Gossypol inhibited DNA synthesis in HeLa cells at low doses, without affecting RNA or protein synthesis. This caused cells to accumulate in S phase without affecting cells in other phases of the cell cycle. The inhibition of DNA synthesis was both dose- and time-dependent. This irreversible block was associated with a decrease in HeLa plating efficiency. Gossypol did bind to DNA but did not measurably affect its ability to serve as a template for DNA polymerase $\alpha$, the major replicative enzyme. Only in the absence of serum could gossypol induce single-strand DNA breaks in HeLa cells; no DNA-DNA or DNA-protein crosslinks were formed.^ Gossypol exhibited dose-dependent inhibition of a number of eukaryotic and prokaryotic replicative DNA polymerases both in vitro and in vivo. This inhibition was kinetically non-competitive with respect to the DNA template and dNTP substrates. Both a filter binding assay and polyacrylamide gel electrophoresis were used to study gossypol binding to DNA polymerase. Inhibition resulted from drug binding to two adjacent amino acid residues on the enzyme. Binding was found to be irreversible and mediated through either non-covalent interactions or by Schiff's base formation between the aldehyde groups of gossypol and the $\varepsilon$-NH$\sb2$ groups of amino acid residues on the polymerase. Structure-function studies using eleven gossypol derivatives revealed that both aldehyde and hydroxyl groups function independently to effect inhibition of DNA polymerase and DNA replication. The activities of DNA polymerase $\beta$ and ribonucleotide reductase were also inhibited by increasing gossypol concentrations.^ These studies demonstrate that the gossypol-mediated inhibition of DNA replication is due in part to inhibition of key replicative enzymes, such as DNA polymerase $\alpha$. The study of DNA polymerase may serve as a model for the interaction of enzymes with gossypol, a drug which may prove useful as a chemotherapeutic agent. ^