AB0582 KNEE OSTEOARTHRITIS PHENOTYPES STRATIFICATION

Background:

Osteoarthritis (OA) relevance is determined by its record prevalence with progredient growth throughout the world [1]. Clinical and pathogenic heterogeneity of disease actualizes problem of its stratification [2]. Lack of unified understanding of OA and its phenotype determination results in incredible number of attempts to group OA, using of different classification criteria in last decade.

Objectives:

To analyze and systematize available OA classifications, proposals and phenotypes, to highlight the most promising of them.

Methods:

We studied publications from MEDLINE / PubMed and Google Scholar databases found by the keywords “osteoarthritis”, “phenotypes”, “subphenotypes”, “classification”, “subtypes”, “subsets”, “subgroups”, “subpopulations”, “profiles” and “endotypes” in various combinations in English and Russian. We did not set a time frame, but aimed to include as many different methods as possible in order to reflect evolution of scientists’ views on structuring of this disease.

Results:

A total of 55 OA grouping methods were covered so that OA was structured by different determinants into 6 big boxes.

First OA classifications were characterized by complex etiopathogenetic approach, while subsequent studies differed in joint-mediated approach, and the knee joint was undisputed “champion” in this “race”. One of the first attempts to group OA was division into primary, or idiopathic, and secondary, due to known causes. It is now obvious that it is becoming obsolete, and criteria for OA primacy are difficult to determining. Genomic highly specialized studies based on isolation of “favorable” and “unfavorable” genes develops prerequisites to genetic OA classifying. Clinical variants occupy central place as they are the most fully consistent with modern phenotype conception [3], considerating as subtypes of disease shared by underlying pathobiological and pain mechanisms and their structural and functional consequences. Trajectories of OA progression are distinguished by longitudinal design, that is, the determinants for grouping here are disease characteristics in dynamics. The ancestor of structural OA trajectories can be considered Kellgren-Lawrence grades; subsequent studies identified complex of clinical, laboratory and morphological factors contributing to development of trajectories. Structural OA variants are diverse depending on visualization methods, and many of them can be naturally considered phenotypes, since they drive certain clinical OA manifestations. Morphological changes were described at macro- and microscopic levels; it is interesting to note the absence of histopathological norm in patients without OA. Laboratory profiles of patients are determined by content of systemic (serum, urinary) or local, “proximal” (in synovial fluid) biomarkers, which seem to be more precise. Metabolomic analysis is perspective new direction of laboratory studies based on joint metabolic products identification in the synovial fluid. New trend in OA research is molecular phenotyping. The specific molecular pathway explaining observed phenotype properties is called “endotype”. Endotype is related to certain pathobiological scenario, and laboratory markers are potentially effective for its diagnosis.

Conclusion:

Thus, a large amount of accumulated information and its diversity soon will probably lead to qualitatively new knowledge level with deep understanding of phenotype-associated strategy for managing OA patients.

References:

[1]Wallace IJ, Worthington S, Felson DT, et al. Knee osteoarthritis has doubled in prevalence since the mid-20th century. Proc Natl Acad Sci USA. 2017 Aug 29;114(35): 9332-9336. doi: 10.1073/pnas.1703856114 Epub 2017 Aug 14.

[2]Deveza LA, Nelson AE, Loeser RF. Phenotypes of osteoarthritis: current state and future implications. Clin Exp Rheumatol 37 Suppl 2019;120(5):64-72.

[3]Van Spil WE, Bierma-Zeinstra SMA, Deveza LA, et al. A consensus-based framework for conducting and reporting osteoarthritis phenotype research. Arthritis Res Ther. 2020;22(1):54. doi:10.1186/s13075-020-2143-0

Disclosure of Interests:

None declared.

BODY COMPONENT COMPOSITION AND STRESS-INDUCED FEATURES OF LEPTIN SECRETION IN FEMALES WITH VARIOUS BODY MASS INDEX

Aim. The article aims to study the dynamics of the adipose tissue hormone leptin in healthy females with different body component composition and body mass index to assess their energy status under stress. Materials and methods. Based on anthropometry and bioimpedancemetry, three groups of girls were formed: the first group included girls with a BMI of less than 18.5 kg/m2, body mass deficiency (BMD), the second – from 18.5 to 24.9 kg/m2, normal body mass (NBM), the third – with a BMI of more than 24.9 kg/m2, which corresponds to overweight (OvBM). The body component composition was determined by bioimpedancemetry. Plasma leptin concentration was evaluated by enzyme-linked immunosorbent assay both in normal conditions and during emotional stress. Results. In the selected subgroups, a significant difference in the body component composition was found with an increase in the series of BMD → NBM → OvBM. When assessing the level of metabolic processes by the magnitude of the phase angle, it was found that 3.41% of females with BMD showed signs of increased protein catabolism. The stress-induced dynamics of the hormone leptin was evaluated. In normal conditions, the concentration of leptin in blood plasma positively correlated with BMI and depended on the component composition of the body. Conclusion. In persons with BMD, the adipose tissue hormone leptin had a negative dynamics under stress conditions, which was an adequate adaptive physiological response to stress. Females with BMD had a significantly high positive dynamics of leptin, while girls with OvBM did not have significant changes in its concentrations, which can be explained by the development of leptin resistance.

Abstract P1-10-08: Development of a first-in-class oral selective ERα covalent antagonist (SERCA) for the treatment of ERαWT and ERαMUT breast cancer

Mutations in estrogen receptor alpha (ERα) are detected in up to 30% of breast cancer patients who have relapsed during endocrine therapy. ERα mutations functionally confer resistance to existing classes of endocrine therapies, likely through gaining constitutive activity. The fact that current ER-directed therapies are only partially effective in the ERα mutant setting, and that a significant proportion of resistant breast cancer metastases continue to remain dependent on ERα signaling for growth/survival, highlights the critical need to develop the next generation of ERα antagonists that can overcome aberrant ERα activity. Using structure-based drug design approaches we have identified a novel class of ERα antagonist referred to as Selective ERα Covalent Antagonist (SERCA) that inactivate both wild-type and mutant ERα by targeting a unique cysteine residue that is not conserved among other steroid hormone receptors. Biophysical, biochemical and cellular analyses confirm the covalent mechanism of action, specific binding to ER and selective inhibition of ERα-dependent transcription of SERCAs. H3B-6545 is a highly selective SERCA that potently antagonizes wild-type and mutant ERα in biochemical and cell based assays demonstrating increased potency over standard of care and other experimental agents. In vivo, H3B-6545 shows superior efficacy to fulvestrant in the MCF-7 xenograft model with once daily oral dosing, achieving maximal antitumor activity at doses >10x below the maximum tolerated dose in mice. In addition, H3B-6545 shows superior antitumor activity to both tamoxifen and fulvestrant in patient derived xenograft models of breast cancer carrying estrogen receptor mutations. In summary, H3B-6545 is a first-in-class, orally available and selective ER covalent antagonist with a compelling pre-clinical profile that is being developed for the treatment of ERα positive breast cancer.

Citation Format: Korpal M, Puyang X, Furman C, Zheng GZ, Banka D, Wu J, Zhang Z, Thomas M, Mackenzie C, Yao H, Rimkunas V, Kumar P, Caleb B, Karr C, Subramanian V, Irwin S, Larsen N, Vaillancourt F, Nguyen T-V, Davis A, Chan B, Hao MH, O'Shea M, Prajapati S, Agoulnik S, Kuznetsov G, Kumar N, Yu Y, Lai G, Hart A, Eckley S, Fekkes P, Bowser T, Joshi JJ, Selvaraj A, Wardell S, Norris J, Smith S, Reynolds D, Mitchell L, Wang J, Yu L, Kim A, Rioux N, Sahmoud T, Warmuth M, Smith PG, Zhu P. Development of a first-in-class oral selective ERα covalent antagonist (SERCA) for the treatment of ERαWT and ERαMUT breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-10-08.

Eribulin mesilate suppresses experimental metastasis of breast cancer cells by reversing phenotype from epithelial-mesenchymal transition (EMT) to mesenchymal-epithelial transition (MET) states

Background:

Eribulin mesilate (eribulin), a non-taxane microtubule dynamics inhibitor, has shown trends towards greater overall survival (OS) compared with progression-free survival in late-stage metastatic breast cancer patients in the clinic. This finding suggests that eribulin may have additional, previously unrecognised antitumour mechanisms beyond its established antimitotic activity. To investigate this possibility, eribulin's effects on the balance between epithelial–mesenchymal transition (EMT) and mesenchymal–epithelial transition (MET) in human breast cancer cells were investigated.

Methods:

Triple negative breast cancer (TNBC) cells, which are oestrogen receptor (ER−)/progesterone receptor (PR−)/human epithelial growth receptor 2 (HER2−) and have a mesenchymal phenotype, were treated with eribulin for 7 days, followed by measurement of EMT-related gene and protein expression changes in the surviving cells by quantitative real-time PCR (qPCR) and immunoblot, respectively. In addition, proliferation, migration, and invasion assays were also conducted in eribulin-treated cells. To investigate the effects of eribulin on TGF-β/Smad signalling, the phosphorylation status of Smad proteins was analysed. In vivo, the EMT/MET status of TNBC xenografts in mice treated with eribulin was examined by qPCR, immunoblot, and immunohistochemical analysis. Finally, an experimental lung metastasis model was utilised to gauge the metastatic activity of eribulin-treated TNBC in the in vivo setting.

Results:

Treatment of TNBC cells with eribulin in vitro led to morphological changes consistent with transition from a mesenchymal to an epithelial phenotype. Expression analyses of EMT markers showed that eribulin treatment led to decreased expression of several mesenchymal marker genes, together with increased expression of several epithelial markers. In the TGF-β induced EMT model, eribulin treatment reversed EMT, coincident with inhibition of Smad2 and Smad3 phosphorylation. Consistent with these changes, TNBC cells treated with eribulin for 7 days showed decreased capacity for in vitro migration and invasiveness. In in vivo xenograft models, eribulin treatment reversed EMT and induced MET as assessed by qPCR, immunoblot, and immunohistochemical analyses of epithelial and mesenchymal marker proteins. Finally, surviving TNBC cells pretreated in vitro with eribulin for 7 days led to decreased numbers of lung metastasis when assessed in an in vivo experimental metastasis model.

Conclusions:

Eribulin exerted significant effects on EMT/MET-related pathway components in human breast cancer cells in vitro and in vivo, consistent with a phenotypic switch from mesenchymal to epithelial states, and corresponding to observed decreases in migration and invasiveness in vitro as well as experimental metastasis in vivo. These preclinical findings may provide a plausible scientific basis for clinical observations of prolonged OS by suppression of further spread of metastasis in breast cancer patients treated with eribulin.

Ion optics of RHIC electron beam ion source

RHIC electron beam ion source has been commissioned to operate as a versatile ion source on RHIC injection facility supplying ion species from He to Au for Booster. Except for light gaseous elements RHIC EBIS employs ion injection from several external primary ion sources. With electrostatic optics fast switching from one ion species to another can be done on a pulse to pulse mode. The design of an ion optical structure and the results of simulations for different ion species are presented. In the choice of optical elements special attention was paid to spherical aberrations for high-current space charge dominated ion beams. The combination of a gridded lens and a magnet lens in LEBT provides flexibility of optical control for a wide range of ion species to satisfy acceptance parameters of RFQ. The results of ion transmission measurements are presented.

Collimation with hollow electron beams

A novel concept of controlled halo removal for intense high-energy beams in storage rings and colliders is presented. It is based on the interaction of the circulating beam with a 5-keV, magnetically confined, pulsed hollow electron beam in a 2-m-long section of the ring. The electrons enclose the circulating beam, kicking halo particles transversely and leaving the beam core unperturbed. By acting as a tunable diffusion enhancer and not as a hard aperture limitation, the hollow electron beam collimator extends conventional collimation systems beyond the intensity limits imposed by tolerable losses. The concept was tested experimentally at the Fermilab Tevatron proton-antiproton collider. The first results on the collimation of 980-GeV antiprotons are presented.

Experimental demonstration of colliding-beam-lifetime improvement by electron lenses

We report the successful application of space-charge forces of a low-energy electron beam for improvement of particle lifetime determined by beam-beam interaction at a high-energy collider. In our experiments, an electron lens, a novel instrument developed for the beam-beam compensation, was set on a 980-GeV proton bunch at the Fermilab Tevatron proton-antiproton collider. The proton-bunch losses due to its interaction with the antiproton beam were reduced by a factor of 2 when the electron lens was operating. We describe the principle of electron lens operation and present experimental results.

In vitro and in vivo anticancer activities of synthetic macrocyclic ketone analogues of halichondrin B

Halichondrin B is a highly potent anticancer agent originally found in marine sponges. Although scarcity of the natural product has hampered efforts to develop halichondrin B as a new anticancer drug, the existence of a complete synthetic route has allowed synthesis of structurally simpler analogues that retain the remarkable potency of the parent compound. In this study, we show that two macrocyclic ketone analogues of halichondrir B, ER-076349 and ER-086526, have sub-nM growth inhibitory activities in vitro against numerous human cancer cell lines as well as marked in vivo activities at 0.1-1 mg/kg against four human xenografts: MDA-MB-435 breast cancer, COLO 205 colon cancer, LOX melanoma, and NIH: OVCAR-3 ovarian cancer. ER-076349 and ER-086526 induce G2-M cell cycle arrest and disruption of mitotic spindles, consistent with the tubulin-based antimitotic mechanism of halichondrin B. This is supported further by direct binding of the biotinylated analogue ER-040798 to tubulin and inhibition of tubulin polymerization in vitro by ER-076349 and ER-086526. Retention of the extraordinary in vitro and in vivo activity off halichondrin B in structurally simplified, fully synthetic analogues establishes the feasibility of developing halichondrin B-based agents as highly effective, novel anticancer drugs.

Multiple molecular chaperones complex with misfolded large oligomeric glycoproteins in the endoplasmic reticulum

Thyroglobulin (Tg), the major protein secreted by thyroid epithelial cells and precursor of thyroid hormones, is a large dimeric glycoprotein with multiple disulfide bonds. The folding and assembly of this complex molecule begins in the endoplasmic reticulum (ER) and is likely to involve a variety of reactions catalyzed by molecular chaperones (Kuznetsov, G., Chen, L. B., and Nigam, S. K. (1994) J. Biol. Chem. 269, 22990-22995). By coimmunoprecipitation in rat thyroid cells, we were able to demonstrate that BiP, grp94, ERp72, and grp170, four proteins believed to function as specific molecular chaperones, complex with Tg during its maturation. The same complex of the four putative chaperones with Tg was observed in cells treated with tunicamycin, indicating that these four ER chaperones stably associate with Tg when it is misfolded/misassembled due to inhibition of its glycosylation. BiP, grp94, and ERp72 were also found to associate with Tg in cells in which misfolding was induced by perturbing ER calcium stores. To determine if the assembly of a complex between the four chaperones and Tg under conditions of misglycosylation was unique to the maturation of this particular secretory protein or a more general phenomenon, adenovirus-transformed rat thyroid cells that do not synthesize Tg were analyzed. In these transformed cells, the only protein these same four chaperones were found to complex with was a protein of approximately 200 kDa. This protein was subsequently identified as thrombospondin, which, like Tg, is a large oligomeric secreted glycoprotein with multiple disulfide bonds. We therefore propose that these ER chaperones complex together with a variety of large oligomeric secretory glycoproteins as they fold and assemble in the ER.

Perturbations in maturation of secretory proteins and their association with endoplasmic reticulum chaperones in a cell culture model for epithelial ischemia

The effects of ischemia on the maturation of secretory proteins are not well understood. Among several events that occur during ischemia-reperfusion are a rapid and extensive decrease in ATP levels and an alteration of cellular oxidative state. Since the normal folding and assembly of secretory proteins are mediated by endoplasmic reticulum (ER) molecular chaperones, the function of which depends on ATP and maintenance of an appropriate redox environment, ischemia might be expected to perturb folding of secretory proteins. In this study, whole animal and cultured cell models for the epithelial ischemic state were used to examine this possibility. After acute kidney ischemia, marked increases in the mRNA levels of the ER chaperones glucose-regulated protein (grp)78/immunoglobulin-binding protein (BiP), grp94, and ER protein (ERp)72 were noted. Likewise, when cellular ATP was depleted to less than 10% of control with antimycin A, mRNA levels of BiP, ERp72, and grp94 were increased in kidney and thyroid epithelial cell culture models. Since the signal for the up-regulation of these stress proteins is believed to be the accumulation of misfolded/misassembled secretory proteins in the ER, their induction after ischemia in vivo and antimycin treatment of cultured cells suggests that maturation of secretory proteins in the ER lumen might indeed be perturbed. To analyze the effects of antimycin A on the maturation of secretory proteins, we studied the fate of thyroglobulin (Tg), a large oligomeric secretory glycoprotein, the folding and assembly of which seems to require a variety of ER chaperones. Treatment of cultured thyroid epithelial cells with antimycin A greatly inhibited ( > 90%) the secretion of Tg. Sucrose density gradient analysis revealed that in antimycin A-treated cells Tg associates into large macromolecular complexes which, by immunofluorescence, appeared to localize to the ER. Furthermore, coimmunoprecipitation studies after antimycin A treatment demonstrated that Tg stably associates with BiP, grp94, and ERp72. Together, our results suggest that a key cellular lesion in ischemia is the misfolding of secretory proteins as they transit the ER, and this leads not only to increased expression of ER chaperones but also to their stable association with and the subsequent retention of at least some misfolded secretory proteins.

Several endoplasmic reticulum stress proteins, including ERp72, interact with thyroglobulin during its maturation

We have previously demonstrated that several endoplasmic reticulum (ER) proteins, including BiP, ERp72, grp94, and protein disulfide isomerase, bind to a denatured thyroglobulin (Tg) affinity column and can be specifically eluted by ATP (Nigam, S.K., Goldberg, A.L., Ho, S., Rohde, M.F., Bush, K.T., and Sherman, M.Y. (1994) J. Biol. Chem. 269, 1744-1749). Using chemical cross-linking, we now demonstrate that BiP, ERp72, and grp94 associate with Tg in two types of cultured thyroid cells, FRTL-5 and PCC13. Whereas BiP could be coimmunoprecipitated with anti-Tg antibodies in the absence of cross-linking, only trace amounts of ERp72 and grp94 were coimmunoprecipitated. Likewise, in both cell types, anti-BiP antibodies were able to coimmunoprecipitate Tg in the absence of cross-linking, though ERp72 and grp94 were only minimally present. Coprecipitation of BiP and Tg was abolished when ATP and Mg2+ were added to cell lysates. In contrast, after cross-linking, there was a large increase in the amount of ERp72 and grp94 that coimmunoprecipitated with anti-Tg antibodies, although there was only a slight increase in BiP. Similarly, in cross-linked lysates, grp94 and ERp72 were also coimmunoprecipitated with anti-BiP antibodies. An apparently novel 200-kDa protein was also consistently immunoprecipitated by anti-BiP antibodies in both cell types. In addition, anti-ERp72 antibodies coimmunoprecipitated Tg, BiP, and grp94 only after cross-linking. Analysis of uncross-linked and cross-linked samples by sucrose density gradient centrifugation confirmed that Tg, BiP, grp94, and ERp72 are present together in high molecular weight complexes only after treatment of cells with cross-linking reagent. These results suggest that ERp72, as well as BiP and grp94, function as molecular chaperones in the maturation of Tg, potentially as part of a macromolecular complex.

Role of endoplasmic reticular calcium in oligosaccharide processing of alpha 1-antitrypsin

Mobilization of Ca2+ from the endoplasmic reticulum (ER) suppresses translational initiation and inhibits post-translational processing and secretion of glycoproteins. This study explores the mechanism whereby ionomycin, a Ca2+ ionophore, and thapsigargin, an ER Ca(2+)-ATPase inhibitor, promote retention of alpha 1-antitrypsin (alpha 1-AT) bearing high mannose, endoglycosidase H (Endo H)-sensitive oligosaccharide side chains within the ER of HepG2 cells. Arrest occurred at the removal of mannose residues such that intermediates with Man7-9GlcNAc2 side chains accumulated with the Man8-9GlcNAc2 structures predominating. Maturation of alpha 1-AT bearing Man5-6GlcNAc2 side chains was unaffected. Inhibition of alpha 1-AT processing by ionomycin occurred independently of translational suppression. Forms of alpha 1-AT identical to those retained with ionomycin or thapsigargin were observed upon treatment with the alpha-1,2-mannosidase inhibitor 1-deoxymannojirimycin whereas castanospermine, an inhibitor of ER alpha-glucosidase I, produced different forms of the glycoprotein. Neither inhibitor impaired transport or secretion of alpha 1-AT. With brefeldin A, which causes redistribution of Golgi enzymes to the ER, alpha 1-AT was retained intracellularly but acquired resistance to Endo H. With ionomycin, thapsigargin, or 1-deoxymannojirimycin-treated cells, however, brefeldin A failed to promote further processing of the glycoprotein. Possible mechanisms for the suppression of alpha 1-AT processing at the alpha-1,2-mannosidase step by Ca(2+)-mobilizing agents are discussed. Excepting tunicamycin, traditional inhibitors of protein processing did not affect amino acid incorporation.

Inhibition of protein synthesis and early protein processing by thapsigargin in cultured cells

Thapsigargin, a tumour-promoting sesquiterpene lactone, selectively inhibits the Ca(2+)-ATPase responsible for Ca2+ accumulation by the endoplasmic reticulum (ER). Mobilization of ER-sequestered Ca2+ to the cytosol and to the extracellular fluid subsequently ensues, with concomitant alteration of cellular functions. Thapsigargin was found to serve as a rapid, potent and efficacious inhibitor of amino acid incorporation in cultured mammalian cells. At concentrations mobilizing cell-associated Ca2+ to the extracellular fluid, thapsigargin provoked extensive inhibition of protein synthesis within 10 min. The inhibition in GH3 pituitary cells involved the synthesis of almost all polypeptides, was not associated with increased cytosolic free Ca2+ concentration ([Ca2+]i), and was not reversed at high extracellular Ca2+. The transient rise in [Ca2+]i triggered by ionomycin was diminished by thapsigargin. Polysomes failed to accumulate in the presence of the drug, indicative of impaired translational initiation. With longer (1-3 h) exposures to thapsigargin, recovery of translational activity was observed accompanied by increased synthesis of the ER protein glucose-regulated stress protein 78 or immunoglobulin heavy-chain binding protein ('GRP78/BiP') and its mRNA. Such inductions were comparable with those observed previously with Ca2+ ionophores which mobilize the cation from all intracellular sequestered sites. Actin mRNA concentrations declined significantly during such treatments. In HepG2 cells processing and secretion of the glycoprotein alpha 1-antitrypsin were rapidly suppressed by thapsigargin. Ca2+ sequestered specifically by the ER is concluded to be essential for optimal protein synthesis and processing. These rapid effects of thapsigargin on mRNA translation, protein processing and gene expression should be considered when evaluating potential mechanisms by which this tumour promoter influences cellular events.

Demonstration of a calcium requirement for secretory protein processing and export. Differential effects of calcium and dithiothreitol

HepG2 cells were employed as model system to investigate potential relationships between early protein processing and Ca2+ storage by the endoplasmic reticulum. Ca2+ was required for glycoprotein processing and export by intact cells. The processing and export of alpha 1-antitrypsin and the secretion of complement factor 3, which are glycosylated proteins, were inhibited by the Ca2+ ionophore ionomycin whereas the export of albumin, a non-glycoprotein, was little affected. Ionomycin blocked processing of alpha 1-antitrypsin at the conversion from the high mannose to the complex glycosylated form without affecting ATP or GTP contents. Pre-existing inhibition of intracellular processing of alpha 1-antitrypsin by ionomycin was fully reversible upon removal of the ionophore with fatty acid-free bovine serum albumin. This reversal required Ca2+. After reversal the arrested form of alpha 1-antitrypsin was fully converted to the mature form and exported to the medium. Inhibitions of alpha 1-antitrypsin processing and complement factor 3 secretion by the metalloendoprotease antagonist Cbz-Gly-Phe-NH2 (where Cbz is benzyloxycarbonyl) were strongest at low extracellular Ca2+ but were reduced or prevented by high extracellular Ca2+. Processing and secretion of alpha 1-antitrypsin were reduced upon incubation in low Ca2+ medium. Exposure to dithiothreitol reduced albumin export while affecting alpha 1-antitrypsin export minimally. Suppression of amino acid incorporation into total cellular proteins of HepG2 cells accompanied inhibitions of protein processing by agents depleting sequestered Ca2+ stores or by dithiothreitol. Putative control of rates of translational initiation by the endoplasmic reticulum through linkage to rates of early protein processing is discussed.

Inhibition of translational initiation by metalloendoprotease antagonists. Evidence for involvement of sequestered Ca2+ stores

Synthetic oligopeptide inhibitors of metalloendoprotease activity have been shown to block membrane fusion events, to slow transport of secretory proteins from the endoplasmic reticulum (ER) to the Golgi, and to perturb Ca2+ homeostasis. Effects of such agents on translational activity, which requires Ca2+ sequestered putatively within the ER, were examined in this study. Cbz-Gly-Phe-NH2 (where Cbz is benzyloxycarbonyl) provoked rapid inhibition of amino acid incorporation into a broad spectrum of proteins in GH3 pituitary, C6 glial, and Neuro-2a cells but not in reticulocytes, which lack ER. Polysome accumulation and incorporation were reduced concurrently, indicating that the dipeptide acted to slow translational initiation. Inhibitions were largest at low extracellular Ca2+, were reversed by increasing extracellular Ca2+, were comparable to those achieved in the presence of EGTA or Ca2+ ionophores, and were observed with assorted metalloendoprotease antagonists but not with leupeptin. At concentrations inhibitory to protein synthesis Cbz-Gly-Phe-NH2 mobilized cell-associated 45Ca, lowered cytosolic free Ca2+, and did not generate inositol phosphates. Cells treated for 3-4 h with Cbz-Gly-Phe-NH2 reacquired the ability to synthesize proteins at nearly normal rates; a phorbol ester or cAMP-elevating agent was necessary for such recovery in GH3, but not C6 or Neuro-2a, cells. GRP78, which may function in the folding and assembly of secretory proteins and in translational accommodation to agents that deplete sequestered Ca2+ stores, was induced during such treatments. Accumulation of GRP78 mRNA in treated preparations was reduced as extracellular Ca2+ was increased. Extended exposure to dipeptide followed by brief recovery in its absence rendered protein synthesis resistant to inhibition by Ca2+ ionophore. It is concluded that metalloendoprotease antagonists suppress translational initiation as a consequence of their capacity to mobilize sequestered Ca2+ stores.