High-throughput screening of small molecules in miniaturized mammalian cell-based assays involving post-translational modifications.
BACKGROUND: Fully adapting a forward genetic approach to mammalian systems requires efficient methods to alter systematically gene products without prior knowledge of gene sequences, while allowing for the subsequent characterization of these alterations. Ideally, these methods would also allow function to be altered in a temporally controlled manner. RESULTS: We report the development of a miniaturized cell-based assay format that enables a genetic-like approach to understanding cellular pathways in mammalian systems using small molecules, rather than mutations, as the source of gene-product alterations. This whole-cell immunodetection assay can sensitively detect changes in specific cellular macromolecules in high-density arrays of mammalian cells. Furthermore, it is compatible with screening large numbers of small molecules in nanoliter to microliter culture volumes. We refer to this assay format as a 'cytoblot', and demonstrate the use of cytoblotting to monitor biosynthetic processes such as DNA synthesis, and post-translational processes such as acetylation and phosphorylation. Finally, we demonstrate the applicability of these assays to natural-product screening through the identification of marine sponge extracts exhibiting genotype-specific inhibition of 5-bromodeoxyuridine incorporation and suppression of the anti-proliferative effect of rapamycin. CONCLUSIONS: We show that cytoblots can be used for high-throughput screening of small molecules in cell-based assays. Together with small-molecule libraries, the cytoblot assay can be used to perform chemical genetic screens analogous to those used in classical genetics and thus should be applicable to understanding a wide variety of cellular processes, especially those involving post-transitional modifications. (+info
Rapamycin causes poorly reversible inhibition of mTOR and induces p53-independent apoptosis in human rhabdomyosarcoma cells.
The mammalian target of rapamycin (mTOR) has been shown to link growth factor signaling and posttranscriptional control of translation of proteins that are frequently involved in cell cycle progression. However, the role of this pathway in cell survival has not been demonstrated. Here, we report that rapamycin, a specific inhibitor of mTOR kinase, induces G1 cell cycle arrest and apoptosis in two rhabdomyosarcoma cell lines (Rh1 and Rh30) under conditions of autocrine cell growth. To examine the kinetics of rapamycin action, we next determined the rapamycin sensitivity of rhabdomyosarcoma cells exposed briefly (1 h) or continuously (6 days). Results demonstrate that Rh1 and Rh30 cells were equally sensitive to rapamycin-induced growth arrest and apoptosis under either condition. Apoptosis was detected between 24 and 144 h of exposure to rapamycin. Both cell lines have mutant p53; hence, rapamycin-induced apoptosis appears to be a p53-independent process. To determine whether induction of apoptosis by rapamycin was specifically due to inhibition of mTOR signaling, we engineered Rh1 and Rh30 clones to stably express a mutant form of mTOR that was resistant to rapamycin (Ser2035-->Ile; designated mTOR-rr). Rh1 and Rh30 mTOR-rr clones were highly resistant (>3000-fold) to both growth inhibition and apoptosis induced by rapamycin. These results are the first to indicate that rapamycin-induced apoptosis is mediated by inhibition of mTOR. Exogenous insulin-like growth factor (IGF)-I protected both Rh1 and Rh30 from apoptosis, without reactivating ribosomal p70 S6 kinase (p70S6K) downstream of mTOR. However, in rapamycin-treated cultures, the response to IGF-I differed between the cell lines: Rh1 cells proliferated normally, whereas Rh30 cells remained arrested in G1 phase but viable. Rapamycin is known to inhibit synthesis of specific proteins but did not inhibit synthesis or alter the levels of mTOR. To examine the rate at which the mTOR pathway recovered, the ability of IGF-I to stimulate p70S6K activity was followed in cells treated for 1 h with rapamycin and then allowed to recover in medium containing > or =100-fold excess of FK506 (to prevent rapamycin from rebinding to its cytosolic receptor FKBP-12). Our results indicate that, in Rh1 cells, rapamycin dissociates relatively slowly from FKBP-12, with a t1/2 of approximately 17.5 h. in the presence of FK506, whereas there was no recovery of p70S6K activity in the absence of this competitor. This was of interest because rapamycin was relatively unstable under conditions of cell culture having a biological t1/2 of approximately 9.9 h. These results help to explain why cells are sensitive following short exposures to rapamycin and may be useful in guiding the use of rapamycin analogues that are entering clinical trials as novel antitumor agents. (+info
Hmo1p, a high mobility group 1/2 homolog, genetically and physically interacts with the yeast FKBP12 prolyl isomerase.
The immunosuppressive drugs FK506 and rapamycin bind to the cellular protein FKBP12, and the resulting FKBP12-drug complexes inhibit signal transduction. FKBP12 is a ubiquitous, highly conserved, abundant enzyme that catalyzes a rate-limiting step in protein folding: peptidyl-prolyl cis-trans isomerization. However, FKBP12 is dispensible for viability in both yeast and mice, and therefore does not play an essential role in protein folding. The functions of FKBP12 may involve interactions with a number of partner proteins, and a few proteins that interact with FKBP12 in the absence of FK506 or rapamycin have been identified, including the ryanodine receptor, aspartokinase, and the type II TGF-beta receptor; however, none of these are conserved from yeast to humans. To identify other targets and functions of FKBP12, we have screened for mutations that are synthetically lethal with an FKBP12 mutation in yeast. We find that mutations in HMO1, which encodes a high mobility group 1/2 homolog, are synthetically lethal with mutations in the yeast FPR1 gene encoding FKBP12. Deltahmo1 and Deltafpr1 mutants share two phenotypes: an increased rate of plasmid loss and slow growth. In addition, Hmo1p and FKBP12 physically interact in FKBP12 affinity chromatography experiments, and two-hybrid experiments suggest that FKBP12 regulates Hmo1p-Hmo1p or Hmo1p-DNA interactions. Because HMG1/2 proteins are conserved from yeast to humans, our findings suggest that FKBP12-HMG1/2 interactions could represent the first conserved function of FKBP12 other than mediating FK506 and rapamycin actions. (+info
Interaction of asparagine and EGF in the regulation of ornithine decarboxylase in IEC-6 cells.
Our laboratory has shown that asparagine (ASN) stimulates both ornithine decarboxylase (ODC) activity and gene expression in an intestinal epithelial cell line (IEC-6). The effect of ASN is specific, and other A- and N-system amino acids are almost as effective as ASN when added alone. In the present study, epidermal growth factor (EGF) was unable to increase ODC activity in cells maintained in a salt-glucose solution (Earle's balanced salt solution). However, the addition of ASN (10 mM) in the presence of EGF (30 ng/ml) increased the activity of ODC 0.5- to 4-fold over that stimulated by ASN alone. EGF also showed induction of ODC with glutamine and alpha-aminoisobutyric acid, but ODC induction was maximum with ASN and EGF. Thus the mechanism of the interaction between ASN and EGF is important for understanding the regulation of ODC under physiological conditions. Therefore, we examined the expression of the ODC gene and those for several protooncogenes under the same conditions. Increased expression of the genes for c-Jun and c-Fos but not for ODC occurred with EGF alone. The addition of ASN did not further increase the expression of the protooncogenes, but the combination of EGF and ASN further increased the expression of ODC over that of ASN alone. Western analysis showed no significant difference in the level of ODC protein in Earle's balanced salt solution, ASN, EGF, or EGF plus ASN. Addition of cycloheximide during ASN and ASN plus EGF treatment completely inhibited ODC activity without affecting the level of ODC protein. These results indicated that 1) the increased expression of protooncogenes in response to EGF is independent of increases in ODC activity and 2) potentiation between EGF and ASN on ODC activity may not be due to increased gene transcription but to posttranslational regulation and the requirement of ongoing protein synthesis involving a specific factor dependent on ASN. (+info
Cyclosporin A treatment alters characteristics of Ca2+-release channel in cardiac sarcoplasmic reticulum.
Chronic treatment with cyclosporin A (CsA) has been reported (H. S. Banijamali, M. H. ter Keurs, L. C. Paul, and H. E. ter Keurs. Cardiovasc. Res. 27: 1845-1854, 1993; I. Kingma, E. Harmsen, H. E. ter Keurs, H. Benediktsson, and L. C. Paul. Int. J. Cardiol. 31: 15-22, 1991) to induce reversible alterations of contractile properties in rat hearts. To define the molecular mechanisms underlying the physiological alterations, the Ca2+-release channel (CRC) and Ca2+-ATPase from sarcoplasmic reticulum in rats were examined. Ryanodine binding to whole homogenates of rat hearts shows time- and dose-dependent alterations in CRC properties by CsA. On 3 wk of treatment with 15 mg CsA. kg body wt-1. day-1, 1) maximal ryanodine binding (Bmax) decreased, 2) the dissociation constant of ryanodine (Kd) increased, 3) caffeine sensitivity of CRC increased, and 4) ruthenium red sensitivity of CRC decreased. On the other hand, Bmax and Kd of ryanodine binding in rat skeletal muscles were not changed. Ryanodine-sensitive oxalate-supported Ca2+ uptake in whole homogenates was lower in CsA-treated rat hearts than in control hearts, whereas total Ca2+ uptake in the presence of 500 M ryanodine was not changed. Functional experiments with rapamycin and Western blot analysis suggest that the CsA-induced alteration of ryanodine binding is due at least in part to an upregulation of calcineurin. The heart muscle-specific alterations of CRC could be responsible for the previously reported contractile changes of CsA-treated rat hearts. (+info
Inhibition of cell cycle progression by rapamycin induces T cell clonal anergy even in the presence of costimulation.
Costimulation (signal 2) has been proposed to inhibit the induction of T cell clonal anergy by either directly antagonizing negative signals arising from TCR engagement (signal 1) or by synergizing with signal 1 to produce IL-2, which in turn leads to proliferation and dilution of negative regulatory factors. To better define the cellular events that lead to the induction of anergy, we used the immunosuppressive agent rapamycin, which blocks T cell proliferation in late G1 phase but does not affect costimulation-dependent IL-2 production. Our data demonstrate that full T cell activation (signal 1 plus 2) in the presence of rapamycin results in profound T cell anergy, despite the fact that these cells produce copious amounts of IL-2. Similar to conventional anergy (induction by signal 1 alone), the rapamycin-induced anergic cells show a decrease in mitogen-activated protein kinase activation, and these cells can be rescued by culture in IL-2. Interestingly, the rapamycin-induced anergic cells display a more profound block in IL-3 and IFN-gamma production upon rechallenge. Finally, in contrast to rapamycin, full T cell activation in the presence of hydroxyurea (which inhibits the cell cycle in early S phase) did not result in anergy. These data suggest that it is neither the direct effect of costimulation nor the subsequent T cell proliferation that prevents anergy induction, but rather the biochemical events that occur upon progression through the cell cycle from G1 into S phase. (+info
p70(S6K) controls selective mRNA translation during oocyte maturation and early embryogenesis in Xenopus laevis.
In mammalian cells, p70(S6K) plays a key role in translational control of cell proliferation in response to growth factors. Because of the reliance on translational control in early vertebrate development, we cloned a Xenopus homolog of p70(S6K) and investigated the activity profile of p70(S6K) during Xenopus oocyte maturation and early embryogenesis. p70(S6K) activity is high in resting oocytes and decreases to background levels upon stimulation of maturation with progesterone. During embryonic development, three peaks of activity were observed: immediately after fertilization, shortly before the midblastula transition, and during gastrulation. Rapamycin, an inhibitor of p70(S6K) activation, caused oocytes to undergo germinal vesicle breakdown earlier than control oocytes, and sensitivity to progesterone was increased. Injection of a rapamycin-insensitive, constitutively active mutant of p70(S6K) reversed the effects of rapamycin. However, increases in S6 phosphorylation were not significantly affected by rapamycin during maturation. mos mRNA, which does not contain a 5'-terminal oligopyrimidine tract (5'-TOP), was translated earlier, and a larger amount of Mos protein was produced in rapamycin-treated oocytes. In fertilized eggs rapamycin treatment increased the translation of the Cdc25A phosphatase, which lacks a 5'-TOP. Translation assays in vivo using both DNA and RNA reporter constructs with the 5'-TOP from elongation factor 2 showed decreased translational activity with rapamycin, whereas constructs without a 5'-TOP or with an internal ribosome entry site were translated more efficiently upon rapamycin treatment. These results suggest that changes in p70(S6K) activity during oocyte maturation and early embryogenesis selectively alter the translational capacity available for mRNAs lacking a 5'-TOP region. (+info
Growth hormone-dependent differentiation of 3T3-F442A preadipocytes requires Janus kinase/signal transducer and activator of transcription but not mitogen-activated protein kinase or p70 S6 kinase signaling.
The signals mediating growth hormone (GH)-dependent differentiation of 3T3-F442A preadipocytes under serum-free conditions have been studied. GH priming of cells was required before the induction of terminal differentiation by a combination of epidermal growth factor, tri-iodothyronine, and insulin. Cellular depletion of Janus kinase-2 (JAK-2) using antisense oligodeoxynucleotides (ODNs) prevented GH-stimulated JAK-2 and signal transducer and activator of transcription (STAT)-5 tyrosine phosphorylation and severely attenuated the ability of GH to promote differentiation. Although p42(MAPK)/p44(MAPK) mitogen-activated protein kinases were activated during GH priming, treatment of cells with PD 098059, which prevented activation of these kinases, did not block GH priming. However, antisense ODN-mediated depletion of mitogen-activated protein kinases from the cells showed that their expression was necessary for terminal differentiation. Similarly, although p70(s6k) was activated during GH priming, pretreatment of cells with rapamycin, which prevented the activation of p70(s6k), had no effect on GH priming. However, rapamycin did partially block epidermal growth factor, tri-iodothyronine, and insulin-stimulated terminal differentiation. By contrast, cellular depletion of STAT-5 with antisense ODNs completely abolished the ability of GH to promote differentiation. These results indicate that JAK-2, acting specifically via STAT-5, is necessary for GH-dependent differentiation of 3T3-F442A preadipocytes. Activation of p42(MAPK)/p44(MAPK) and p70(s6k) is not essential for the promotion of differentiation by GH, although these signals are required for GH-independent terminal differentiation. (+info