A peptide from the GAP-binding domain of the ras-p21 protein as well as azatyrosine block ras-induced maturation of Xenopus oocytes

Lysyl oxidase regulates MMTV promoter: indirect evidence of histone H1 involvement

Lysyl oxidase (LOX) is the enzyme that facilitates the cross-linking of collagen and elastin, although other functions for this enzyme have been indicated. Of these other functions, we describe herein the ability of LOX to regulate several gene promoters, like collagen III, elastin, and cyclin D1. We have previously demonstrated a specific binding between LOX and histone H1, in vitro. Therefore, we investigated whether LOX would affect the mouse mammary tumor virus (MMTV) promoter and its glucocorticoid regulation, which depends on the phophorylation status of histone H1. Our results show that the over-expression of recombinant human LOX was able to trigger MMTV activity, both in the presence and absence of glucocorticoids. Moreover, we demonstrated that histone H1 from cells expressing recombinant LOX contained isodesmosine and desmosine, indicating specific lysyl-oxidase-dependent lysine modifications. Finally, we were able to co-immunoprecipitate the exogenous LOX and histone H1 from the LOX transfected cells. The data are compatible with a decreased positive charge of histone H1, owing to deamination by LOX of its lysine residues. This event would favor H1 detachment from the target DNA, and consequent opening of the MMTV promoter structure to the activating transcription factors. The presented data, therefore, suggest a possible histone-H1-dependent mechanism for the modulation of MMTV promoter by LOX.

Comparative effects of insulin on the activation of the Raf/Mos-dependent MAP kinase cascade in vitellogenic versus postvitellogenic Xenopus oocytes

Xenopus postvitellogenic oocytes resume meiosis in vitro upon exposure to insulin or insulin-like growth factor 1 (IGF-1) via a ras-dependent pathway, whereas stage IV (600 micron < diameter < 1000 micron) oocytes cannot. The aim of the present study was to determine which event(s) of the transduction pathway from IGF-1 receptor to maturation-promoting factor (MPF) activation is deficient in the small, vitellogenic, oocytes to explain their inability to undergo germinal vesicle breakdown (GVB) after insulin treatment. We thus analyzed the effect of insulin on the Ras/Raf-dependent mitogen-activated protein kinase cascade because of its crucial role prior to MPF activation. The effect of insulin on pp39mos synthesis in stage IV oocytes was also studied since this protein kinase participates in the mitogen-activated protein kinase (MAPK) pathway as a MAPKK kinase like Raf. Contrary to what is observed in postvitellogenic oocytes, MAPK was not activated in insulin-treated stage IV oocytes even 20 hr after the stimulation. This was not caused by the absence of MAPK activators like MEK (MAPKK), Raf, or Ras, but rather by the inability of insulin to activate Ras. Interestingly, injection of constitutively active raf mRNA as well as oncogenic Ras protein, Ha-Ras lys12, in stage IV oocytes resulted in MAPK activation, whereas neither Mos accumulation nor GVB occurred, suggesting that the Ras --> Raf --> MAPKK --> MAPK cascade was functional but that MAPK activation alone was not sufficient for the mitogenic signal to proceed further down in the pathway leading to MPF activation. Treatment of stage IV oocytes with insulin did not stimulate Mos synthesis either, indicating a dysfunction in the "Mos synthesis machinery." The present results show that incompetence of Xenopus stage IV oocytes to activate MPF in response to insulin is primarily due to the inability of the peptide to activate Ras and to stimulate pp39mos synthesis and secondarily to a deficiency in the mitogenic pathway that connects MAPK to MPF activation.

Reversion of human prostate tumorigenic growth by azatyrosine

OBJECTIVES

Azatyrosine, an antibiotic isolated from a Streptomyces species, has been previously shown to have antitumor activity against ras- and neu-transformed fibroblasts and human epithelial cells. In this study, we investigated the effect of azatyrosine on human prostate cancer cell growth and the reversion potential of this antibiotic on prostate tumorigenic cell lines.

METHODS

Three androgen-independent human prostate cancer cell lines (TSU-Prl, DU-145, and PC-3) were cultured in the presence of azatyrosine and their growth rates were determined over a 7-day period. Following exhaustive treatment with azatyrosine for 5 weeks, three azatyrosine-resistant colonies were cloned from the PC-3 cell line and were subsequently established as stable cell lines. The growth characteristics of these azatyrosine-resistant clones were examined both in vitro and in vivo to establish their "potentially revertant" profiles.

RESULTS

Incubation with azatyrosine (for 7 days) resulted in greater than 95% in vitro growth inhibition of the three parental prostate cancer cell lines. Analysis of the biologic properties of these azatyrosine-resistant cell lines revealed: (1) a significant reduction in in vitro growth rates; (2) a decreased rate of DNA synthesis as measured by thymidine uptake; and (3) a decreased ability for colony formation in soft agar. Moreover all three azatyrosine-resistant clones exhibited suppressed tumorigenicity in severe combined immunodeficient (SCID) mice when compared with the parental cell line. An important observation was that one revertant clone demonstrated complete loss of tumorigenicity. On the basis of this biologic behavior, these cell lines were characterized as revertants. Cytogenetic analysis revealed gross chromosomal differences between the revertant clones and the parental cell line. Northern hybridization analysis demonstrated elevated expression of the K-rev-1 and bcl-2 but not the rrg mRNA transcripts in the revertant cell lines.

CONCLUSIONS

These results suggest that azatyrosine inhibits prostate tumorigenic growth; it has a high reversion efficiency on human prostate cancer cells; and the K-rev-1 suppressor gene and the bcl-2 proto-oncogene could be potentially involved in the reversion mechanism mediated by azatyrosine. This reversion of prostate cancer cells to an apparently nontumorigenic phenotype points to a potentially significant therapeutic role for azatyrosine in the treatment of advanced prostate cancer.

Ras-dependent maturation of Xenopus oocytes is blocked by modified peptides of GTPase activating protein (GAP)

Guanosine triphosphatase activating protein (GAP) is an important modulator of p21ras (Ras)-dependent signal transduction in mammalian cells and in insulin-induced maturation of Xenopus oocytes. A synthetic octapeptide from the catalytic domain of GAP, residues 899-906 (F899VFLRLIC906), inhibited GAP-stimulated hydrolysis of GTP to GDP by Ras in an in vitro biochemical assay (IC50 = 12 microM). The peptide was assayed for its ability to block insulin- (Ras-dependent) and progesterone- (Ras-independent) induced maturation of stage VI Xenopus laevis oocytes, marked by germinal vesicle breakdown (GVBD). Microinjection of 50 pmol of the peptide inhibited insulin- but not progesterone-induced GVBD by 50%. A 7-residue peptide lacking F899, GAP(900-906)-NH2, failed to inhibit GAP-stimulated GTPase activity and did not block GVBD. Replacement of the cysteine residue at position 906 with methionine resulted in a peptide with prolonged inhibitory activity in the oocyte. Moreover, sequential replacement of specific L-amino acid residues with the corresponding D-amino acids produced a peptide with a two-fold increased half-life after injection into oocytes. None of the peptides tested affected progesterone induced GVBD, suggesting that the modifications did not result in loss of specificity. These studies show that (a) peptides that were able to inhibit GAP-stimulated Ras GTPase activity in vitro were also able to block Ras-dependent GVBD in oocytes, and (b) specific substitutions in these peptides can result in improved stability in oocytes.

Species specificity and organ, cellular and subcellular localization of the 100 kDa Ras GTPase activating protein

A p100-GAP isoform, generated by an alternative splicing mechanism that eliminates the 180 hydrophobic amino acids at the amino terminus of p120-GAP, has been described in human placenta, in addition to the known p120GAP and neurofibromin. This p100-GAP possesses full Ras-GTPase stimulating activity. p120-GAP is ubiquitously localized in the cytosol while the localization of p100-GAP is unknown. Here we have explored the precise localization of p100-GAP and show that p100-GAP is present only in extracts of primate placenta. It is abundant in both human and Maccaca Rhesus placentae, where it is present in far larger amounts than p120-GAP. The p100-GAP is species-specific since it was not detected in the placenta of pig, sheep, mouse or rat. p100-GAP was also found to be organ-specific, since it was not detectable in organs other than the placenta. In this connection, we substantiated our previous finding that p100-GAP is mainly localized in the trophoblasts. Both subcellular trophoblast fractionation and immunofluorescence analyses showed that this protein was distributed between the cytosol, plasma membrane and a fraction bound to the nucleus, but not inside it. This highly restrictive specificity of p100-GAP localization in relation to species, organ and cell type, confirms the extreme singularity of this protein, and strongly suggests a particular specific function in the trophoblast.

Rad: a member of the Ras family overexpressed in muscle of type II diabetic humans

To identify the gene or genes associated with insulin resistance in Type II (non-insulin-dependent) diabetes mellitus, subtraction libraries were prepared from skeletal muscle of normal and diabetic humans and screened with subtracted probes. Only one clone out of 4000 was selectively overexpressed in Type II diabetic muscle as compared to muscle of non-diabetic or Type I diabetic individuals. This clone encoded a new 29-kilodalton member of the Ras-guanosine triphosphatase superfamily and was termed Rad (Ras associated with diabetes). Messenger ribonucleic acid of Rad was expressed primarily in skeletal and cardiac muscle and was increased an average of 8.6-fold in the muscle of Type II diabetics as compared to normal individuals.

DNA modification and repair by 2-nitropropane is extensive in hepatocytes of rats compared to those of humans and mice

Hepatocytes from rats, mice or humans were exposed to either 2-nitropropane or propane 2-nitronate and the extent of unscheduled DNA synthesis (UDS) was measured. At a concentration of 2-nitropropane (0.1 mM) that produced a marked induction of UDS in rat hepatocytes, a negative or minimal response was seen in hepatocytes from mice and humans. A 10-fold higher concentration of 2-nitropropane was required in mouse hepatocytes for an equivalent UDS response to that seen in rat-liver cells. All three species showed UDS after exposure of hepatocytes to propane 2-nitronate at 0.1 mM although the effect in human cells was variable. In rat hepatocytes the induction of UDS by 2-nitropropane was not inhibited by the antioxidant dimethyl sulphoxide (1%). In these cells, 2-nitropropane produced an electrochemically active modified deoxynucleoside, similar to that reported to be formed in livers of rats which had received 2-nitropropane in vivo. A smaller but significant production of this altered deoxynucleoside was found in mouse hepatocytes but not in human hepatocytes treated identically. The level of 8-oxo-7,8-dihydrode-oxyguanosine was not increased in hepatocytes from any of the three species exposed to 2-NP under the same conditions. It is suggested that the UDS caused by 2-nitropropane may be in response to damage other than that produced by oxygen radicals.

Evidence that oocyte maturation induced by an oncogenic ras-p21 protein and insulin is mediated by overlapping yet distinct mechanisms

We have recently shown that a peptide (residues 35-47) from a functional region of the ras p21 protein, thought to be involved in the binding of p21 to GTPase activating protein, the antibiotic azatyrosine, known to induce the ras-recision gene, and the selective protein kinase C inhibitor, CGP 41,251, all inhibit oncogenic p21 protein-induced maturation of oocytes in a dose-dependent manner. We now show that these three agents only partially inhibit insulin-induced oocyte maturation, known to be dependent on activation of cellular p21 protein. On the other hand, the anti-p21 protein antibody Y13-259 completely inhibits both insulin- and oncogenic p21 protein-induced maturation as does a tetrapeptide, CVIM, known to block the enzyme farnesyl transferase which covalently attaches the farnesyl moiety to the p21 protein allowing it to attach to the cell membrane. Our results suggest that while the oncogenic and insulin-activated normal p21 proteins share certain elements of their signal transduction pathways in common, these pathways diverge and allow for selective inhibition of the oncogenic pathway.

Non–Neutralizing Monoclonal Antibodies Against RAS GTPase–Activating Protein: Production, Characterization and Use in an Enzyme Immunometric Assay

We studied several monoclonal antibodies (mAbs) raised against the 100 kD Ras GTPase activating protein (p100-GAP), which was purified from human placenta. These antibodies recognized p120-GAP and p100-GAP in native and in denatured forms. The most reactive, GP15 and GP200, both recognized distinct epitopes and did not neutralize GTPase stimulatory activity. These two mAbs were selected for a two-site enzyme immunoassay, using covalent conjugates of the antibodies coupled to the tetrameric form of acetylcholinesterase as tracer. This assay was used to quantify Ras-GAP in both normal and tumor tissues and cell extracts.

The antibiotic azatyrosine suppresses progesterone or [Val12]p21 Ha-ras/insulin-like growth factor I-induced germinal vesicle breakdown and tyrosine phosphorylation of Xenopus mitogen-activated protein kinase in oocytes

The antibiotic azatyrosine [DL-3-(5-hydroxy-2-pyridyl)alanine] suppressed meiotic maturation in oocytes induced by progesterone or the combination of [Val12]p21Ha-ras microinjection and insulin-like growth factor I. The suppression was dose-dependent in the range of 20-250 microM azatyrosine. In addition, azatyrosine blocked the tyrosine phosphorylation of Xp42, a member of the mitogen-activated protein kinase family, after progesterone or [Val12]p21Ha-ras/insulin-like growth factor I stimulation. Activation of maturation-promoting factor, as shown by a decrease in the tyrosine phosphorylation of the Xenopus homolog of p34cdc2, was also suppressed by azatyrosine. Azatyrosine had no effect in vivo or in vitro on the growth factor-induced autophosphorylation of the oocyte insulin-like growth factor I receptor. Azatyrosine has been shown by others [Shindo-Okada, N., Makabe, O., Nagahara, H. & Nishimura, S. (1989) Mol. Carcinog. 2, 159-167] to inhibit the growth of ras-transformed cells without affecting that of nontransformed cells. In oocytes, the antibiotic exerts an inhibitory action on both a ras-dependent and a ras-independent pathway. Lack of an effect of azatyrosine on germinal vesicle breakdown induced by the microinjection of an extract from mature oocytes, however, suggests that azatryosine is acting upstream of maturation-promoting factor activation.