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S Abel

Publications and source records attributed to S Abel.

At least 19 recordsLinked to original sources
PubMed1999

Biochemical characterization of recombinant polypeptides corresponding to the predicted betaalphaalpha fold in Aux/IAA proteins.

The plant hormone indoleacetic acid (IAA or auxin) transcriptionally activates a select set of early genes. The Aux/IAA class of early auxin-responsive genes encodes a large family of short-lived, nuclear proteins. Aux/IAA polypeptides homo- and heterodimerize, and interact with auxin-response transcription factors (ARFs) via C-terminal regions conserved in both protein families. This shared region contains a predicted betaalphaalpha motif similar to the prokaryotic beta-ribbon DNA binding domain, which mediates both protein dimerization and DNA recognition. Here, we show by circular dichroism spectroscopy and by chemical cross-linking experiments that recombinant peptides corresponding to the predicted betaalphaalpha region of three Aux/IAA proteins from Arabidopsis thaliana contain substantial alpha-helical secondary structure and undergo homo- and heterotypic interactions in vitro. Our results indicate a similar biochemical function of the plant betaalphaalpha domain and suggest that the betaalphaalpha fold plays an important role in mediating combinatorial interactions of Aux/IAA and ARF proteins to specifically regulate secondary gene expression in response to auxin.

Amino Acid Sequence↗
PubMed1998

Oxidative damage and fumonisin B1-induced toxicity in primary rat hepatocytes and rat liver in vivo.

Dietary fumonisin B1 (FB1) levels of 250 and 500 mg FB1/kg increased the level of thiobarbituric acid reactive substances (TBARS) significantly (P < 0.05) in the liver of rats fed FB1 over 21 days. Levels of 10, 50 and 100 mg FB1/kg also markedly (not significantly) increased the level of TBARS in the liver homogenate. Subcellular fractionation of the liver of the rats fed the 250 mg FB1/kg diet, showed a marginally significant increase of TBARS in the plasma membranes (0.05 < P < 0.1) and a significant increase in the microsomes (P < 0.05). In vitro investigations in primary rat hepatocytes indicated that the level of TBARS was increased in a dose dependent manner associated with an increase in cytotoxicity. Addition of the antioxidant, alpha-tocopherol, significantly decreased the cytotoxicity whereas the level of TBARS was decreased to basal levels, suggesting that lipid peroxidation is likely to contribute to the cytotoxic effect of FB1. Addition of cumene hydroperoxide (CMHP) to primary hepatocytes exposed to FB1 for 44 h, enhanced the CMHP-induced TBARS release suggesting that the hepatocytes exposed to FB1 are more susceptible to chemically-induced oxidative stress. Free radical production could result in excessive cellular damage and/or metabolic abnormalities that are likely to be involved in FB1-induced altered growth responses and cell death in primary hepatocytes. The hepatotoxic effects and resultant oxidative damage induced by FB1 may be important during cancer induction in rat liver by this apparently non-genotoxic compound.

Animals↗
PubMed1998

Inhibition of sphingolipid biosynthesis in rat primary hepatocyte cultures by fumonisin B1 and other structurally related compounds.

The fumonisins and toxins produced by Alternaria alternata f. sp. lycopersici (AAL toxins) are structurally related mycotoxins that disrupt sphingolipid biosynthesis by inhibiting the rate-limiting enzyme, ceramide synthase. Rat primary hepatocytes were exposed to fumonisin B1 (FB1), its N-acetyl analogue, FA1, its fully hydrolysed analogue, AP1 and the AAL toxins (TA and TB) at concentrations of 1 microM for 40 hr in culture. The extent to which these compounds disrupt sphingolipid biosynthesis in hepatocytes in vitro was investigated by analysing the sphingosine (So) and sphinganine (Sa) levels by HPLC. The inhibition of ceramide synthase was irreversible as the Sa:So ratio was maximally increased by FB1 after 24 hr of exposure and the subsequent removal of FB1 had no effect on the ratio as compared with the 40-hr incubation period in the presence of FB1. The Sa concentration was significantly (P < 0.01) increased in all the cultures treated with the different structurally related compounds, while only AP1 increased the So concentration significantly (P < 0.05) above the control. As AP1 was found to be less effective in disrupting sphingolipid biosynthesis it would appear that the tricarballylic (TCA) moiety is required for maximal inhibition of ceramide synthase. The presence of an amino group appears not to be a requisite for activity, since FA1 increased the Sa:So ratio to the same extent as FB1. The AAL toxins TA and TB increased the Sa concentration significantly (P < 0.01) above that of FB1 and FA1, while the Sa:So ratios were altered to the same extent. The structural requirements for the induction of cytotoxicity differ from those required for ceramide synthase inhibition as TA and TB were significantly (P < 0.05 to P < 0.01) less toxic to primary hepatocytes than FB1 at all the concentrations tested.

Alternaria↗
PubMed1997

Effect of fumonisin B1 on the levels and fatty acid composition of selected lipids in rat liver in vivo.

The modulating role of fumonisin B1 (FB1) on lipid biosynthesis was evaluated in a short-term (21 day) experiment using male Fischer rats fed high dietary levels (50, 100 and 250 mg FB1/kg) and in a long-term (2 yr) experiment using male BD IX rats fed low dietary levels (1, 10 and 25 mg FB1/kg) of FB1. The total serum and liver cholesterol was significantly (P < 0.01) increased in the rats fed 250 mg FB1/kg diet for 21 days, while the liver phospholipids, sphingomyelin and phosphatidylethanolamine (PE) were significantly decreased (P < 0.01) and increased (P < 0.05), respectively. In the long-term study, only PE was significantly (P < 0.05) increased in all the FB1-treated animals. Fatty acid (FA) analysis of PE indicated that C18:2n-6 was significantly increased (P < 0.05 to P < 0.01) in the FB1-treated rats of the short-term study, while it was markedly (not significantly) increased in phosphatidylcholine (PC). The same pattern was observed in the PC and PE fractions of the liver of the FB1-treated rats from the long-term studies, but the changes were not significant due to the small number (three rats per group) of rats analysed. The levels of C22:5n-6 and C22:6n-3 were also markedly decreased and increased respectively in the 10 and 25 mg FB1/kg-treated groups. When the FAs were determined in the total lipids in a larger number of rats (four to six animals per group) the level of C18:2n-6 was significantly increased in the 10 (P < 0.01) and 25 (P < 0.05) mg FB1/kg-treated groups. Similar effects were noticed in plasma PC with respect to the C18:5n-6 and C22:55n-6 in both the long- and short-term treated groups, except that C20:4n-6 was also lower in both cases. The total n-6 FAs and polyunsaturated FAs were significantly (P < 0.01) and markedly reduced in PC and PE, respectively, of the rats fed the 250 mg FB1/kg diet. In the long-term experiment the n-6/n-3 ratio was significantly (P < 0.01) decreased in PE and markedly lowered in PC due to a significant (P < 0.05) increase in the n-3 FAs of both phospholipid fractions. The sphinganine/sphingosine ratio was significantly (P < 0.05) altered in the liver of the rats fed the 100 and 250 mg FB1/kg diets for 21 days, while in the long-term study no significant changes were noticed in either the liver or sera. The present data indicate that FB1 affects lipid biosynthesis in rat liver and plasma differently, depending on the dietary level and duration of treatment. Alterations to the n-3 and n-6 FA biosynthetic pathways, detected in rats fed relatively low dietary levels of FB1, are likely to be important mediators for FB1-induced effects on hepatocyte cell proliferation.

Animals↗
PubMed1997

Thresholds and kinetics of fatty acid replacement in different cellular compartments in rat liver as a function of dietary n-6/n-3 fatty acid content.

The kinetics of fatty acid (FA) replacement in different membrane compartments in the rat liver were investigated using diets with varying n-6/n-3 FA ratios. Rats at different stages of growth, i.e. after weaning and at 150 g body weight, were either fed a modified AIN 76A diet containing sunflower oil as fat source or the same diet containing sunflower oil and fish oil to achieve n-6/n-3 FA ratios of 12:1 and 6:1 (diets A and B, respectively). In the adult rats, fed diet A for 8 weeks, C18:2n-6 increased significantly at week 2 in the phosphatidylcholine (PC) fraction of the plasma membranes, microsomes and plasma but not in phosphatidylethanolamine (PE), C20:3n-6 increased significantly at week 2 in the plasma membrane and microsomal PC, but did only increased in PE of both compartments by week 8. C20:4n-6 and the n-3 FAs significantly decreased and increased, respectively, at week 2 in PC and PE of both membrane compartments and plasma PC. The experimental diets led to a change in the plasma membrane fluidity but not in the microsomes. The FA changes in the weaned rats followed a similar pattern as in the adult rats although the changes were greater, depending on the phospholipid fraction and specific FA. The decrease in C20:4n-6 was significantly greater in the microsomal PC and PE and plasma PC but not in the plasma membrane PC and PE. The n-3 FAs increased significantly above the adult levels in the plasma membrane PC and PE respectively but not in the microsomal phospholipid fractions. A plateau for maximal n-3 and n-6 FA incorporation was achieved in the adult rats fed diet A in the microsomes after 2 weeks with no further alterations occurring with diet B. In the plasma PC and plasma membranes most of the n-3 FAs achieved a threshold incorporation after 2 weeks on diet A, except for C22:6n-3 in the plasma membranal PE and certain n-6 FAs in the plasma membrane PC and PE. The present data shows that differences exist in the kinetics of FA incorporation and replacement depending on the specific phospholipid fraction, membrane compartment, age and to a certain extent the dietary n-6/n-3 FA ratio.

Aging↗
PubMed1996

DNA elements responsive to auxin.

Genes induced by the plant hormone auxin are probably involved in the execution of vital cellular functions and developmental processes. Experimental approaches designed to elucidate the molecular mechanisms of auxin action have focused on auxin perception, genetic dissection of the signaling apparatus and specific gene activation. Auxin-responsive promoter elements of early genes provide molecular tools for probing auxin signaling in reverse. Functional analysis of several auxin-specific promoters of unrelated early genes suggests combinatorial utilization of both conserved and variable elements. These elements are arranged into autonomous domains and the combination of such modules generates uniquely composed promoters. Modular promoters allow for auxin-mediated transcriptional responses to be revealed in a tissue- and development-specific manner.

Base Sequence↗
PubMed1996

Effect of fumonisin B1 on protein and lipid synthesis in primary rat hepatocytes.

The effect of fumonisin B1 (FB1) on protein and lipid synthesis was evaluated in primary rat hepatocytes. FB1 did not affect incorporation of [3H]leucine into hepatocytes at either non-toxic (150 microM) or cytotoxic (500 microM) concentrations indicating that protein synthesis was not affected. However, FB1 significantly (P < 0.01 to P < 0.0001) inhibited incorporation of [14C]palmitic acid into hepatocyte cultures implying that lipid synthesis was altered. Incorporation of the radiolabel was significantly (P < 0.05 to P < 0.0001) lowered in triacylglycerol (TAG) and sphingomyelin fractions and increased in phosphatidylcholine (PC) and phosphatidylethanolamine (PEA) in both FB1 concentrations. The incorporation pattern of [14C]palmitic acid closely resembles the changes in phospholipid levels in the treated cells. The sphingolipid, sphinganine (Sa), was significantly (P < 0.0001) increased in treated cells but there was no significant difference between the toxic and non-toxic dose levels implying that the increased Sa level alone is not responsible for the in vitro toxicity. FB1 significantly (P < 0.01 to P < 0.001) decreased the level of free cholesterol within the cell, resulting in an increased PC:cholesterol ratio suggesting a more rigid membrane structure. Subsequent studies on the fatty acid (FA) profiles in PC and the neutral lipid, TAG, indicated that FB1 significantly (P < 0.05 to P < 0.0001) increased the levels of the polyunsaturated FAs C18:2n-6 and C20:4n-6 at both concentrations. The FB1-induced changes to cellular membranes, specifically those related to FA changes in the major membrane phospholipids, and the altered FA content of the hepatocytes are likely to be key events in explaining the cytotoxic effects and altered growth responses induced by fumonisins in primary hepatocytes.

Animals↗
PubMed1996

Differential activation of the primary auxin response genes, PS-IAA4/5 and PS-IAA6, during early plant development.

The plant growth hormone auxin typified by indoleacetic acid (IAA) transcriptionally activates early genes in pea, PS-IAA4/5 and PS-IAA6, that are members of a multigene family encoding short-lived nuclear proteins. To gain first insight into the biological role of PS-IAA4/5 and PS-IAA6, promoter-beta-glucuronidase (GUS) gene fusions were constructed and their expression during early development of transgenic tobacco seedlings was examined. The comparative analysis reveals spatial and temporal expression patterns of both genes that correlate with cells, tissues, and developmental processes known to be affected by auxin. GUS activity in seedlings of both transgenic lines is located in the root meristem, sites of lateral root initiation and in hypocotyls undergoing rapid elongation. In addition, mutually exclusive cell-specific expression is evident. For instance, PS-IAA4/5-GUS but not PS-IAA6-GUS is expressed in root vascular tissue and in guard cells, whereas only PS-IAA6-GUS activity is detectable in glandular trichomes and redistributes to the elongating side of the hypocotyl upon gravitropic stimulation. Expression of PS-IAA4/5 and PS-IAA6 in elongating, dividing, and differentiating cell types indicates multiple functions during development. The common and yet distinct activity patterns of both genes suggest a combinatorial code of spatio-temporal co-expression of the various PS-IAA4/5-like gene family members in plant development that may mediate cell-specific responses to auxin.

Cell Differentiation↗
PubMed1996

Hepatotoxicity and -carcinogenicity of the fumonisins in rats. A review regarding mechanistic implications for establishing risk in humans.

Cancer induction by the non-genotoxic mycotoxin, fumonisin B1, has been investigated by studying the mechanisms involved during cancer initiation and promotion in rat liver. Cancer initiation is effected through a toxic-proliferative response while the inhibitory effect on hepatocyte cell proliferation appears to be a key aspect determining cancer promotion. Dose-response effects of the fumonisins on the induction of early neoplastic lesions in both long- and short-term animal experiments have been established. The biphasic response of FB1 on hepatocyte proliferation will be discussed in relation to the known mechanisms of cancer induction by the genotoxic hepatocarcinogens. Recent investigations regarding the effect of the fumonisins on lipid biosynthesis and its inhibitory effect on hepatocyte growth stimulatory responses in vitro will be highlighted. Integration of our current knowledge regarding the carcinogenic potential of the fumonisins in setting a realistic and applicable risk assessment model for this non-genotoxic carcinogen will finally be addressed.

Animals↗
PubMed1995

The PS-IAA4/5-like family of early auxin-inducible mRNAs in Arabidopsis thaliana.

The plant hormone auxin transcriptionally activates early genes. We have isolated a 14-member family of DNA sequences complementary to indoleacetic acid (IAA)-inducible transcripts in Arabidopsis thaliana. The corresponding genes, IAA1 to IAA14, are homologs of PS-IAA4/5 and PS-IAA6 from pea, Aux22 and Aux28 from soybean, ARG3 and ARG4 from mungbean, and AtAux2-11 and AtAux2-27 from Arabidopsis. The members of the family are differentially expressed in mature Arabidopsis plants. Characterization of IAA gene expression in etiolated seedlings demonstrates specificity for auxin inducibility. The response of most family members to IAA is rapid (within 4 to 30 minutes) and insensitive to cycloheximide. Cycloheximide alone induces all the early genes. Auxin-induction of two late genes, IAA7 and IAA8, is inhibited by cycloheximide, indicating requirement of protein synthesis for their activation. All IAA genes display a biphasic dose response that is optimal at 10 microM IAA. However, individual genes respond differentially between 10 nM and 5 microM IAA. Expression of all genes is defective in the Arabidopsis auxin-resistant mutant lines axr1, axr2 and aux1. The encoded polypeptides share four conserved domains, and seven invariant residues in the intervening regions. The spacers vary considerably in length, rendering the calculated molecular mass of IAA proteins to range from 19 kDa to 36 kDa. Overall sequence identity between members of the family is highly variable (36 to 87%). Their most significant structural features are functional nuclear transport signals, and a putative beta alpha alpha-fold whose modeled three dimensional structure appears to be compatible with the prokaryotic beta-ribbon DNA recognition motif. The data suggest that auxin induces in a differential and hierarchical fashion a large family of early genes that encode a structurally diverse class of nuclear proteins. These proteins are proposed to mediate tissue-specific and cell-type restricted responses to the hormone during plant growth and development.

Amino Acid Sequence↗
PubMed1995

ACS4, a primary indoleacetic acid-responsive gene encoding 1-aminocyclopropane-1-carboxylate synthase in Arabidopsis thaliana. Structural characterization, expression in Escherichia coli, and expression characteristics in response to auxin [corrected].

1-Aminocyclopropane-1-carboxylic acid (ACC) synthase is the key regulatory enzyme in the biosynthetic pathway of the plant hormone ethylene. The enzyme is encoded by a divergent multigene family in Arabidopsis thaliana, comprising at least five genes, ACS1-5 (Liang, X., Abel, S., Keller, J.A., Shen,N. N.F., and Theologis, A. (1992) Poc. Natl. Acad. Sci. U.S.A. 89, 11046-11050). In etiolated seedlings, ACS4 is specifically induced by indoleacetic acid (IAA). The response to IAA is rapid (within 25 min) and insensitive to protein synthesis inhibition, suggesting that the ACS4 gene expression is a primary response to IAA. The ACS4 mRNA accumulation displays a biphasic dose-response curve which is optimal at 10 microM of IAA. However, IAA concentrations as low as 100 microM are sufficient to enhance the basal level of ACS4 mRNA. The expression of ACS4 is defective in the Arabidopsis auxin-resistant mutant lines axr1-12, axr2-1, and aux1-7. ACS4 mRNA levels are severely reduced in axr1-12 and axr2-1 but are only 1.5-fold lower in aux1-7. IAA inducibility is abolished in axr2-1. The ACS4 gene was isolated and structurally characterized. The promoter contains four sequence motifs reminiscent of functionally defined auxin-responsive cis-elements in the early auxin-inducible genes PS-IAA4/5 from pea and GH3 from soybean. Conceptual translation of the coding region predicts a protein with a molecular mass of 53,795 Da and a theoretical isoelectric point of 8.2. The ACS4 polypeptide contains the 11 invariant amino acid residues conserved between aminotransferases and ACC synthases from various plant species. An ACS4 cDNA was generated by reverse transcriptase-polymerase chain reaction, and the authenticity was confirmed by expression of ACC synthase activity in Escherichia coli.

Amino Acid Sequence↗
PubMed1995

cDNA structure and regulatory properties of a family of starvation-induced ribonucleases from tomato.

In previous work we have determined the primary structure of two of the five ribonucleases which are induced by phosphate starvation in cultured tomato cells. Here, we present the isolation and characterization of the cDNAs for the extracellular ribonuclease LE and the intracellular, but extravacuolar ribonuclease LX. Structural analysis of these cDNAs together with partial protein-sequencing of vacuolar ribonucleases LV1, LV2 and LV3 revealed a family of very similar ribonucleases. From these data we assume identify between ribonucleases LE and LV3 for which the targeting mechanism has to be shown. Furthermore, RNase LV1 and RNase LV2 might be posttranslational processing products of RNase LX which travel to the vacuoles after splitting off the putative ER retention signal present at RNase LX. Additionally, we show by northern blot analysis that phosphate starvation in plant cells leads to an increase in the steady-state level of this type of enzymes revealing close similarities of the plant response to a limited supply of inorganic phosphate with the PHO regulation in bacteria and fungi.

Amino Acid Sequence↗
PubMed1995

A polymorphic bipartite motif signals nuclear targeting of early auxin-inducible proteins related to PS-IAA4 from pea (Pisum sativum).

The plant hormone, indoleacetic acid (IAA), transcriptionally activates two early genes in pea, PS-IAA4/5 and PS-IAA6 that encode short-lived nuclear proteins. The identification of the nuclear localization signals (NLS) in PS-IAA4 and PS-IAA6 using progressive deletion analysis and site-directed mutagenesis is reported. A C-terminal SV40-type NLS is sufficient to direct the beta-glucuronidase reporter to the nucleus of transiently transformed tobacco protoplasts, but is dispensible for nuclear localization of both proteins. The dominant and essential NLS in PS-IAA4 and PS-IAA6 overlap with a bipartite basic motif which is polymorphic and conserved in related proteins from other plant species, having the consensus sequence (KKNEK)KR-X(24-71)-(RSXRK)/(RK/RK). Both basic elements of this motif in PS-IAA4, (KR-X41-RSYRK), function interdependently as a bipartite NLS. However, in PS-IAA6 (KKNEKKR-X36-RKK) the upstream element of the corresponding motif contains additional basic residues which allow its autonomous function as an SV40-type monopartite NLS. The spacer-length polymorphism, X(24-70), in respective bipartite NLS peptides of several PS-IAA4-like proteins from Arabidopsis thaliana does not affect nuclear targeting function. The structural and functional variation of the bipartite basic motif in PS-IAA4-like proteins supports the proposed integrated consensus of NLS.

Amino Acid Sequence↗