Tissue-specific alternative splicing of spermidine/spermine N1-acetyltransferase.
ABSTRACT The polyamines, spermidine and spermine, are abundant organic cations participating in many important cellular processes. We have previously shown that the rate-limiting enzyme of polyamine catabolism, spermidine/spermine N(1)-acetyltransferase (SSAT), has an alternative mRNA splice variant (SSATX) which undergoes degradation via nonsense-mediated mRNA decay (NMD) pathway, and that the intracellular polyamine level regulates the ratio of the SSATX and SSAT splice variants. The aim of this study was to investigate the effect of SSATX level manipulation on SSAT activity in cell culture, and to examine the in vivo expression levels of SSATX and SSAT mRNA. Silencing SSATX expression with small interfering RNA led to increased SSAT activity. Furthermore, transfection of SSAT-deficient cells with mutated SSAT gene (which produced only trace amount of SSATX) yielded higher SSAT activity than transfection with natural SSAT gene (which produced both SSAT and SSATX). Blocking NMD in vivo by protein synthesis inhibitor cycloheximide resulted in accumulation of SSATX mRNA, and like in cell culture, the increase of SSATX mRNA was prevented by administration of polyamine analog N(1),N(11)-diethylnorspermine. Although SSATX/total SSAT mRNA ratio did not correlate with polyamine levels or SSAT activity between different tissues, increasing polyamine levels in a given tissue led to decreased SSATX/total SSAT mRNA ratio and vice versa. Taken together, the regulated unproductive splicing and translation of SSAT has a physiological relevance in modulating SSAT activity. However, in addition to polyamine level there seems to be additional factors regulating tissue-specific alternative splicing of SSAT.
- SourceAvailable from: jbc.org[show abstract] [hide abstract]
ABSTRACT: Calf liver contains two nuclear N-acetyltransferases which are separated by chromatography on hydroxylapatite. Both acetyltransferase A and acetyltransferase B will transfer acetate from acetyl-CoA to either histone or spermidine. The same protein catalyzes the reaction with both substrates; this is shown by a constant ratio of spermidine to histone activity over a 5,000-fold purification and identical heat denaturation kinetics for both spermidine and histone acetyltransferase activity with each enzyme. Histone is preferentially acetylated when both acceptors are present. Both enzymes preferentially acetylate polyamines (spermidine, spermine, and diaminodipropylamine) to diamines. Acetyltransferase A acetylates histones in the order: whole histone greater than H4 greater than H2A greater than H3 greater than H2B greater than H1; acetyltransferase B in the order: whole histone greater than H4 = H3 greater than H2A greater than H2B greater than H1. Michaelis constants are 2 X 10(-4)M for spermidine and 9 X 10(-6)M for acetyl-CoA. Acetyltransferase A has a molecular weight of 150,000; acetyltransferase B 175,000. Both enzymes are strongly inhibited by p-chloromercuribenzoate and weakly inhibited by EDTA.Journal of Biological Chemistry 02/1978; 253(1):233-7. · 4.65 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The naturally occurring polyamines, spermine, spermidine and the diamine putrescine are widespread in nature. They have been implicated in growth and differentiation processes. Polyamines accumulate in cancerous tissues and their concentration is elevated in body fluids of cancer patients. Assays of urinary and blood polyamines have been used to detect cancer and to determine the success of therapy. Drugs which inhibit the synthesis of polyamines can prevent cancer and may also be used for therapeutic purposes. Ornithine decarboxylase, which catalyzes the rate limiting step in polyamine synthesis, can serve as a marker of proliferation. Recently, a new in vitro chemosensitivity test, based on the disappearance of ornithine decarboxylase in drug-treated cancer cells has been developed. The increasing interest in polyamines and their physiological functions may lead to a more extensive application of these compounds or their derivatives in cancer diagnosis and treatment.Amino Acids 08/2004; 26(4):307-9. · 3.91 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: In eukaryotes, an elaborate set of mechanisms has evolved to ensure that the multistep process of gene expression is accurately executed and adapted to cellular needs. The mRNA surveillance pathway works in this context by assessing the quality of mRNAs to ensure that they are suitable for translation. mRNA surveillance facilitates the detection and destruction of mRNAs that contain premature termination codons by a process called nonsense-mediated decay. Moreover, recent studies have shown that a distinct mRNA surveillance process, called nonstop decay, is responsible for depleting mRNAs that lack in-frame termination codons. mRNA surveillance thereby prevents the synthesis of truncated and otherwise aberrant proteins, which can have dominant-negative and other deleterious effects.Journal of Cell Science 09/2002; 115(Pt 15):3033-8. · 5.88 Impact Factor