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1 Department of Surgery, University of Maryland Medical School and Veterans Affairs Medical Center, Baltimore, Maryland 21201; and 2 Department of Physiology and Biophysics, University of Tennessee College of Medicine, Memphis, Tennessee 38163
Polyamines serve
as natural substrates for the transglutaminase that catalyzes covalent
cross-linking of proteins and is involved in cellular adhesion and
proliferation. This study tests the hypothesis that intracellular
polyamines play a role in the regulation of transglutaminase expression
in rat small intestinal crypt cells (IEC-6 cell line) and human colon
carcinoma cells (Caco-2 cell line). Treatment with
-difluoromethylornithine (DFMO; a specific inhibitor of polyamine
synthesis) significantly depleted the cellular polyamines putrescine,
spermidine, and spermine in both cell lines. In IEC-6 cells, polyamine
depletion was associated with a decrease in the levels of
transglutaminase mRNA. In Caco-2 cells, however, polyamine depletion
significantly increased the levels of transglutaminase mRNA and enzyme
activity. In both cell lines, ornithine decarboxylase mRNA levels
increased and protooncogene c-myc mRNA
decreased in the presence of DFMO. Addition of polyamines to cells
treated with DFMO reversed the effect of DFMO on the levels of mRNA for these genes in both lines. There was no significant change in the
stability of transglutaminase mRNA between control and DFMO-treated IEC-6 cells. In contrast, the half-life of mRNA for transglutaminase in
Caco-2 cells was dramatically increased after polyamine depletion. Spermidine, when given together with DFMO, completely prevented increased half-life of transglutaminase mRNA in Caco-2 cells. These
results indicate that 1) expression
of transglutaminase requires polyamines in IEC-6 cells but is inhibited
by these agents in Caco-2 cells, 2)
polyamines modulate transglutaminase expression at the level of mRNA
through different pathways in these two cell lines, and
3) posttranscriptional regulation
plays a major role in the induction of transglutaminase mRNA in
polyamine-deficient Caco-2 cells.
ornithine decarboxylase; IEC-6 cell line; Caco-2 cell line; posttranscription
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