Abstract
We have investigated polyamine metabolism in primary cultures of mouse epidermal cells. These cells, which grow at low Ca2+ levels as a monolayer with characteristics of basal cells, terminally differentiate when the extracellular Ca2+ level is raised above 1 mM. The cellular levels of free polyamines were measured, and, after incubation of cell cultures with [3H]putrescine, the distribution of label in both acid-soluble and acid-insoluble cellular components was examined. Free polyamine levels were reduced in cells induced to differentiate. Treatment with retinoic acid, which prevents differentiation and causes increased proliferation, resulted in an increase in free putrescine. Upon adjustment of the calcium concentration to a level that induces differentiation, the enzyme transglutaminase was activated, and a concomitant increase in the level of both protein-bound mono- and bis-gamma-glutamyl derivatives of putrescine and spermidine was observed. Isolation of a material of apparent molecular weight about 6000 which contains only mono-gamma-glutamylpolyamines and the finding of both mono- and bis-gamma-glutamylpolyamines in the protein fraction containing cornified cell envelopes provided the basis for speculation on polyamines in envelope formation. Our data suggest that polyamines play a role during epidermal cell differentiation through transglutaminase-mediated post-translational modification.
Highlights
From the $Department of Bwlogy, Second University of Rome, Via 0.Rainondo, 00173 Rome, Italy and the llLaboratoryof Oral Biology andPhysiology, Natwml Institute of Dental Research, Natioml Institutes of Health, Bethesda, Maryland 20892
The findings suggest that polyamines are involved in cell differentiation, no conclusions as totheir specific rolecan tion, resulted in an increase in free putrescine
A distinctive aspect of epidermal cell differentiation isthe establishment of the cornified envelope beneath the plasma membrane. This envelope is composed of cross-linked proteins [27], andits formation has been related to the progressive increase in particulate epidermal transglutaminase activity [36,37].During differentiation the transglutaminase catalyzes the production of isopeptide bonds between precursor proteins to form the envelope [38, 39].Cells growing inlow calcium form very low levels of envelopes [27,40]
Summary
There are other reports, that polyamine depletion stimulates differentiation in tumor cell lines, such as neuroblastoma [10] and melanoma [11],and more recent reports claim stimulation in various embryonal carcinoma lines [12,13,14]. Despite these evident discrepancies, the findings suggest that polyamines are involved in cell differentiation, no conclusions as totheir specific rolecan tion, resulted in an increase in free putrescine. Formation of these protein components through exchange of the polyamine for ammonia at the carboxamide
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