DIFFERENTIAL EXPRESSION OF TISSUE TRANSGLUTAMINASE DURING IN-VIVO APOPTOSIS OF THYMOCYTES INDUCED VIA DISTINCT SIGNALING PATHWAYS

A significant increase in the expression and activity of tissue transg lutaminase (tTG), one of the effector elements of apoptosis, was obser ved during involution of thymus elicited by treatment with either anti -CD3 antibody or dexamethasone or by irradiation. The blood plasma con centration of epsilon(gamma-glutamyl)lysine isodipeptide, the end-prod uct of the digestion of transglutaminase cross-linked proteins, was al so elevated in each of these cases, tTG was localized in cells of the cortical layer of the thymus and immunofluorescence double staining re vealed that the enzyme appeared in the apoptotic cells. None of these observations could be made when apoptosis was induced by fas-receptor stimulation. The lack of tTG activity in fas-stimulated cells was acco mpanied with a less organized apoptotic morphology. Our data suggest t hat distinct signalling pathways, which induce apoptosis within the sa me cell type, can differentially regulate the expression of tTG, and t his enzyme may be involved in structural stabilization of the apoptoti c cells. (C) 1997 Federation of European Biochemical Societies.