MODULATION OF RESISTANCE TO ANTI-APO-1-INDUCED APOPTOSIS IN OSTEOSARCOMA CELLS BY CYTOKINES

The CD95/APO-I/Fas receptor/ligand system plays a crucial role in grow th control by mediating apoptosis in lymphoid and non-lymphoid cells, To investigate the role of CD95-mediated apoptosis in osteosarcoma, we studied 3 human osteosarcoma cell lines (HOS/TE 85, MG 63 and Saos-2) and osteoblasts derived from bone biopsies. In contrast to osteoblast -like cells, all cell lines were resistant to anti-APO-1-induced apopt osis despite constitutive CD95 expression at intermediate levels. Bloc king of macromolecular synthesis by cycloheximide or actinomycin D or modulation of CD95 expression by cytokines (TNF-alpha and/or gamma-int erferon) restored sensitivity to anti-APO-l-induced cell death, PCR an alysis of the CD95 transcripts revealed the production of a truncated splice variant that codes for a soluble form of the CD95 receptor. Syn thesis and secretion of soluble CD95 protein into the culture supernat ant was demonstrated by Western blot analysis, Treatment with sensitiz ing cytokines led to up-regulation of full-length CD95 transcripts and the encoded membrane-bound CD95 protein but not the truncated mRNA sp lice variant and the corresponding soluble receptor, as shown by PCR a nd Western blot analysis. The biological activity of soluble CD95 secr eted by osteosarcoma cells was demonstrated by the ability of osteosar coma supernatants to protect the sensitive T-cell line Jurkat from ant i-APO-1-mediated apoptosis, Our results suggest that the production of soluble CD95 by osteosarcoma cell lines that may block physiological death signals and the production of membrane-bound CD95 are differentl y regulated by cytokines via modulation of RNA splicing. (C) 1997 Wile y-Liss, Inc.