INDUCTION OF APOPTOSIS BY THE N-ACETYL-GALACTOSAMINE-SPECIFIC TOXIC LECTIN FROM VISCUM-ALBUM L. IS ASSOCIATED WITH A DECREASE OF NUCLEAR P53 AND BCL-2 PROTEINS AND INDUCTION OF TELOMERIC ASSOCIATIONS

The ribosome-inhibiting proteins from Viscum album L., i.e, the mistle toe lectins (ML), were recognized to induce apoptosis in various tumou r cell lines and human lymphocytes. However, several aspects of ML-ind uced cell death are unclear. We report that the galNAc-binding ML III incubated with human lymphocytes mediates a very effective death signa l resulting in the binding of Annexin-V and expression of mitochondria l membrane proteins Apo2.7, but also in an influx of the DNA intercala ting dye propidium iodide. The addition of the ribosome-inhibiting pro tein Vollcensin also induced Apo2.7 molecules, while Momordin, lacking a carbohydrate-binding chain, did not enter the cell membrane and thu s did not affect the cells. However, we observed ML III to preferentia lly affect CD8(+) cells with a memory phenotype (CD62L(lo)) as compare d to their CD8(+) CD62L(hi) counterparts, CD4(+) T cells and CD19(+) B cells. Furthermore, ML III did not induce sister chromatid exchange-i nducing DNA lesions but reduced the intensity of telomeric signals, in creased the frequencies of telomeric associations and C-anaphases and reduced nuclear Bcl-2 and p53 proteins. Whatever the exact mechanisms are, our results provide strong evidence that the hit III-mediated cyt otoxicity involves distinct killing pathways, i.e. (1) primary cell de ath via an induction of apoptosis which may not be dependent on protei n and/or RNA synthesis and may not involve p53 and Bcl-2 proteins and (2) a loss of telomeres resulting in chromosomal instability in the su rviving cells which is incompatible with life. However, we cannot excl ude the possibility that this effect is due to a decrease in nuclear p 53 proteins. (C) 1998 Elsevier Science Ireland Ltd. All rights reserve d.