ANTI-IGM-MEDIATED REGULATION OF C-MYC AND ITS POSSIBLE RELATIONSHIP TO APOPTOSIS

Anti-IgM treatment of Burkitt's lymphoma cells is followed by either g rowth arrest or induction of apoptosis, In this study we have explored the role of c-myc in these events, Our results in Ramos cells indicat e the following, (a) The decline in c-myc mRNA occurs at about 4 h; in hibition of about 80% being observed, (b) The stability of c-myc messa ge is involved since the half life of c-my mRNA is decreased from abou t 30 min in untreated cells to about 15 min following treatment with a nti-IgM, In the presence of cycloheximide, a protein synthesis inhibit or, the half-life is increased to about 50 min and was unaltered by tr eatment with anti-IgM. (c) By contrast, nuclear run-on experiments ind icated no change in transcription rates for c-myc message due to treat ment with anti-IgM, (d) A decrease in c-myc causes apoptosis since spe cific repression of c-myc with antisense oligonucleotides decreases th e levels of c-Myc, inhibits growth rate, decreases viability, and indu ces apoptosis, (e) Anti-CD40 inhibition of apoptosis occurs without al teration in anti-IgM-induced down-regulation of c-myc mRNA, suggesting that it acts distally to c-myc down regulation, Other cell lines were also investigated, In Epstein-Barr virus (EBV)-positive cell lines (D audi, Raji, and Namalwa), anti-IgM treatment for 24 h results in growt h inhibition without induction of apoptosis, In EBV-negative cell line s (ST486 and CA46, as well as Ramos), a more heterogeneous pattern of responses to anti-IgM are observed, Ramos and ST486 cells both show gr owth inhibition and apoptosis upon anti-IgM treatment; CA46 cells show n only growth inhibition but not apoptosis, Anti-IgM causes a decline in c-myc mRNA levels in all of these lines, as well as in c-Myc protei n level in the two lines investigated, Daudi and Ramos, regardless of apoptosis, Addition of antisense c-myc oligonucleotides to the cells r educed growth in both Daudi and Ramos cells lines, however it resulted in substantial apoptosis only in Ramos cells, These results suggest t hat anti-IgM destabilizes c-myc mRNA by a process that involves mRNA t urnover, rather than transcription rates, However anti-IgM exerts diff erential effects in EBV positive and EBV-negative cell lines, EBV-posi tive cells are uniformly resistant to apoptosis, while EBV-negative ce ll lines show a tendency 60 apoptosis but with exceptions, Growth inhi bition can be uncoupled from apoptosis in EBV-positive cell lines, but not in those EBV-negative cell lines prone to apoptosis, Furthermore, down-regulation of c-myc message correlates with growth inhibition in these cells, but is an insufficient link to apoptosis, By contrast in hibition of apoptosis by anti-CD40 occurs even though c-myc mRNA is de creased.