The antagonistic role of activated oncogenes and apoptosis : studies of n-myc and p53 in lymphoid tumorigenesis

Inactivation of the p53 tumor suppressor gene and activation of proto-oncogenes have been implicated in the initiation and/or progression of a number of neoplasms. In this thesis, the involvement of N-myc gene activation has been studied in mouse plasmacytomagenesis and the role of wild-type (wt)p53 and oncogene activation has been evaluated with particular emphasis on apoptosis. Nearly all mouse plamacytomas (MPC) carry Ig/c-myc translocations.

We showed that MPCs could be induced in Emy-N-myc transgenic mice. All the tumors expressed the N-myc transgene but not c-myc. None of them carried the MPC-associated Ig/c-myc translocations. In the conventional MPC-induction system, an exceptional MPC with a novel t(6;12) translocation was identified that juxtaposed N-myc to the Igk locus. A functional equivalence between c-myc and N-myc in plasmacytomagenesis was suggested. We have found that a v-myc retrovirus-induced T cell Iymphoma line (J3D) has lost one of its p53 alleles, whereas the other has become inactivated due to an insertion of a Moloney murine leukemia provirus in intron 4 with an opposite transcriptional orientation.

No p53 protein could be detected. Reconstitution of these cells with wt p53 using a temperature-sensitive (ts) murine Val135 mutant construct demonstrated that wt p53 can trigger both G1 cell cycle arrest andapoptosis. A kinetic analysis showed that wt p53-induced G1 arrest preceded apoptosis. We have hypothesized that apoptosis is triggered by the contradictory signals of the growth-promoting activated myc and the growth-inhibiting wtp53. Introduction of a bcl-2 expression vector into the J3D ts p53 cells inhibited wtp53-induced apoptosis. However, bc1-2 had no effect on wt pS3-induced Gl arrest and did not interfere with wt p53-mediated transactivation of p21WAFl and bax.

We suggest that inhibition of p53-induced apoptosis by bcl-2 could be due to neutralization of the apoptosis-promoting effect of bax. These findings are consistent with the possibility that p21WAFl induction is responsible for pS3-mediated Gl arrest and that p53-mediated induction of bax accelerates apoptosis. An expression vector carrying the human papillomavirus (HPV) 16 E7 gene was introduced into the ts p53 J3D cells. A partial relief from p53-induced Gl arrest by overexpression of E7 affected neither the kinetics nor the frequency of p53-triggered apoptosis. Expression of E7 protein did not interfere with pS3-mediated transactivation of p21WAFl and bax. This indicates that pS3-induced apoptosis is independent of p53-induced Gl arrest.