Exon 2-Mediated c-mycmRNA Decay In Vivo Is Independent of Its Translation

Abstract

We have previously shown that the steady-state level of c-myc mRNA in vivo is primarily controlled by posttranscriptional regulatory mechanisms. To identify the sequences involved in this process, we constructed a series of H-2/myc transgenic lines in which various regions of the human c-MYC gene were placed under the control of the quasi-ubiquitous H-2K class I regulatory sequences. We demonstrated that the presence of one of the two coding exons, exon 2 or exon 3, is sufficient to confer a level of expression of transgene mRNA similar to that of endogenous c-myc in various adult tissues as well as after partial hepatectomy or after protein synthesis inhibition. We now focus on the molecular mechanisms involved in modulation of expression of mRNAs containing c-myc exon 2 sequences, with special emphasis on the coupling between translation and c-myc mRNA turnover. We have undertaken an analysis of expression, both at the mRNA level and at the protein level, of new transgenic constructs in which the translation is impaired either by disruption of the initiation codon or by addition of stop codons upstream of exon 2. Our results show that the translation of c-myc exon 2 is not required for regulated expression of the transgene in the different situations analyzed, and therefore they indicate that the mRNA destabilizing function of exon 2 is independent of translation by ribosomes. Our investigations also reveal that, in the thymus, some H-2/myc transgenes express high levels of mRNA but low levels of protein. Besides the fact that these results suggest the existence of tissue-specific mechanisms that control c-myc translatability in vivo, they also bring another indication of the uncoupling of c-myc mRNA translation and degradation.

Healthy mice with an altered c-myc gene: role of the 3′ untranslated region revisited

“…Although the polII promoter is far less e cient than the c-myc promoters, the intensities of the polIIneo signals are not signi®cantly di erent between Myc* neo3'myc or Myc* neo3'SV tissues Table 2 mycneo3'myc/ mycneo3'SV/ Genotype tissue/cell +/mycneo3'myc +/mycneo3'SV mycneo3'myc mycneo3'SV Myc primers 1 ± 3 1 ± 4 1 ± 3 1 ± 4 1 ± 3 1 ± 4 1 ± 3 1 ± 4 Spleen TBP 123 113 100 92 252 223 204 225 P0 45 80 92 85 241 213 139 153 Thymus TBP 80 75 97 77 201 190 119 138 P0 112 104 119 99 255 242 140 163 See legend to Table 1 Targeted c-myc 3' UTR modifications F Langa et al myc RNA half-life might be ensured by the determinants located within the coding region. This hypothesis is supported by the studies highlightning the role of exon 2 and/or exon 3 in the control of c-myc mRNA stability ex vivo (Bernstein et al, 1992;Herrick and Ross, 1994;Yeilding and Lee, 1997;Yeilding et al, 1996) or in vivo (Lavenu et al, 1995;Pistoi et al, 1996).…”

Developmental expression of AUF1 and HuR, two c-myc mRNA binding proteins

“…Like AUF1 and HuR, CRD-BP is expressed during liver development but absent in both quiescent adult liver and regenerating liver (Leeds et al, 1997). Both coding region determinants in c-myc mRNA were shown to act in vivo independently of the 3'-UTR instability determinant (Lavenu et al, 1995;Pistoi et al, 1996). The level of mRNA expression we observe in a cell at a particular stage of di erentiation and in a given environment is the result of the interplay between these di erent sequences and their binding factors.…”

“…The use of H-2K/myc transgenic mice expressing c-myc transgenes under the control of the constitutive H-2K promoter has con®rmed that c-myc mRNA expression is mainly controlled post-transcriptionally in the various situations analysed (Morello et al, 1989. Control of c-myc mRNA stability has been hypothetized to be mainly responsible for di erential accumulation of cmyc transcripts (Pistoi et al, 1996). The pattern of cmyc expression in various tissues was analysed by S1 mapping using a c-myc speci®c probe (see Materials and methods).…”

Purification and Characterization of a Polysome-associated Endoribonuclease That Degrades c-myc mRNA in Vitro

“…c-myc mRNA levels increase transiently and then decrease during the liver regeneration process that occurs following partial hepatectomy in adult rodents (20,21). These fluctuations in c-myc mRNA abundance are regulated post-transcriptionally and are dependent on the c-myc mRNA coding region, not the c-myc gene promoter (22)(23)(24)(25). (iv) The CRD is an mRNA instability element in cells independent of other c-myc mRNA regions.…”

“…(i) c-myc mRNA abundance is regulated post-transcriptionally in rodent liver cells during fetal development and during liver regeneration (20,21). The c-myc coding region is required for proper regulation to occur (22)(23)(24)(25)(26). Therefore, it seemed reasonable to search for an endonucleolytic c-myc mRNase in liver tissue.…”