Both Coding Exons Of The c-myc Gene Contribute to Its Posttranscriptional Regulation in the Quiescent Liver and Regenerating Liver and after Protein Synthesis Inhibition

Abstract

In vivo, the steady-state level of c-myc mRNA is mainly controlled by posttranscriptional mechanisms. Using a panel of transgenic mice in which various versions of the human c-myc proto-oncogene were under the control of major histocompatibility complex H-2K b class I regulatory sequences, we have shown that the 5 and the 3 noncoding sequences are dispensable for obtaining a regulated expression of the transgene in adult quiescent tissues, at the start of liver regeneration, and after inhibition of protein synthesis. These results indicated that the coding sequences were sufficient to ensure a regulated c-myc expression. In the present study, we have pursued this analysis with transgenes containing one or the other of the two c-myc coding exons either alone or in association with the c-myc 3 untranslated region. We demonstrate that each of the exons contains determinants which control c-myc mRNA expression. Moreover, we show that in the liver, c-myc exon 2 sequences are able to down-regulate an otherwise stable H-2K mRNA when embedded within it and to induce its transient accumulation after cycloheximide treatment and soon after liver ablation. Finally, the use of transgenes with different coding capacities has allowed us to postulate that the primary mRNA sequence itself and not c-Myc peptides is an important component of c-myc posttranscriptional regulation.In recent years, it has been realized that the rates of RNA decay in the nucleus and in the cytoplasm are important in determining the level of expression of a gene (reviewed in reference 5). In eukaryotic cells, the range of mRNA stability can vary over several orders of magnitude, and numerous studies have indicated that several mRNAs contain within them the information necessary to determine their stability. Transiently expressed genes such as those encoding proto-oncogenes and growth factors have far shorter half-lives than others, such as those encoding ␤-globin or albumin (15,32). The mechanisms of differential degradation are an attractive problem which covers several distinct aspects, such as analysis of the structural features of mRNA that determine its susceptibility to decay, identification of the locations of the destabilizing determinants, determination of the trans-acting factors with which they interact, and discovery of the physiological signals which alter rates of mRNA decay (reviewed in reference 30).

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.…”

“…One is localized to exon 2 (Lavenu et al, 1995;Yeilding and Lee, 1997;Yielding et al, 1996), and another one to exon 3 at the most C-terminal 60 amino acids (Wisdom and Lee, 1991). This region of c-myc mRNA interacts with CRD-BP, a protein that shields it from endonucleolytic attack in vitro (Bernstein et al, 1992;Herrick and Ross, 1994).…”

Developmental regulation of CRD-BP, an RNA-binding protein that stabilizes c-myc mRNA in vitro

“…This model could account for the rapid uctuations in c-myc mRNA abundance observed during cell di erentiation. While this mechanism for CRD-BP action has not been con®rmed, it is strengthened by studies demonstrating that the CRD a ects c-myc mRNA metabolism in intact cells Lavenu et al, 1995;Morello et al, 1993;Wisdom and Lee, 1991;Yeilding et al, 1996).…”

“…(ii) It is also stabilized fourfold by 6 h posthepatectomy . (iii) c-myc upregulation during liver regeneration depends only on sequences within the coding region, because c-myc transgenes driven by a constitutive promoter and lacking the 3'-and 5'-untranslated regions (UTR's) are regulated in parallel with the endogenous c-myc gene (Lavenu et al, 1995;Morello et al, 1990aMorello et al, ,b, 1993Sobczak et al, 1989). In other words, transgenederived mRNA containing only the c-myc coding region is undetectable in resting adult liver but is induced within 1 h following partial hepatectomy.…”

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

“…c-myc ARE was shown to be actively involved in the regulated expression of c-myc in cultured cells (Alberta et al, 1994;Brewer and Ross, 1989;Cole and Mango, 1990;Herrick and Ross, 1994) and review in (Chen and Shyu, 1995). However, other non-ARE elements exist in the coding regions of c-myc mRNA which play an important role in the control of mRNA stability, ex vivo (Yeilding and Lee, 1997) and in vivo (Lavenu et al, 1995).…”