Enforced expression of the c-myc oncogene inhibits cell differentiation by precluding entry into a distinct predifferentiation state in G0/G1.

Abstract

A broad base of data has implicated a role for the c-myc proto-oncogene in the control of the cell cycle and cell differentiation. To further define the role of myc in these processes, I examined the effect of enforced myc expression on several events that are thought to be important steps leading to the terminally differentiated state:(i) the ability to arrest growth in GWG1, (ii) the ability to replicate the genome upon initiation of the differentiation program, and (iii) the ability to lose responsiveness to mitogens and withdraw from the cell cycle. 3T3-L1 preadipocyte cell lines expressing various levels of myc mRNA were established by transfection with a recombinant myc gene under the transcriptional control of the Rous sarcoma virus (RSV) promoter.Cells that expressed high constitutive levels of pRSVmyc mRNA arrested in GO/G1 at densities siiMilar to those of normal cells at confluence. Upon initiation of the diffekrentiation program, such cells traversed the cell cycle with kinetics similar to those of normal cells and subsequently arrested in G0/Gl. Thus, enforced expression of myc had no effect on the ability of cells to arrest growth in G0/G1 or to replicate the genome upon initiation of the differentiation program. Cells were then tested for their ability to reenter the cell cycle upon exposure to high concentrations of serum and for their capacity to differentiate. In contrast to normal cells, cells expressing high constitutive levels of myc RNA reentered the cell cycle when challenged with 30% serum and failed to terminally differentiate. The block to differentiation could be reversed by high expression of myc antisense RNA, showing that the induced block was specifically due to enforced expression of pRSVmyc. These findings indicate that 3T3-L1 preadipocytes enter a specific state in GWG1 after treatment with differentiation inducers, into which cells expressing high constitutive levels of myc RNA are precluded from entering. I propose that myc acts as a molecular switch and directs cells to a pathway that can lead to continued proliferation or to terminal differentiation.

Direct association of YY-1 with c-Myc and the E-box binding protein in regulation of glycophorin gene expression

“…(b) EMSA was performed with 32 P-labeled EY and competed with EY (lanes 2 ± 4), EDY (lanes 5 ± 7), DEY (lanes 8 ± 10) and DEDY (lanes 11 ± 13) (c) EMSA was performed with 32 P-labeled EDY and competed with EY (lanes 2 ± 4), EDY (lanes 5 ± 7), DEY (lanes 8 ± 10) and DEDY (lanes 11 ± 13). The nucleotide sequences of the oligonucleotides used in this experiment is described in Table 1 (Spotts and Hann, 1990;Lachman and Skoultchi, 1984) and many other cell lines (Maruyama et al, 1987;Freitag, 1988;Miner and Wold, 1991; and expression of glycophorin gene as well as globin genes was inhibited by overexpression of c-Myc during MEL cell di erentiation (Shoji et al, 1993). We demonstrated here a physical direct interaction of c-Myc with YY-1 in MEL cells.…”

Patterns of methylation of the c-myc gene in human colorectal cancer progression

“…Over-expressed myc protein in dysplastic and tumour cells, accumulating in the cytoplasm and transferring continuously to the nucleus, may alter cellular response to growth factors and abrogate normal growth control mechanisms by preventing cells from escaping from the proliferation cycle (Freytag, 1988). C-myc protein may also control its own expression by binding, directly or indirectly, to the c-myc gene; if this interaction is affected by DNA methylation (see Prendergast & Ziff, 1991), there may be a feedback effect between hypomethylation of the third exon of myc and deregulation of expression.…”

Repression of c-Myc responsive genes in cycling cells causes G1 arrest through reduction of cyclin E/CDK2 kinase activity

“…Activation of c-myc is su cient to induce cell cycle entry in quiescent cells (Eilers et al, 1991). Moreover, constitutive expression of c-myc inhibits di erentiation and prevents cells from leaving the cell cycle (Coppola and Cole, 1986;Freytag, 1988). Conversely, inhibition of c-myc expression using an antisense approach leads to growth arrest and induction of di erentiation (Heikkila et al, 1987;Prochownik et al, 1988;Biro et al, 1993).…”