Methionine Deprivation Regulates the Translation of Functionally-Distinct c-Myc Proteins

Abstract

Numerous studies have demonstrated a critical role for the c-myc gene in the control of cellular growth. Alterations of the c-myc gene have been found associated with many different types of tumors in several species, including humans. The increased synthesis of one of the major forms of c-Myc protein, c-Myc 1, upon methionine deprivation provides a link between the regulation of oncogenes and the nutritional status of the cell. While deregulation or overexpression of the other major form, c-Myc 2, has been shown to cause tumorigenesis, the synthesis of c-Myc 1 protein is lost in many tumors. This suggests that the c-Myc 1 protein is necessary to keep the c-Myc 2 protein “in check” and prevent certain cells from becoming tumorigenic. Indeed, we have shown that overproduction of c-Myc 1 can inhibit cell growth. We have also shown that c-Myc 1 and 2 proteins have a differential molecular function in the regulation of transcription through a new binding site for Myc/Max heterodimers. We have also recently identified new translational forms of the c-Myc protein which we term Δ-c-Myc. These proteins arise from translational initiation at downstream start sites which yield N-terminally-truncated c-Myc proteins. Since these proteins lack a significant portion of the transactivation domain of c-Myc, they behave as dominant-negative inhibitors of the full-length c-Myc 1 and 2 proteins. The synthesis of Δ-c-Myc proteins is also regulated during cell growth and is repressed by methionine deprivation. Therefore, the synthesis of c-Myc 1 and Δ-c-Myc proteins are reciprocally regulated by methionine availability. We have also found some tumor cell lines which synthesize high levels of the Δ-c-Myc proteins. Taken together, our data suggest that c-Myc function is dependent on the levels of these different translational forms of c-Myc protein which are regulated by the nutritional status of the cell during growth.

Numerous reports have demonstrated a fundamental and diverse role for the myc gene in cellular events, including proliferation, differentiation and apoptosis (Cole 1986; Spencer and Groudine 1991; Askew et al. 1991; Evan et al. 1992). This is dramatically illustrated by the frequent occurrence of a variety of tumors in many species having alterations of myc genes and the transduction of c-myc sequences by retroviruses (Spencer and Groudine 1991). The diverse biological activity of myc is demonstrated by its ability to contribute to cellular proliferation (Spencer and Groudine 1991), inhibit terminal differentiation (Cole 1986), and promote apoptosis (Evan et al. 1992). Despite intensive study, however, the mechanism by which Myc proteins perform such diverse cellular roles is unknown (Luscher and Eisenman 1990).

A distinctive feature of the myc gene is that it encodes multiple N-terminally-distinct proteins. Alternative translational forms of the Myc protein exist for all species of c-Myc examined thus far (Hann and Eisenman 1984; Hann et al, 1988), as well as for N-Myc (Ramsay et al. 1986) and L-Myc proteins (Dosaka-Akita et al. 1991). The c-Myc 1 and 2 proteins have been found in all vertebrate species examined (Hann et al. 1988). In mammalian and avian cells, c-Myc 1 protein arises from an upstream non-AUG translational start site and thus contains an N-terminal extension of 14 amino acids compared to c-Myc 2 protein (Hann et al. 1988). Recently we have found that human, murine and avian cells also express smaller-sized c-Myc proteins to the full-length c-Myc 1 and 2 proteins (Spotts and Hann unpublished). These smaller-sized proteins, which we term Δ-c-Myc proteins, arise from translational initiation at a doublet of AUG codons downstream of the initiation sites for c-Myc 1 and 2 yielding proteins lacking the first 100 amino acids of c-Myc 2. Figure 1 diagrams the initiation of the different c-Myc proteins.