Expression of the L-myc gene is under positive control by short-lived proteins.

The c-myc oncogene is the most extensively studied member of the myc gene family which now consists of three characterized members, namely the c-myc, N-myc and L-myc genes. These genes are often found amplified in cell lines from small cell carcinomas of the lung (SCLC). In this study the L-myc gene was examined in a panel of human SCLC cell lines and tumors. One of the cell lines (U-1690) expressed abundant L-myc RNA and had an about 40-fold amplification of the L-myc gene. In contrast, no amplifications were found in clinical tumor material of ten SCLC specimens. The regulation of L-myc gene expression was studied with regard to mRNA stability and transcriptional control. The two L-myc transcripts observed in U-1690 cells were found to have different half-lives. Unlike c-myc mRNA in other cells, no significant stabilization of the L-myc mRNA occurred when protein synthesis was inhibited. Instead, transcription of the L-myc gene was found to be dependent on the synthesis of short-lived proteins.