EXPRESSION OF MYC FAMILY ONCOPROTEINS IN SMALL-CELL LUNG-CANCER CELL-LINES AND XENOGRAFTS

A number of genes have altered activity in small-cell lung cancer (SCLC), but especially genes of the myc family (c-myc, L-myc and N-myc) are expressed at high levels in SCLC. Most studies have explored expression at the mRNA level, whereas studies of myc family oncoprotein expression are sparse. We examined the expression of myc proto-oncogenes at the mRNA and protein level in 23 cell lines or xenografts. In the cell lines, the doubling time and the cell-cycle distribution, as determined by flow-cytometric DNA analysis, were examined to establish whether the level of myc-gene-family expression correlated with proliferative parameters. All tumours expressed at least one myc family member at the mRNA level. Exclusive c-myc mRNA expression was demonstrated in 8 tumours, L-myc in 7 and N-myc in I. Five tumours expressed both c-myc and L-myc, and 2 tumours expressed both c-myc and N-myc. In general, the level of expression of c-myc and N-myc was similar at the mRNA and the protein level. Expression of c-myc was positively correlated with the proliferative index (sum of S and G2 + M phases) of cell lines, but not with the population doubling time. in general, L-myc-expressing cell lines had a low proliferative index. There was no systematic difference in myc expression between cell lines and xenografts of individual tumours.