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 (SCL C), but especially genes of the myc family (c-myc, L-myc and N-myc) ar e expressed at high levels in SCLC. Most studies have explored express ion at the mRNA level, whereas studies of myc family oncoprotein expre ssion are sparse. We examined the expression of myc proto-oncogenes at the mRNA and protein level in 23 cell lines or xenografts. In the cel l lines, the doubling time and the cell-cycle distribution, as determi ned by flow-cytometric DNA analysis, were examined to establish whethe r the level of myc-gene-family expression correlated with proliferativ e 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, t he 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, bu t not with the population doubling time. in general, L-myc-expressing cell lines had a low proliferative index. There was no systematic diff erence in myc expression between cell lines and xenografts of individu al tumours.