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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.

Etoposide (VP16) is a potent inducer of DNA double-strand breaks (DSBs) and is efficiently used in small cell lung cancer (SCLC) therapy. However, acquired VP16 resistance remains an important barrier to effective treatment. To understand the underlying mechanisms for VP16 resistance in SCLC, we investigated DSB repair and cellular VP16 sensitivity of SCLC cells. VP16 sensitivity and RAD51, DNA-PK(cs), topoisomerase IIalpha and P-glycoprotein protein levels were determined in 17 SCLC cell lines. In order to unravel the role of RAD51 in VP16 resistance, we cloned the human RAD51 gene, transfected SCLC cells with RAD51 sense or antisense constructs and measured the VP16 resistance. Finally, we measured VP16-induced DSBs in the 17 SCLC cell lines. Two cell lines exhibited a multidrug-resistant phenotype. In the other SCLC cell lines, the cellular VP16 resistance was positively correlated with the RAD51 protein level. In addition, downregulation or overexpression of the RAD51 gene altered the VP16 sensitivity. Furthermore, the levels of the RAD51 and DNA-PK(cs) proteins were related to VP16-induced DSBs. The results suggest that repair of VP16-induced DSBs is mediated through both RAD51-dependent homologous recombination and DNA-PK(cs)-dependent nonhomologous end-joining and may be a determinant of the variation in clinical treatment effect observed in human SCLC tumors of identical histologic subtype. Finally, we propose RAD51 as a potential target to improve VP16 efficacy and predict tumor resistance in the treatment of SCLC patients.

We investigated the influence of cellular heterogeneity on the response to low-dose BCNU chemotherapy of an artificially mixed human small-cell lung cancer (SCLC) xenograft in nude mice containing a BCNU-sensitive and dominating sub-population and a BCNU-resistant and undetectable (dominated) sub-population. The cell lines differed in DNA content, making them distinguishable by DNA flow cytometry (FCM). After 3 weeks of tumor growth, the mice were stratified according to tumor size and randomized to 2 different low-dose treatments with BCNU or no treatment. After a further 3 to 4 weeks, a high-dose treatment (LD10) was given to both groups of treated tumors. Changes in the relative proportions of and cell lines in the tumors were measured by FCM on fine-needle tumor aspirates. At the time of low-dose treatment, all the tumors were totally dominated by the sensitive cells. A temporary response was seen after low-dose treatment. After the highdose treatment, a similar short response was seen. In the non-treated group, the sensitive cells continued to dominate. At the time of tumor regrowth after the low-dose treatment, most of the tumors continued to be dominated by the sensitive population. At the time of progression after the response to the high-dose treatment, the resistant cell line was the predominant population. If compared with a single high-dose BCNU treatment, the response of tumors treated with a low dose was superior, indicating that the presence of a dominating and slower growing sub-population influenced the outcome of the treatment. © 1994 Wiley-Liss, Inc.

A study was made on the effect of ovariectomy, 17 beta-oestradiol, and tamoxifen on the oestrogen and progesterone receptor-positive T61 human breast carcinoma grown in nude mice. The effect of the treatment was evaluated by the specific growth delay calculated on the basis of Gompertz growth curves, and by the changes in the cell cycle distribution monitored by flow cytometric DNA analysis. The results demonstrated that both oestradiol and tamoxifen induced a temporary growth delay, whereas ovariectomy of the host had no effect on the growth of the tumour. The oestradiol-induced tumour growth delay was accompanied by a decrease in the G1 fraction, an accumulation of cells in the S-phase, and polyploidy, whereas neither treatment with tamoxifen nor ovariectomy influenced cell cycle distribution. The results indicate that oestradiol and tamoxifen have different mechanisms of action. In addition, they were interpreted as indicating different mechanisms regulating ovarian-dependent tumour growth, on the one hand, and oestrogen and tamoxifen-induced tumour growth inhibition, on the other. The results support the view that the presence of receptors may be of importance but is not a sufficiently clear marker to allow prediction of the endocrine sensitivity of individual breast tumours.