Thymidylate synthase as a determinant of pemetrexed sensitivity in non-small cell lung cancer

Abstract

Background: Although a high level of thymidylate synthase (TS) expression in malignant tumours has been suggested to be related to a reduced sensitivity to the antifolate drug pemetrexed, no direct evidence for such an association has been demonstrated in non-small cell lung cancer (NSCLC). We have now investigated the effect of TS overexpression on pemetrexed sensitivity in NSCLC cells.

Methods: We established NSCLC cell lines that stably overexpress TS and examined the effects of such overexpression on the cytotoxicity of pemetrexed both in vitro and in xenograft models. We further examined the relation between TS expression in tumour specimens from NSCLC patients and the tumour response to pemetrexed by immunohistochemical analysis.

Results: The sensitivity of NSCLC cells overexpressing TS to the antiproliferative effect of pemetrexed was markedly reduced compared with that of control cells. The inhibition of DNA synthesis and induction of apoptosis by pemetrexed were also greatly attenuated by forced expression of TS. Furthermore, tumours formed by TS-overexpressing NSCLC cells in nude mice were resistant to the growth-inhibitory effect of pemetrexed observed with control tumours. Finally, the level of TS expression in tumours of non-responding patients was significantly higher than that in those of responders, suggestive of an inverse correlation between TS expression and tumour response to pemetrexed.

Conclusion: A high level of TS expression confers a reduced sensitivity to pemetrexed. TS expression is thus a potential predictive marker for response to pemetrexed-based chemotherapy in NSCLC patients.

Figure 1

Abundance and enzymatic activity of TS in TS-overexpressing NSCLC cell lines. Parental (P) A549, H1299, or PC9 cells or corresponding sublines either stably overexpressing TS (TS1 and TS2) or harbouring the empty vector (Mock) were cultured overnight in complete medium, after which cell lysates were prepared and either subjected to immunoblot analysis with antibodies to TS and to β-actin (loading control) (A) or assayed for TS activity. (B) Data are means±s.d. of triplicates from experiments that were repeated on two additional occasions with similar results. ^*P<0.05 vs the corresponding value for Mock cells (Student's two-tailed t-test).

Figure 2

Effect of TS depletion on pemetrexed sensitivity in TS-overexpressing NSCLC cells. (A) Cells of the indicated lines were transfected or not (−) with nonspecific (Con) or TS siRNAs for 48 h, after which cell lysates were subjected to immunoblot analysis with antibodies to TS and to β-actin. (B) Cells transfected as in A were cultured for 72 h in complete medium containing various concentrations of pemetrexed, after which cell viability was assessed as described in Materials and methods section, and the median inhibitory concentration (IC₅₀) of pemetrexed was determined. Data are means of triplicates from experiments that were repeated on two additional occasions with similar results. ^*P<0.05 (Student's two-tailed t-test).

Figure 3

Effects of pemetrexed and cisplatin on DNA synthesis in NSCLC cells overexpressing TS. The indicated NSCLC cell lines were cultured for 48 h in complete medium containing various concentrations of pemetrexed (A) or cisplatin (B), after which BrdU incorporation was assessed as described in Materials and methods section. Data are means±s.d. of triplicates from experiments that were repeated a total of three times with similar results. ^*P<0.05 vs the corresponding value for Mock cells (Student's two-tailed t-test). NS, not significant.

Figure 4

Effects of pemetrexed and cisplatin on apoptosis in NSCLC cells overexpressing TS. (A and C) The indicated NSCLC cell lines were cultured for 72 h in complete medium containing various concentrations of pemetrexed (A) or cisplatin (C), after which the proportion of apoptotic cells was assessed by staining with fluorescein isothiocyanate-conjugated annexin V and propidium iodide followed by flow cytometry. (B) The indicated NSCLC cell lines were cultured for 72 h in complete medium containing the indicated concentrations of pemetrexed with or without nonspecific (control) or TS siRNAs, after which the proportion of apoptotic cells was assessed as in A and C. All data are means±s.d. of triplicates from experiments that were repeated a total of three times with similar results. ^*P<0.05 (Student's two-tailed t-test). NS, not significant.

Figure 5

Effect of pemetrexed on the growth of TS-overexpressing NSCLC cells in vivo. Nude mice with tumour xenografts established by subcutaneous implantation of tumour fragments derived from the indicated NSCLC cell lines were treated with vehicle (control) or pemetrexed (100 mg kg⁻¹, intraperitoneal) on days 1, 8, 15, and 22. Tumour volume was determined at the indicated times after the onset of treatment. Data are means±s.e.m. of values from eight mice per group. ^*P<0.05 for pemetrexed vs the corresponding value for vehicle (Student's two-tailed t-test).

Figure 6

Relation of TS expression level to tumour response in NSCLC patients treated with pemetrexed and either carboplatin or cisplatin. (A) Representative sections of carcinomas including cells with the indicated intensities of TS immunostaining. Scale bars, 125 μm. (B) TS expression level (HSCORE) for the clinical specimens of 24 patients classified according to tumour response (response=CR or PR, n=7; non-response=SD or PD, n=17). Horizontal lines indicate mean values. The P value was determined by Student's two-tailed t test. (C) Progression-free survival of the NSCLC patients according to the expression level of TS in tumour specimens. The P-value was determined with the log-rank test.