Fig. 1

The factor stimulating the growth of small tumor cells population is found in the conditioned medium (CM) from dying tumor cells following chemotherapy. (A) Tumor cells treated with etoposide continued to die even in the absence of a chemotherapeutic drug. Mouse colon carcinoma CT-26 cells were treated with 3 µM etoposide for 24 h, then cells were washed and allowed to be in culture. Cell viability was evaluated each 24 h with aid of MTT assay. * p < 0.05, **p < 0,001. (B) Scheme of in vivo experiments with CT-26 cells: CT-26wt cells were treated with 3µM etoposide for 24 h, then were washed out and mixed with CT-26luc in a ratio of 106:2 × 104 and subcutaneously inoculated to Balb/c mice. Other mice groups received either 2 × 104 CT-luc cells or 106 dying CT-26wt (data not shown) cells. (C) CT-26luc tumors were evaluated with the aid of IVIS Spectrum imaging system on day 28. (D) Luminenscence count of tumor lesions from (C), *p < 0.05. (E) Scheme of in vivo experiments with A549 cells: A549 cells were treated with 100µM etoposide for 18 h, then washed out and for the next 48 h were kept without drug. Then, CM from these cells was collected and added to naive A549-luc cells for 96 h. Naive A549luc and A549luc were subcutaneously injected to Balb/c Nude mice. (F) A549-luc tumor growth from naive A549-luc and A549-luc cells incubated with CM was estimated using IVIS Spectrum imaging system on day 28. (G) Luminescence count of tumor lesions from D. *p < 0.05. (H) A549wt cells were seeded into wells of E-plates at a concentration of 500 cells per well and were allowed to grow in CM from untreated A549 cells, or with full (100%) or half-diluted (50%) Eto-CM. Recording on xCELLigence equipment lasted 180 h. (I) Data of PGE2 measurement in the CM of untreated A549 cells and Eto-CM. ** p < 0.001