Mouse stromal cell line MS-5 inhibits proliferation and prevents apoptotic cell death of leukemhc cells line K562

The objective of this study was to evaluate the influence of the stromal microenvironment on the prolifération and apoptosis of the hum leukemic ceils As a model mouse stromal cell line MS-5 known to be able to support human hemopoietic cells (kindly provided by Dr. Itoh) and human erythromyeloid leukemic cell line K562 were used. The increase of aie number of K362 cells was reduced by cocultivation with stromal cells in comparison to control (threefold vs. tenfold after one week, res., p<O.OOI). Cell viability was more than 96% in both groups. Colony formation of K562 cells was also significantly less (7,2 vs. 22 colonies per 10 cells after 1 week, res.). This effect was reproduced, when 0,5% agar layer was placed between stromal and leukemic cells (4.03±0.61xlOVml vs. 9.7l±2.I2xloVmlafter 1 week, res.,p<0.00l)and when leukemic cells were cultured in liquid culture on agar layer which contained SO % condition medium <8.31±1.89xlOs/ml vs. 15.28±2.35xlOs/ml, res., p<0.005). Apoptosis of leukemic cells, induced by serum deprivation, as significantly lower by cocultivation with MS-5 than in control, was detected by acrujin-orange staining (14.33±3.69% in nonadhercnt fraction vs. 7 S. 7 5 ±7 64% in control, p<0.00t ; and only 4% of adherent cells showed morphological features characteristic for programmed cell death after 72 h of culture). The bcl-2 protein was not detected in K562 cells, cultured both with or without stromal cells. In conclusion, this result suggests that prevention of apoptosis of leukemic cells by cocultivation with stromal cells was mediated through mechanism not involving bcl-2 gene pathway; stromal cell mediated inhibition of proliferation was conducted by humoral mechanisms.