Effect of the irradiated microenvironment on the expression and retrotransposition of intracisternal type A particles in hematopoietic cells

Objective. We have previously demonstrated that the frequency of transforma tion of the factor-dependent hematopoietic cell line FDCP-1JL26 was dramati cally increased when cells were cocultured with the irradiated bone marrow cell line D2XRII. In many of our factor-independent subclonal cell lines th at we examined, transformation to factor independence appeared to be due to the retrotransposition of intracisternal type A particles (IAP) into the g rowth factor genes that are normally required for survival and growth of FD CP-1JL26 cells. To determine the role of the irradiated microenvironment in the evolution of factor-independent cells, we have examined the expression and retrotransposition of IAPs after exposure to the irradiated bone marro w stromal cell line D2XRII. Methods and Results. Differential display and Northern blot analysis demons trated that IAPs were overexpressed in a nonautocrine factor-independent su bclonal cell line, F17CL2. The frequency of retrotransposition was determin ed by the introduction of the IAP-neo(RT) plasmid into FDCP-1JL26 cells. Th e IAP-neo(RT) contains a neomycin resistance gene (neo) that only becomes a ctive after retrotransposition, and thus the frequency of retrotranspositio n in FDCP-1JL26 cells was quantified hy determining the frequency of neo-re sistant cells. Conclusions. No significant increases in the expression of IAPs were observ ed after the cells were exposed to the irradiated stromal cells. This obser vation is in agreement with the observation that no increase in the frequen cy of retrotransposition could be detected.These results suggest that the i rradiated bone marrow may have a passive role in the selection of factor-in dependent cells. During cocultivation, bane marrow stromal cells may provid e a factor(s) to hematopoietic cells that allow it to survive in medium lac king IL-3. At random, a retrotransposition may occur that provides a select ive advantage to the hematopoietic cells. In the absence of the irradiated stromal cells, the hematopoietic cells are perhaps more likely to die and t herefore are not available for a random retrotransposition event to occur. This model is to be distinguished from an active role in which the irradiat ed microenvironment would synthesize or activate a factor(s) that promotes retrotransposition. (C) 2000 International Society for Experimental Hematol ogy. Published by Elsevier Science Inc.