Erythropoietin and IL-3 receptor cell surface expression is decreased under conditions that model some aspects of microgravity.

Human and experimental animals in space develop the anemia of space flight (Tavassoli, M., 1982; Udden et al., 1995). Studies have shown that erythropoietin (Epo) is implicated in this microgravity-induced abnormality (Udden et al., 1995). The status of the Epo receptor (EpoR) in microgravity, however, is unknown. The Rotary Cell Culture System (RCCS), based on NASA rotating wall vessel (RWV) technology and manufactured by Synthecon Inc., is an in vitro culture system. Within this system individual cells, to a certain extent, experience an environment with similarities to true microgravity (Gao et al., 1997; Battle et al., 1999). BaF3 cells stably expressing the transfected human Epo receptor (BaF3EpoR cells) were cultured in RPMI-1640 supplemented with 5% fetal bovine serum and either 1 unit/mL of recombinant human Epo (rhEpo) or 5 ng of recombinant mouse interleukin-3/mL (rmIL-3) in either the RCCS or in 175 cm standard tissue culture flasks (control) at 37 C in a humidified atmosphere of 95% air/5% CO2, 37 C for 48 hours. Cells were then harvested by centrifugation, incubated with I-labeled rhEpo or I-labeled rmIL-3, and the bound/free ligands were separated by centrifuging the cells through a serum cushion according to published methods (Yonekura, 1991). The radioactivity of the cell pellet was quantified by gamma scintillation spectrometry. Starting from 0.1 nM of I-Epo (the lowest concentration tested) upward, the specific binding of radiolabeled ligand to flask-cultured cells ( ) was significantly greater than that to RCCS-cultured cells ( ) (p<0.01) (Figure 1). At 3.2 nM (the highest concentration tested), the bound radioactivity (surface EpoR density) of RCCS-cultured cells was only 56.6% of that of the flaskcultured counterpart. The non-specific binding of radiolabeled Epo to the flask( ) and RCCS ( )cultured cells were similar.