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Growth In Modeled/Simulated Microgravity Reduces Cell-Surface Erythropoietin  Receptors By Enhancing Receptor-Mediated Endocytosis.  K. Xu, L. Feldman, K.L. Davis and A.J. Sytkowski.   The Laboratory for Cellular and Molecular Biology, Division of Hematology and Oncology, Beth Israel Deaconess Medical Center, Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215

   Probing the differential expression of the erythropoietin receptor (EpoR) under low shear conditions (LSC) (modeled/simulated microgravity) and static culture conditions (SCC) should provide useful insights into mechano- and gravisensing in mammalian cells. In our current studies, Ba/F3-EpoR cells were cultured: 1) under LSC (in the NASA-designed Rotating Wall Vessel) or SCC (in flasks);    2) at low density (LD, 5.0 x 105 cells/mL) or high density (HD, 1.0 x 106 cells/mL); and 3) at low (1 mg/mL) or high (2 mg/mL) glucose.  The binding of radiolabeled recombinant human erythropoietin (125I-rhEpo) and interleukin-3 (125I-IL-3) to the cell was measured, and northern blotting was used to detect the expression of Epo mRNA by these cells. We observed that: 1) at HD  125I-Epo and 125I-IL-3 binding under LSC were only half of that under SCC (p0.01); 2) at HD the number of 125I-rhEpo binding sites/cell was significantly lower (p0.05) under LSC than under SCC; 3) EpoR internalization under SCC was significantly less (p0.05) than under LSC at HD;              4) glucose level did not affect 125I-rhEpo binding under LSC at HD; 5) no effect on EpoR mRNA expression by shear force or cell density existed; and 6) culture vessels were not the cause of the differences. In conclusion, culture in RWV modeled microgravity reduced surface EpoR number at HD. The reduction was due to enhanced receptor-mediated endocytosis.

(Supported by NASA: NAG9-1368 and NAG2-1592.)