Thrombopoietin derived from human embryonic kidney cells stimulates an increase in DNA content of murine megakaryocytes in vivo

Abstract

A thrombocytopoiesis-stimulating factor (TSF or thrombopoietin) is known to increase the size and number of mouse bone marrow megakaryocytes, to increase the proportion of megakaryocytes in endomitosis, and to increase the number of small acetylcholinesterase-positive cells. Megakaryocyte ploidy values have not previously been measured in mice treated with TSF from human embryonic kidney (HEK) cells. Therefore, in the present study C3H mice were injected with approximately 4 U of step II TSF; platelet production indices and megakaryocyte ploidy values were measured 1-5 days after treatment. For controls, other mice were injected with saline, human serum albumin (HSA), normal rabbit serum (NRS), or rabbit anti-mouse platelet serum (RAMPS). Platelet counts, platelet sizes, and percent 35S incorporation into platelets were measured using standard techniques. For measurement of megakaryocyte DNA content, bone marrow cells were collected into CATCH medium and incubated with RAMPS, followed by labeling with a saturating concentration of fluorescein-conjugated goat anti-rabbit immunoglobulin F(ab')2 fragment. After washing, the cells were resuspended in propidium iodide, and DNA content was measured by flow cytometry. When compared to suitable control values, the results showed that TSF caused a significant (p less than 0.025) increase in platelet counts of treated mice by 3 days; both TSF and RAMPS caused significant (p less than 0.0005) increases in platelet sizes and percent 35S incorporation into platelets of mice at 2 and 3 days after treatment. The most frequent polyploid DNA class of megakaryocytes from untreated C3H mice was 32N, confirming our previous observation. Both TSF and RAMPS caused significant (p less than 0.0005) increases in the average polyploid megakaryocyte DNA content, with peak values on days 2 and 3. These data show that TSF administered in vivo significantly increases DNA content of mouse bone marrow megakaryocytes.