[Changes in cyclin expression during proliferation and differentiation of CD34(+) cells derived from fetal liver induced by thrombopoietin]

In order to elucidate the intrinsic mechanism underlying proliferation and differentiation of megakaryocytes during ontogenesis, CD34(+) cells were isolated from human fetal liver (FL) with a high-gradient magnetic sorting system (MACS) and were incubated in liquid suspension with 50 and 100 ng/ml of thrombopoietin (TPO) and in MegaCult(Tm) -C semi-solid culture system with 0, 12.5, 25, 50, 100, and 200 ng/ml of TPO. The cell number, colony number of CFU-Mk, platelet-associated antigen phenotype, and DNA ploidy of CD41(+) cells were examined from d 0 to d 12 in culture. The expression patterns of cyclins B1, D1, and D3 were also analyzed by using immunoblot and flow cytometry. TPO stimulated proliferation of CD34(+) cells of FL from 1 x 10(5)/ml to 13.12 +/-4.06 10(5)/ml with 95% of CD41a(+) cells and 3% of CD34(+) cells after 12 d of culture. Most of the megakaryocytes (MKs) derived from FL were in 2 N ploidy class, and few in 4 N ploidy class, but no megakaryocytes ploidy class was higher than 4 N. The effect of TPO on the formation of CFU-Mk colonies from FL derived CD34(+) cells is shown in a dose-response curve. The expression of cyclin B1 increased progressively and the high level of cyclin B1 was maintained in FL CD34(+) cells induced by TPO during 12 d of culture. A high level of cyclin B1 appeared on FL derived MKs of G1 phase at d 12. The expression of cy-of cyclins D1 and D3 gradually increased in FL CD34(+) cells, which was induced by TPO during the initial 6-day incubation. Afterwards, the level of cyclins D1 and D3 decreased progressively, particularly in MKs which were in G2+M phases. These data suggest that (1) TPO induced proliferation and differentiation of FL derived CD34(+) cells through upregulation of cyclin B1 in G2+M phases and cyclins D1 and D3 in all phases of cell cycle, and (2) Continuing high level of cyclin B1 and decreases of cyclins D1 and cyclin D3 on MKs in G2+M phases may contribute to a retardation of MK endoreduplication.

Crystal structure of rabbit phosphoglucose isomerase complexed with its substrate D-fructose 6-phosphate

Phosphoglucose isomerase (PGI, EC 5.3.1.9) catalyzes the interconversion of D-glucose 6-phosphate (G6P) and D-fructose 6-phosphate (F6P) and plays important roles in glycolysis and gluconeogenesis. Biochemical characterization of the enzyme has led to a proposed multistep catalytic mechanism. First, the enzyme catalyzes ring opening to yield the open chain form of the substrate. Then isomerization proceeds via proton transfer between C2 and C1 of a cis-enediol(ate) intermediate to yield the open chain form of the product. Catalysis proceeds in both the G6P to F6P and F6P to G6P directions, so both G6P and F6P are substrates. X-ray crystal structure analysis of rabbit and bacterial PGI has previously identified the location of the enzyme active site, and a recent crystal structure of rabbit PGI identified Glu357 as a candidate functional group for transferring the proton. However, it was not clear which active site amino acid residues catalyze the ring opening step. In this paper, we report the X-ray crystal structure of rabbit PGI complexed with the cyclic form of its substrate, D-fructose 6-phosphate, at 2.1 A resolution. The location of the substrate relative to the side chains of His388 suggest that His388 promotes ring opening by protonating the ring oxygen. Glu216 helps to position His388, and a water molecule that is held in position by Lys518 and Thr214 accepts a proton from the hydroxyl group at C2. Comparison to a structure of rabbit PGI with 5PAA bound indicates that ring opening is followed by loss of the protonated water molecule and conformational changes in the substrate and the protein so that a helix containing amino acids 513-520 moves in toward the substrate to form additional hydrogen bonds with the substrate.

CD34+ cells derived from fetal liver contained a high proportion of immature megakaryocytic progenitor cells

Endoreplication and maturation of the megakaryocyte (MK) may be retarded or delayed during ontogenesis. In this study, CD34+ cells were isolated from both human fetal liver and adult bone marrow and incubated with thrombopoietin (TPO). The cell number, morphological characteristics, platelet-associated antigen phenotype, maturation stage and DNA ploidy of CD41+ cells were examined from day 0 to day 12 in culture. 1) TPO stimulated the proliferation of fetal liver (FL)-derived CD34+ cells with a mean 73.14-fold increase of CD41+ cells after 12 d in culture. Adult BM-derived CD34+ cells increased only slightly, with a mean 8.18-fold increase of CD41+ cells. 2) Although the membrane phenotype of both FL CD34+-derived MKs and BM CD34+ -derived MKs analyzed with CD41a, CD42a, CD61 and CD34 were similar, all FL CD34+-derived MKs were in maturation stage I and II and in low ploidy (<4N) class. By comparison, BM CD34+ MKs possessed 15% MKs in maturation stage III and IV and with 23% MKs in high ploidy class ( > 4N). 3) Most of cultured FL-derived CD34+ cells did not have a well developed demarcation system (DM) and numerous alpha-granules after 12 d incubation. von Willebrand factor (vWF) appeared earlier on the cultured BM-derived CD34+ cells than on FL-derived CD34+ cells. 4) The expression of both cyclin E and cyclin B1 progressively increased in FL CD34+ cells induced by TPO during 12 d in culture. 5) The expression of cyclin D1 gradually decreased in FL CD34+ cells induced by TPO over 12 d incubation. 6) Immunocytochemical analysis showed that cyclin D3 was detected only in cytoplasm of cultured FL-derived CD34+ cells, whereas in both cytoplasm and nuclei of cultured BM-derived CD34+ cells. These data suggest that FL-derived CD34+ cells contain a high proportion of immature megakaryocytic progenitor cells. It further suggests that TPO can push these progenitor cells into proliferation by upregulating the expression of cyclins B1 and E, and drive a high proportion of cells into megakaryocytic lineage.

Selective induction of apoptosis of NB4 cells from G2+M phase by sodium arsenite at lower doses

Apoptosis of NB4 cells induced by sodium arsenite and arsenate was studied using flow cytometry and DNA gel electrophoresis in order to investigate their effects on cell cycle and determine the relationship between apoptosis and cell cycle. In this study, we found that: 1) at low doses, sodium arsenite selectively induced apoptosis of NB4 cells in G2+M phase of cell cycle after its being arrested in G2 phase. With increment of the cells blocked in G2 phase, dUTP-specifically labeling cells in G2+M phase increased without concomitant increment of dUTP-labeling cells in other two phases of cell cycle; 2) at high doses, extensive apoptosis was induced in NB4 cells from all phases of cell cycle without cell cycle preference and cell cycle blockade; 3) sodium arsenite-induced apoptosis of NB4 cells occurred in the presence of bcl-2 expression as the unapoptotic cells; 4) sodium arsenite with As3+ induced apoptosis of NB4 cells more strongly than sodium arsenate with As5+ did although both of them affected NB4 cells in the same pattern. These results not only suggested that both arsenite and arsenate induced apoptosis of NB4 cells through 2 different mechanisms--at low doses, arsenical might directly induce apoptosis through regulation of cell cycle checkpoint, while at high doses they might directly induce it, but also indicated that bcl-2 might not play an important role in arsenite or arsenate-induced apoptosis of NB4 cells, whereas chemical valence of As in a compound might be related to efficiency in arsenical induction of apoptosis.

[In vitro growth characteristics of rIL3 stimulated megakaryocytic progenitor cells (CFU-MK) of fetal liver]

In this study, plasma clot and methycellulose semi-solid and liquid culture technigues were employed to observe the in vitro growth characteristics of proliferation and differetiation of 4-5 months fetal liver and adult bone marrow CFU-MKs. Fetal liver MK colonies in plasma clot in the presence of rIL3 or Meg-CSA were larger (usually with > 50 cells/colony) than that of the adult (usually with < 50 cells/colony). The number (36.40 +/- 16.60/1 x 10(5) cells) of fetal liver Mk colonies in presence of 2 ng/ml rIL3 was larger than that (10.05 +/- 2.81/1 x 10(5) cells) of human adult bone marrow MK colonies (P < 0.01). In contrast, the major DNA ploidy classes of megakaryocytes derived from fetal liver CFU-MK were those of 2N and 4N cells and the major DNA ploidy classes of megakaryocytes derived from human adult bone marrow were those of 8N and 4N cells. The mean (5.45 +/- 0.86) of the ploidy of the former was lower than that (10.13 +/- 1.30) of the latter (P < 0.01). The same results were obtained with the presence of 10% Meg-CSA. These present results indicated that CFU-MK in fetal liver has a high ability of proliferation and low capacity of differentiation (polyploidization). Interestingly, the number of fetal liver MK colonies increased in the range of rIL3 concentration from 0.5 ng/ml to 2 ng/ml. At higher rIL3 concentration (2-8 ng/ml), the colony growth showed a steady decrease from the maximum value instead of an increase. However, in the same range of rIL3 concentration, the numbers of adult bone marrow MK colonies numbers and fetal liver CFU-GM colonies increased steadily and finally reached to a plateau. Furthermore, both fetal liver and adult bone marrow MK colonies showed dose-dependent response in the range of Meg-CSA concentration from 5% to 25%. In addition, there was no difference on DNA ploidy distributions of megakaryocytes derived from fetal liver CFU-MK between rIL3 and Meg-CSA as growth factors. Moreover, the DNA ploidy distribution of fetal liver derived megakaryocytes stimulited by rIL3 could not be changed by addition of rIL6 (100 u/ml). In summary, the above data suggest that CFU-MK in the fetal liver undergoes some intrinsic cellular modification in order to suit the need of ontogensis.

Developmental change of megakaryocyte maturation and DNA ploidy in human fetus

Megakaryocytes in fetal livers obtained from 30 water-balloon aborted normal fetuses of 3 to 6 months' gestation, in the bone marrow from the same 30 fetuses, and another 9 fetuses of 7 to 8 months' gestation and in the normal bone marrow of adults were analyzed by immunocytochemical staining for size and maturation stage distribution and by flow cytometry for ploidy distribution simultaneously. In human fetuses, megakaryocytes showed a shift during ontogenesis from smaller towards larger size and from less mature towards a more mature stage with advancement of gestation. This was accompanied by a significant progressive shift to higher ploidy. However, the proportions (78.64%) of hypoploidy (< or = 8N) megakaryocytes in bone marrow of 7-8 months' gestation fetuses was still much higher than that (33.32%) in human adults (p < 0.05), with the proportion of hyperploidy (> or = 16N) megakaryocytes lower than that (67.86%) in human adults. This result indicated that megakaryocyte polyploidization may be retarded or inhibited during development.