The roles of phosphatidylinositol 3-kinase and protein kinase Czeta for thrombopoietin-induced mitogen-activated protein kinase activation in primary murine megakaryocytes.

Thrombopoietin (TPO) stimulates a network of intracellular signaling pathways that displays extensive cross-talk. We have demonstrated previously that the ERK/mitogen-activated protein kinase pathway is important for TPO-induced endomitosis in primary megakaryocytes (MKs). One known pathway by which TPO induces ERK activation is through the association of Shc with the penultimate phosphotyrosine within the TPO receptor, Mpl. However, several investigators found that the membrane-proximal half of the cytoplasmic domain of Mpl is sufficient to activate ERK in vitro and support base-line megakaryopoiesis in vivo. Using BaF3 cells expressing a truncated Mpl (T69Mpl) as a tool to identify non-Shc/Ras-dependent signaling pathways, we describe here novel mechanisms of TPO-induced ERK activation mediated, in part, by phosphoinositide 3-kinase (PI3K). Similar to cells expressing full-length receptor, PI3K was activated by its incorporation into a complex with IRS2 or Gab2. Furthermore, the MEK-phosphorylating activity of protein kinase Czeta (PKCzeta) was also enhanced after TPO stimulation of T69Mpl, contributing to ERK activity. PKCzeta and PI3K also contribute to TPO-induced ERK activation in MKs, confirming their physiological relevance. Like in BaF3 cells, a TPO-induced signaling complex containing p85PI3K is detectable in MKs expressing T61Mpl and is probably responsible for PI3K activation. These data demonstrate a novel role of PI3K and PKCzeta in steady-state megakaryopoiesis.

Kinetics of endomitosis in primary murine megakaryocytes.

Megakaryocytes (MKs) develop from diploid progenitor cells via successive rounds of DNA synthesis in the absence of cell division, a process termed endomitosis (EnM). While the mechanism underlying EnM is not known, studies in yeast and leukemic cell lines have suggested that it may be due to reduced levels of cyclin B1 or cdc2, leading to a decrease in mitotic kinase activity. Using flow cytometry to study EnM highly purified marrow-derived MK precursors, we found that: (1) on average, 36% of 8N-32N MKs expressed abundant cyclin B during G2/M. The percentage of cells in G2/M decreased in >64N MKs, suggesting the limit of EnM, (2) the level of cyclin B per G2/M MK increased linearly with ploidy, (3) cyclin B expression oscillated normally in polyploid MKs, (4) MPM-2, a phosphoepitope created by the action of mitotic kinases and specific to M-phase cells, was expressed in a significant fraction of polyploid MKs, and (5) there was an apparent increase of cyclin B in G1-phase in polyploid MKs. This study provides the first qualitative kinetic data regarding the cell cycle status of MKs within individual ploidy classes. It also demonstrates the feasibility of using anti-cyclin B antibody and flow cytometry to resolve G1 from G2/M populations in polyploid MKs. Finally, these findings establish that neither a relative nor absolute deficiency of mitotic kinase components is responsible for EnM, suggesting that the departure from normal cell division kinetics seen in polyploid MKs is likely due to alterations in other cell cycle regulators.

Phosphatidylinositol 3-kinase is necessary but not sufficient for thrombopoietin-induced proliferation in engineered Mpl-bearing cell lines as well as in primary megakaryocytic progenitors.

Thrombopoietin and its receptor (Mpl) support survival and proliferation in megakaryocyte progenitors and in BaF3 cells engineered to stably express Mpl (BaF3/Mpl). The binding of thrombopoietin to Mpl activates multiple kinase pathways, including the Jak/STAT, Ras/Raf/MAPK, and phosphatidylinositol 3-kinase pathways, but it is not clear how these kinases promote cell cycling. Here, we show that thrombopoietin induces phosphatidylinositol 3-kinase and that phosphatidylinositol 3-kinase is required for thrombopoietin-induced cell cycling in BaF3/Mpl cells and in primary megakaryocyte progenitors. Treatment of BaF3/Mpl cells and megakaryocytes with the phosphatidylinositol 3-kinase inhibitor LY294002 inhibited mitotic and endomitotic cell cycl-ing. BaF3/Mpl cells treated with thrombopoietin and LY294002 were blocked in G(1), whereas megakaryocyte progenitors treated with thrombopoietin and LY294002 showed both a G(1) and a G(2) cell cycle block. Expression of constitutively active Akt in BaF3/Mpl cells restored the ability of thrombopoietin to promote cell cycling in the presence of LY294002. Constitutively active Akt was not sufficient to drive proliferation of BaF3/Mpl cells in the absence of thrombopoietin. We conclude that in BaF3/Mpl cells and megakaryocyte progenitors, thrombopoietin-induced phosphatidylinositol 3-kinase activity is necessary but not sufficient for thrombopoietin-induced cell cycle progression. Phosphatidylinositol 3-kinase activity is likely to be involved in regulating the G(1)/S transition.

[Interferon-alpha directly inhibits thrombopoietin-induced megakaryocyte proliferation and differentiation].

OBJECTIVE: To characterize the inhibitory effect of interferon-alpha (IFN-alpha) on megakaryocyte proliferation and differentiation stimulated by thrombopoietin (Tpo). METHODS: Normal murine megakaryocytes were prepared by three different cell purification procedures. Megakaryocyte development was studied by semi-solid and suspension cultures and assessed with megakaryocyte colony forming unit (CFU-MK), acetylcholinesterase activity (AChE) as well as megakaryocyte count, diameter and DNA content. RESULTS: IFN-alpha prohibited CFU-MK formation and decreased AChE of megakaryocytes. The inhibitory effects were positively related to doses of IFN-alpha and could not be overcome by escalating exogenous Tpo or depriving accessory cells in bone marrow culture. IFN-alpha affected all aspects of megakaryocyte development, including decreases in megakaryocyte count, diameter and geometric mean polyploidy. CONCLUSION: IFN-alpha directly inhibits Tpo-induced megakaryocyte proliferation and differentiation in a dose-dependent manner.

Immunohistochemical localization of xanthine oxidase in human retina.

Xanthine oxidase has been established as an important source of oxygen free radicals in ischemia-reperfusion injury. It has been localized in many different tissues such as heart and intestine, but it has not yet been localized in the eye. Xanthine oxidase was detected using immunohistochemistry on paraformaldehyde/glutaraldehyde fixed cryosections. Antibodies used included rabbit antibovine xanthine oxidase antibody and rabbit antihuman xanthine oxidase antibody. Xanthine oxidase was detected in the capillary endothelium cells of blood vessels in the retina of bovine and post mortem human eyes. Whole mount preparation of human retinas showed xanthine oxidase present throughout the small capillary network. Furthermore, whole mounts showed that xanthine oxidase was present in cones. This was confirmed by using mouse anticalbindin antibody for co-labelling. It is possible that xanthine oxidase can be a source of oxidative damage in the retina following ischemia-reperfusion injury.

Thrombopoietin signal transduction in purified murine megakaryocytes.

Thrombopoietin (TPO) is a recently cloned cytokine that binds to its receptor, Mpl, and promotes hematopoietic expansion and maturation, primarily of the megakaryocyte lineage. The signaling pathways responsible for these events are thought to involve the Janus family of nonreceptor tyrosine kinases (JAKs) and the signal transducers and activators of transcription (STATs), which are activated by tyrosine phosphorylation. Previous investigators have studied these molecules in engineered and naturally occurring cell lines. To investigate the molecular basis for TPO signal transduction in a more physiologic target, we determined the pattern of JAK and STAT activation in purified, normal urine megakaryocytes. These results are compared with those of established cell lines that only proliferate (Ba/F3- mMPL and DA-1-TPO) or only differentiate (L8057) in response to TPO. From these findings, a model is proposed to explain the physiologic roles of JAK2, TYK2, STAT3, and STAT5 in TPO signaling. Furthermore, previous studies of the physical interaction between Mpl and the JAKs are extended, showing a difference in the association of JAK2 and TYK2 with the TPO receptor. Finally, we show that, in the cell line Ba/F3-mMPL, the closely related proteins STAT5A and STAT5B are both activated by TPO stimulation and are capable of heterodimerization. Together, these results further our understanding of the early stages of megakaryocyte and platelet development.

Murine thrombopoietin mRNA levels are modulated by platelet count.

The activity of the c-Mpl ligand hematopoietic progenitors meets criteria expected for thrombopoietin (TPO). Bio-assays have shown that blood TPO levels are inversely related to platelet mass. We sought to identify the molecular basis for this regulation. To determine if TPO mRNA levels respond to platelet demand, RNA from selected organs of mice with high, normal or low platelet counts was subjected to semiquantitative reverse transcriptase-polymerase chain reaction. Although no differences in TPO mRNA levels between control and treated mice could be detected in liver or kidney, TPO-specific bands were more intense after 25 to 30 polymerase chain reaction cycles in marrow-derived mRNA from thrombocytopenic mice. The TPO-specific bands were less intense in thrombocytotic mouse marrow and spleen than control mouse marrow and spleen after 30 cycles. These data support the hypothesis that TPO levels are regulated, at least in part, by modulating mRNA levels in response to platelet demand.

Characterisation of a mouse tumour cell line with in vitro derived resistance to verapamil.

We have established a subline (EMT6/VRP) of the mouse tumour cell line EMT6/P with acquired resistance to the calcium transport blocker verapamil (VRP). The subline was 4-fold resistant to the cytoxicity of VRP alone compared with the parent line but of similar sensitivity to adriamycin, vincristine or colchicine. EMT6/VRP cells growing in 75 micrograms ml-1 VRP were morphologically different from and larger in diameter than EMT6/P cells, but these two parameters reverted almost to normal within 3 days of VRP removal, although resistance was retained. Expression of an mRNA coding for P-glycoprotein was similar in EMT6/VRP and the parent cell line, although considerable hyperexpression was seen in a multidrug resistant subline, EMT6/AR1.0. Cellular accumulation of both 3H-daunorubicin and 3H-VRP were greater in EMT6/VRP than in the parent line. Sensitisation to adriamycin by 3.3 micrograms ml-1 VRP was, however, somewhat reduced in EMT6/VRP (i.e. to 6.1-fold) compared with the 11-fold sensitisation seen in the parent line. It is clear that resistance to VRP seen in this cell line occurs via a different mechanism from the resistance to drugs such as adriamycin, vincristine and colchicine seen in multidrug resistant cell lines.

Chemosensitivity testing of fresh leukaemia cells using the MTT colorimetric assay.

We have determined the feasibility of using the MTT colorimetric assay for measuring the sensitivity of fresh leukaemia cells to cytotoxic agents. A linear relationship was seen between the number of leukaemic cells plated and the resulting absorbance in the MTT assay at day 4. For 2 X 10(5) cells/well from CLL peripheral lymphocytes, the mean absorbance (measured at 540 nm) was 0.41. Similar curves were obtained with the assay carried out between days 3 and 7. ID50 values for CLL lymphocytes exposed to adriamycin (ADM) were in the range of 0.02-0.29 micrograms/ml, a similar range to that given by the DiSC dye exclusion assay. Sensitization of leukaemia cells to ADM by verapamil or cyclosporin A could be demonstrated in some patients but not in others. The lymphocytes from one patient with prolymphocytic leukaemia were abnormally resistant to ionizing radiation. These cells also showed resistance to melphalan and adriamycin but not to vincristine. The MTT assay appears to offer an attractive option for in vitro chemosensitivity testing in leukaemia, determination of cross-resistance profiles and studies of resistance modifiers.

A comparison of clonogenic and radionuclide uptake assays for determining the radiation response of human small-cell lung cancer xenografts and cell lines.

Previous work has suggested that a radionuclide (3HTdR) uptake assay can provide a measure of drug and radiation sensitivity comparable with that given by the clonogenic assay. We have now extended this work to examine the radiation response of human small-cell lung cancer xenografts and cell lines with a wide range of plating efficiencies. Preliminary experiments using cultured cells indicated that irradiation of a single-cell suspension following disaggregation generally produced similar response data to those obtained when cultures were irradiated in situ and subsequently disaggregated. The response of eight cell lines to a single dose of 2 Gy was measured using both radionuclide uptake and clonogenic assays. There was no correlation between the two sets of data. Agreement between the radiation-response curves obtained using 3HTdR uptake and clonogenic assays was better for high plating efficiency xenografts (NCI-H69, COR-L51) than for low plating efficiency xenografts (COR-L24, COR-L31). Radionuclide uptake indicated very shallow response curves for the low plating efficiency lines. Altering the time of radionuclide addition from the standard Day 4 to other times between Day 2 and Day 6 did not greatly change the indications of radioresponsiveness provided by the assay. Growth-curve experiments for line COR-L24 showed that cell numbers at Day 4 after irradiation with 1.5 Gy or 3 Gy were very similar to those in unirradiated cultures. For NCI-H69, however, the cell numbers were very different for the different radiation doses. It appears that a high proportion of cells in lines such as COR-L24 take many days to show radiation damage as measured by reduced 3HTdR uptake.

Drug resistance in human lung cancer cell lines: cross-resistance studies and effects of the calcium transport blocker, verapamil.

We have developed multi-drug resistant variants of three human lung cancer cell lines by growth in increasing concentrations of adriamycin (ADM). Each of the lines shows reduced ADM content after acute drug exposure when compared with the corresponding parent line. The resistance to ADM may in each case be partially overcome by the concurrent use of 6.6 microM verapamil (VRP) (a calcium transport blocker). In small cell line NCI-H69 the growth inhibitory effect of VRP alone is greater in the resistant variant than in the parent line. Little or no change in the ADM sensitivity of parent cells is brought about by concentrations of VRP up to 15-20 microM. In the resistant line, a dose-related effect of VRP is seen with little effect below 1.0 microM and a gradual loss of resistance at higher doses. ADM-resistant cells of line NCI-H69 show only a small degree of resistance to two anthracyclines (aclacinomycin A and Ro 31-1215). This can be further reduced when these drugs are combined with verapamil.

The in vitro effects and cross-resistance patterns of some novel anthracyclines.

A range of new anthracyclines, structurally related to adriamycin (ADM), has been synthesised and studied in vitro. Three compounds described in this paper (Ro 31-1215; Ro 31-1741; Ro 31-2035) are all 4-demethoxyanthracyclines. In the mouse mammary tumour cell line, EMT6/Ca/VJAC, using a 1 h drug exposure followed by colony formation as the response endpoint, we found Ro 31-1215 and Ro 31-1741 to be 2-3 x and 4-7 x more potent then ADM, whilst Ro 31-2035 was 3-4 x less potent. For continuous drug exposure and suppression of population growth as the endpoint, the potency of Ro 31-1741 was similar to that of ADM, whereas that of Ro 31-1215 was 1.5-2 x higher and that of Ro 31-2035 was 10-20 x lower. The potency ratios for continuous drug exposure of a human small cell lung cancer line were similar to those for continuous exposure of EMT6. Variants of the two cell lines selected for resistance to ADM were also studied. These variants also showed considerable resistance to Ro 31-1741 and Ro 31-2035 but much less resistance to Ro 31-1215 (a 9-methyl derivative). A variant of EMT6 made resistant to Ro 31-1215 by continuous growth in this drug was more resistant to ADM than it was to Ro 31-1215. Human cells resistant to ADM contained 6 x less ADM after 24 h exposure than did the parent line, whereas the ratio of drug content for Ro 31-1215 was only 2.

Derivation and preliminary characterisation of adriamycin resistant lines of human lung cancer cells.

We have produced adriamycin (ADM)-resistant variants of the human lung cancer cell lines NCI-H69 (small cell), MOR (adenocarcinoma) and COR-L23 (large cell) but have failed to produce resistant variants of two other small cell lines. In each case, the derivation protocol took 7-9 months and included a period of drug-free growth. All three resistant lines show reduced cellular content of ADM after 1 h exposure when compared with their controls. During prolonged incubation of control and resistant NCI-H69 cells in 0.4 microgram ml-1 ADM, the ADM content of resistant cells was 6-7 times lower than that of control cells. The ratio of ADM doses to suppress growth of the two lines, however, was in the range of 40-200X. The ADM-resistant variant of NCI-H69 was also resistant to vincristine, colchicine, VP16, mitozantrone, 4' epiadriamycin and 4' deoxyadriamycin, somewhat resistant to melphalan but not resistant to aclacinomycin A, bleomycin of CCNU. The resistance to ADM could be partially overcome by the use of verapamil, an inhibitor of calcium transport.

Establishment and characterisation of cell lines from patients with lung cancer (predominantly small cell carcinoma).

Tissue samples from 59 patients with lung cancer have been used to establish cell lines in culture. The primary diagnosis was small cell carcinoma in all except four. Most of the samples were of bone marrow but pleural effusions, lymph node biopsies and skin metastases were also included. The samples were usually split between HITES serum-free medium and HITES plus 2.5% foetal calf serum. A total of 19 cell lines were established and characterised. One line is large cell anaplastic lung carcinoma, four are B-lymphoblastoid and fourteen are small cell lung cancer. Considerable heterogeneity in gross morphology, neuroendocrine differentiation (by electron microscopy) and content of the enzyme L-dopa decarboxylase was seen. The use of HITES plus 2.5% foetal calf serum resulted in better establishment of cultures than did serum-free HITES.