Chronic stress induces CD99, suppresses autophagy, and affects spontaneous adipogenesis in human bone marrow stromal cells

BACKGROUND

Bone marrow-derived mesenchymal stromal cells (MSCs) are multipotent cells with a high constitutive level of autophagy and low expression of CD99. Under certain conditions, MSCs may develop tumorigenic properties. However, these transformation-induced conditions are largely unknown. Recently, we have identified an association between Hsp70, a main participant in cellular stress response and tumorigenesis, and CD99. Preliminary observations had revealed upregulation of both proteins in stressed long-term cultured MSCs. And so we hypothesized that CD99 is implicated in stress-induced mechanisms of cellular transformation in MSCs. Hence, we investigated the effects of prolonged stress on MSCs and the role of CD99 and autophagy in their survival.

METHODS

Human telomerase reverse transcriptase (hTERT) overexpressing immortalized MSCs and primary bone marrow stromal cells were used to investigate the influence of long-term serum deprivation and hypoxia on growth and differentiation of MSCs. Cell proliferation and apoptosis were evaluated using flow cytometry, differentiation capabilities of MSCs were assessed by immunohistochemical staining followed by microscopic examination. CD99, Hsp70 expression were analyzed using flow cytometry, western blotting, and reverse transcriptase polymerase chain reaction. Autophagy was explored with specific inhibitors using cell morphology examination and western blotting.

RESULTS

Chronic stress factors are able to change the morphology of MSCs and to inhibit spontaneous differentiation into adipocyte lineage. Furthermore, CD99 elevation and downregulation of p53 and p21 accompanied defective autophagy, which is usually associated with tumor formation. We found that inhibition of autophagy by chloroquine promoted cell detachment and modulated CD99 expression level whereas incorporation of CD99 recombinant protein into the cells suppressed autophagy.

CONCLUSIONS

Obtained results provide a model for chronic stress-induced transformation of MSCs via CD99 and may therefore be highly relevant to mesenchymal tumorigenesis.

CD99 ligation upregulates HSP70 on acute lymphoblastic leukemia cells and concomitantly increases NK cytotoxicity

CD99 is present in many human cell types, including high-level surface expression on pediatric B and T leukemias and Ewing tumors (ETs). On B lymphocytes and respective malignancies, its level decreases with the stage of maturation. Inter-individual variability of CD99 on B-cell precursor acute lymphoblastic leukemia (BCP-ALL) blasts was shown recently to be associated with distinct cytogenetic backgrounds. However, CD99 targets remain mainly unknown. Here, we show that administration of an anti-CD99 antibody to B- and T-leukemia cell lines induces heat shock protein 70 (HSP70), both on the cell surface and in the cytoplasm. Investigation of primary BCP-ALL cells rendered similar results. Intriguingly, CD99-induced modulation of HSP70 on ET cells had profiles different from that on leukemia cells. Since HSP70 expression on tumor cells is a prerequisite for natural killer (NK) cell-mediated tumor lysis, we hypothesized that CD99-induced HSP70 may allow targeting of some CD99-positive malignancies via NK-cell cytotoxicity. Our experiments with NK92 cell line demonstrated that leukemia cells with upregulated HSP70 can be successfully killed by effector cells. We consider our data as a new view of CD99 functions and as a basis for the development of a potential anti-tumor strategy based on heat-shock protein activation via CD99 triggering.

Modulation of antigen expression in B-cell precursor acute lymphoblastic leukemia during induction therapy is partly transient: evidence for a drug-induced regulatory phenomenon. Results of the AIEOP-BFM-ALL-FLOW-MRD-Study Group

BACKGROUND

Changes of antigen expression on residual blast cells of acute lymphoblastic leukemia (ALL) occur during induction treatment. Many markers used for phenotyping and minimal residual disease (MRD) monitoring are affected. Glucocorticoid (GC)-induced expression modulation has been causally suspected, however, subclone selection may also cause the phenomenon.

METHODS

We investigated this by following the phenotypic evolution of leukemic cells with flow cytometry from diagnosis to four time points during and after GC containing chemotherapy in the 20 (of 360 consecutive) B-cell precursor patients with ALL who had persistent MRD throughout.

RESULTS

The early expression changes of CD10 and CD34 were reversible after stop of GC containing chemotherapy. Modulation of CD20 and CD45 occurred mostly during the GC phase, whereas CD11a also changed later on. Blast cells at diagnosis falling into gates designed according to "shifted" phenotypes from follow-up did not form clusters and were frequently less numerous than later on.

CONCLUSIONS

Our data support the idea that drug-induced modulation rather than selection causes the phenomenon. The good message for MRD assessment is that modulation is transient in at least two (CD10 and CD34) of the five prominent antigens investigated and reverts to initial aberrant patterns after stop of GC therapy, whereas CD20 expression gains new aberrations exploitable for MRD detection.

Death induction by CD99 ligation in TEL/AML1-positive acute lymphoblastic leukemia and normal B cell precursors

Our study was performed to examine the role of CD99 in normal and leukemia BCPs. CD99 is strongly expressed by certain pediatric cancers including BCP-ALL. Modulation of the antigen in ETs and T cells induces apoptosis, hence implicating CD99 as a potential target for anti-cancer therapy. However, nothing is known about these aspects in BCPs. We investigated BCP-ALL cases and normal BCP cells from pediatric BM for CD99 protein and RNA expression as well as for effects of CD99 modulation by mAb. Immunophenotypes, recovery, apoptosis, and aggregation were assessed. Flow cytometry, light microscopy, and qRT-PCR were used in our experiments. An association of CD99 expression levels with the cytogenetic background of pediatric BCP-ALLs was found. Highest CD99 levels were observed in hyperdiploid, followed by TEL/AML1 and random karyotype leukemias. CD99 ligation moderately induced cell death only in TEL/AML1 cases. Stroma cell contact mitigated this effect. Very immature normal BCPs were the most sensitive to CD99-mediated death induction. Type I CD99 mRNA was the main isoform in ALLs and was expressed differentially during BCP maturation. Our data suggest that clinical targeting of CD99 may be effective in BCP-ALL-bearing TEL/AML1 but also may elicit negative effects on normal B-lymphopoiesis. We consider our results as an indication that CD99 may play a physiologic role in the clonal deletion processes necessary for B-lymphoid selection.

Standardization of flow cytometric minimal residual disease evaluation in acute lymphoblastic leukemia: Multicentric assessment is feasible

BACKGROUND

Single-laboratory experience showed that flow cytometric (FCM) assessment of minimal residual disease (MRD) in acute lymphoblastic leukemia (ALL) is feasible in most patients and gives independent prognostic information. It is, however, not known whether FCM analysis can reliably be standardized for multicentric application.

METHODS

An extensive standardization program was installed in four collaborating laboratories, which study FCM-MRD in children treated with the AIEOP-BFM-ALL 2000 protocol. This included methodological alignment, continuous quality monitoring, as well as personnel education by exchange and performance feed-back.

RESULTS

Blinded inter-laboratory tests of list-mode data interpretation concordance (n = 202 blood and bone marrow samples from follow-up during induction of 31 randomly selected patients of a total series of n = 395) showed a very high degree of inter-rater agreement among the four centers despite differences in cytometers and software usage (intraclass correlation coefficient [ICC] 0.979 based on n= 800 single values). Lower concordance was reached with amounts of MRD below 0.1%. Comparing data from sample exchange experiments (n = 42 samples; ICC 0.98) and from independent patient cohorts from the four centers (regarding positive samples per time-point of follow-up as well as risk estimates) concordance was also good.

CONCLUSION

MRD-evaluation by FCM in ALL can be standardized for reliable multicentric assessment in large trials.

Transient activation of Ras-dependent signalling at the early stages of Herbimycin A induced erythroid differentiation of human K562 cells

AIM

To study the dynamics of Ras-dependent signalling in the course of Herbimycin A induced erythroid differentiation of human erythroleukemia K562 cells.

METHODS

p21Ras functional activity was analyzed by direct measurement of GTP/GDP ratio in anti-p21Ras immunoprecipitates of K562 cells previously incubated with H3(32)PO4. Dynamics of protein tyrosine phosphorylation was studied using Western blotting. Electrophoretic mobility shift assay was used to monitor Erk2 activation. Phosphotyrosine (pY)-containing proteins bound to recombinant glutathione-S-tranferase (GST)-fused form of adaptor protein Grb2 were identified using GST in vitro binding assay.

RESULTS

It was shown that the relative quantity of GTP associated with Ras protein in non-induced cells varied from 27% to 37% upon 72 h of cell culturing. Herbimycin A caused 15% increase of GTP/GDP ratio at 3rd h. This index decreased during further investigated periods, although it did not reach control values even at 72nd h. Transient rise of Ras-GTP level at 3rd h of incubation in the presence of Herbimycin A correlated with the increase in tyrosine phosphorylation of proteins with apparent molecular weight of 210, 160, 140, 116 and 42 kDa, as well as with the activation of Erk2 and increase of binding of a set of pY-containing proteins with recombinant GST-fusion form of Ras activator, adaptor protein Grb2. Dramatic inhibition of interaction between docking protein Shc and GST-Grb2 was observed at late stages of cell induction (48-72 h) while binding of pY-containing proteins during this period did not differ significantly in control and differentiated cells.

CONCLUSION

The obtained results suggest that time-dependent changes in Grb2-mediated network of protein-protein interaction events might define implication of Ras-dependent signalling in Herbimycin A-induced erythroid differentiation of K562 cells.

Raf-1 antagonizes erythroid differentiation by restraining caspase activation

The Raf kinases are key signal transducers activated by mitogens or oncogenes. The best studied Raf isoform, Raf-1, was identified as an inhibitor of apoptosis by conventional and conditional gene ablation in mice. c-raf-1(-)(/)(-) embryos are growth retarded and anemic, and die at midgestation with anomalies in the placenta and fetal liver. Here, we show that Raf-1-deficient primary erythroblasts cannot be expanded in culture due to their accelerated differentiation into mature erythrocytes. In addition, Raf-1 expression is down-regulated in differentiating wild-type cells, whereas overexpression of activated Raf-1 delays differentiation. As recently described for human erythroid precursors, we find that caspase activation is necessary for the differentiation of murine fetal liver erythroblasts. Differentiation-associated caspase activation is accelerated in erythroid progenitors lacking Raf-1 and delayed by overexpression of the activated kinase. These results reveal an essential function of Raf-1 in erythropoiesis and demonstrate that the ability of Raf-1 to restrict caspase activation is biologically relevant in a context distinct from apoptosis.