Caspase-3 is activated during both terminal differentiation and erythropoietin-starvation-induced apoptosis of human erythroid precursors. The transcription factor GATA-1, which performs an essential function in erythroid differentiation by positively regulating promoters of erythroid and anti-apoptotic genes, is cleaved by caspases in erythroid precursors undergoing cell death upon erythropoietin starvation or engagement of the death receptor Fas. In contrast, by an unknown mechanism, GATA-1 remains uncleaved when these cells undergo terminal differentiation upon stimulation with Epo. Here we show that during differentiation, but not during apoptosis, the chaperone protein Hsp70 protects GATA-1 from caspase-mediated proteolysis. At the onset of caspase activation, Hsp70 co-localizes and interacts with GATA-1 in the nucleus of erythroid precursors undergoing terminal differentiation. In contrast, erythropoietin starvation induces the nuclear export of Hsp70 and the cleavage of GATA-1. In an in vitro assay, Hsp70 protects GATA-1 from caspase-3-mediated proteolysis through its peptide-binding domain. The use of RNA-mediated interference to decrease the Hsp70 content of erythroid precursors cultured in the presence of erythropoietin leads to GATA-1 cleavage, a decrease in haemoglobin content, downregulation of the expression of the anti-apoptotic protein Bcl-X(L), and cell death by apoptosis. These effects are abrogated by the transduction of a caspase-resistant GATA-1 mutant. Thus, in erythroid precursors undergoing terminal differentiation, Hsp70 prevents active caspase-3 from cleaving GATA-1 and inducing apoptosis.

Differentiating agents have been proposed to overcome the impaired cellular differentiation in acute myeloid leukemia (AML). However, only the combinations of all-trans retinoic acid or arsenic trioxide with chemotherapy have been successful, and only in treating acute promyelocytic leukemia (also called AML3). We show that iron homeostasis is an effective target in the treatment of AML. Iron chelating therapy induces the differentiation of leukemia blasts and normal bone marrow precursors into monocytes/macrophages in a manner involving modulation of reactive oxygen species expression and the activation of mitogen-activated protein kinases (MAPKs). 30% of the genes most strongly induced by iron deprivation are also targeted by vitamin D3 (VD), a well known differentiating agent. Iron chelating agents induce expression and phosphorylation of the VD receptor (VDR), and iron deprivation and VD act synergistically. VD magnifies activation of MAPK JNK and the induction of VDR target genes. When used to treat one AML patient refractory to chemotherapy, the combination of iron-chelating agents and VD resulted in reversal of pancytopenia and in blast differentiation. We propose that iron availability modulates myeloid cell commitment and that targeting this cellular differentiation pathway together with conventional differentiating agents provides new therapeutic modalities for AML.

Anemia because of insufficient production of and/or response to erythropoietin (Epo) is a major complication of chronic kidney disease and cancer. The mechanisms modulating the sensitivity of erythroblasts to Epo remain poorly understood. We show that, when cultured with Epo at suboptimal concentrations, the growth and clonogenic potential of erythroblasts was rescued by transferrin receptor 1 (TfR1)-bound polymeric IgA1 (pIgA1). Under homeostatic conditions, erythroblast numbers were increased in mice expressing human IgA1 compared to control mice. Hypoxic stress of these mice led to increased amounts of pIgA1 and erythroblast expansion. Expression of human IgA1 or treatment of wild-type mice with the TfR1 ligands pIgA1 or iron-loaded transferrin (Fe-Tf) accelerated recovery from acute anemia. TfR1 engagement by either pIgA1 or Fe-Tf increased cell sensitivity to Epo by inducing activation of mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) signaling pathways. These cellular responses were mediated through the TfR1-internalization motif, YXXΦ. Our results show that pIgA1 and TfR1 are positive regulators of erythropoiesis in both physiological and pathological situations. Targeting this pathway may provide alternate approaches to the treatment of ineffective erythropoiesis and anemia.

Metastatic lymph nodes (LNs) are the major prognostic factor in resected non small cell lung carcinoma (NSCLC). However, almost 50% of pN0 patients relapse, suggesting metastatic cells undetected by current staging procedures. A combination of markers [cytokeratins 19 and 7 (CK19, CK7) and mucin type 1 (MUC1) mRNAs] was therefore evaluated by real-time RT-PCR in order to detect occult cancer cells. Forty-three NSCLC tumor samples, 4 micrometastatic, 6 metastatic and 84 histologically negative mediastinal LNs from 19 patients with NSCLC were evaluated as well as blood mononuclear cells from 29 healthy volunteers and 17 benign LNs. When tested on cell lines, RT-PCR was particularly efficient for evaluation of CK19, CK7 and MUC1 mRNA expression. All tumor samples were positive for at least 1 marker and 74% of samples were positive for all 3 markers. CK7 and CK19 mRNA were not detected in benign LN and blood cells from healthy donors in contrast with MUC1 mRNA. Only CK7 and CK19 mRNA were therefore used for evaluation of mediastinal LNs: the 6 histologically metastatic and the 4 micrometastatic LNs were positive for at least one marker. Among the 84 histologically negative LNs, 6 (7%) were positive for at least one marker, potentially changing the stage of 2 out of 19 patients. In conclusion, in our feasibility study, parallel molecular detection of CK19 and CK7 mRNA can be considered a specific diagnostic tool for the assessment of microscopic lymphatic spread. Its prognostic impact remains to be evaluated in a prospective study. ' 2005 Wiley-Liss, Inc.

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Mantle cell lymphoma (MCL) is one of the most frequent of the newly recognized non-Hodgkin's lymphomas. The major problem of MCL therapy is the occurrence of relapse and subsequent resistance to chemotherapy and immunotherapy in virtually all cases. Here, we show that one injection of antihuman transferrin receptor (TfR) monoclonal antibody A24 totally prevented xenografted MCL tumor establishment in nude mice. It also delayed and inhibited tumor progression of established tumors, prolonging mice survival. In vitro, A24 induced up to 85% reduction of MCL cell proliferation (IC 50 = 3.75 nmol/L) independently of antibody aggregation, complement-dependent or antibody-dependent cell-mediated cytotoxicity. A24 induced MCL cell apoptosis through caspase-3 and caspase-9 activation, either alone or synergistically with chemotherapeutic agents. A24 induced TfR endocytosis via the clathrin adaptor protein-2 complex pathway followed by transport to lysosomal compartments. Therefore, A24-based therapies alone or in association with classic chemotherapies could provide a new alternative strategy against MCL, particularly in relapsing cases. [Cancer Res 2007;67(3):1145-54]

Stem cell factor (SCF) and erythropoietin are strictly required for preventing apoptosis and stimulating proliferation, allowing the differentiation of erythroid precursors from colony-forming unit-E to the polychromatophilic stage. In contrast, terminal maturation to generate reticulocytes occurs independently of cytokine signaling by a mechanism not fully understood. Terminal differentiation is characterized by a sequence of morphological changes including a progressive decrease in cell size, chromatin condensation in the nucleus and disappearance of organelles, which requires transient caspase activation. These events are followed by nucleus extrusion as a consequence of plasma membrane and cytoskeleton reorganization. Here, we show that in early step, SCF stimulates the Rho/ROCK pathway until the basophilic stage. Thereafter, ROCK-1 is activated independently of Rho signaling by caspase-3-mediated cleavage, allowing terminal maturation at least in part through phosphorylation of the light chain of myosin II. Therefore, in this differentiation system, final maturation occurs independently of SCF signaling through caspase-induced ROCK-1 kinase activation.

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