Impact of rising seawater temperature on a phagocytic cell population during V. parahaemolyticus infection in the sea anemone E. pallida

Climate change is increasing ocean temperatures and consequently impacts marine life (e.g., bacterial communities). In this context, studying host-pathogen interactions in marine organisms is becoming increasingly important, not only for ecological conservation, but also to reduce economic loss due to mass mortalities in cultured species. In this study, we used Exaiptasia pallida (E. pallida), an anemone, as an emerging marine model to better understand the effect of rising temperatures on the infection induced by the pathogenic marine bacterium Vibrio parahaemolyticus. The effect of temperature on E. pallida was examined at 6, 24, or 30 h after bath inoculation with 108 CFU of V. parahaemolyticus expressing GFP (Vp-GFP) at 27°C (husbandry temperature) or 31°C (heat stress). Morphological observations of E. pallida and their Hsps expression demonstrated heat stress induced increasing damage to anemones. The kinetics of the infections revealed that Vp-GFP were localized on the surface of the ectoderm and in the mucus during the first hours of infection and in the mesenterial filaments thereafter. To better identify the E. pallida cells targeted by Vp-GFP infection, we used spectral flow cytometry. E. pallida cell types were identified based on their autofluorescent properties. corresponding to different cell types (algae and cnidocytes). We identified an AF10 population whose autofluorescent spectrum was identical to that of human monocytes/macrophage, suggesting that this spectral print could be the hallmark of phagocytic cells called "amebocytes''. AF10 autofluorescent cells had a high capacity to phagocytize Vp-GFP, suggesting their possible role in fighting infection. This was confirmed by microscopy using sorted AF10 and GFP-positive cells (AF10+/GFP+). The number of AF10+/GFP+ cells were reduced at 31°C, demonstrating that increased temperature not only damages tissue but also affects the immune response of E. pallida. In conclusion, our study provides a springboard for more comprehensive studies of immune defense in marine organisms and paves the way for future studies of the dynamics, activation patterns, and functional responses of immune cells when encountering pathogens.

Nanoblades allow high-level genome editing in murine and human organoids

Genome engineering has become more accessible thanks to the CRISPR-Cas9 gene-editing system. However, using this technology in synthetic organs called "organoids" is still very inefficient. This is due to the delivery methods for the CRISPR-Cas9 machinery, which include electroporation of CRISPR-Cas9 DNA, mRNA, or ribonucleoproteins containing the Cas9-gRNA complex. However, these procedures are quite toxic for the organoids. Here, we describe the use of the "nanoblade (NB)" technology, which outperformed by far gene-editing levels achieved to date for murine- and human tissue-derived organoids. We reached up to 75% of reporter gene knockout in organoids after treatment with NBs. Indeed, high-level NB-mediated knockout for the androgen receptor encoding gene and the cystic fibrosis transmembrane conductance regulator gene was achieved with single gRNA or dual gRNA containing NBs in murine prostate and colon organoids. Likewise, NBs achieved 20%-50% gene editing in human organoids. Most importantly, in contrast to other gene-editing methods, this was obtained without toxicity for the organoids. Only 4 weeks are required to obtain stable gene knockout in organoids and NBs simplify and allow rapid genome editing in organoids with little to no side effects including unwanted insertion/deletions in off-target sites thanks to transient Cas9/RNP expression.

Deubiquitinase Inhibitors Impair Leukemic Cell Migration Through Cofilin Oxidation and Alteration of Actin Reorganization

Actin networks are dynamically regulated through constant depolymerization and polymerization cycles. Although the fundamental mechanisms that govern these processes have been identified, the nature and role of post-translational modifications (PTMs) of actin and actin regulatory proteins are not completely understood. Here, we employed Actin CytoFRET, a method that we developed for real time detection of fluorescence resonance energy transfer (FRET) signals generated by actin dynamics, to screen a small library of PTM-interfering compounds on a biosensor leukemic T cell line. This strategy led to the identification of small molecule inhibitors of deubiquitinating enzymes (DUBs) as potent inducers of actin polymerization and blockers of chemotactic cell migration. The examination of the underlying mechanism further revealed that the actin depolymerizing protein cofilin represents a major effector of DUB inhibitor (DUBi)-induced actin reorganization. We found that DUB blockade results in the accumulation of polyubiquitinated proteins and ROS production, associated with cofilin oxidation and dephosphorylation on serine 3, which provokes uncontrolled actin polymerization impairing cell migration. Together, our study highlights DUBs as novel regulators of actin dynamics through ROS-dependent cofilin modulation, and shows that DUBi represent attractive novel tools to impede leukemic cell migration.

Targeting Discoidin Domain Receptors DDR1 and DDR2 overcomes matrix-mediated tumor cell adaptation and tolerance to BRAF-targeted therapy in melanoma

Resistance to BRAF/MEK inhibitor therapy in BRAF^V600‐mutated advanced melanoma remains a major obstacle that limits patient benefit. Microenvironment components including the extracellular matrix (ECM) can support tumor cell adaptation and tolerance to targeted therapy; however, the underlying mechanisms remain poorly understood. Here, we investigated the process of matrix‐mediated drug resistance (MMDR) in response to BRAF^V600 pathway inhibition in melanoma. We demonstrate that physical and structural cues from fibroblast‐derived ECM abrogate anti‐proliferative responses to BRAF/MEK inhibition. MMDR is mediated by drug‐induced linear clustering of phosphorylated DDR1 and DDR2, two tyrosine kinase collagen receptors. Depletion and pharmacological targeting of DDR1 and DDR2 overcome ECM‐mediated resistance to BRAF‐targeted therapy. In xenografts, targeting DDR with imatinib enhances BRAF inhibitor efficacy, counteracts drug‐induced collagen remodeling, and delays tumor relapse. Mechanistically, DDR‐dependent MMDR fosters a targetable pro‐survival NIK/IKKα/NF‐κB2 pathway. These findings reveal a novel role for a collagen‐rich matrix and DDR in tumor cell adaptation and resistance. They also provide important insights into environment‐mediated drug resistance and a preclinical rationale for targeting DDR signaling in combination with targeted therapy in melanoma.

A Feed-Forward Mechanosignaling Loop Confers Resistance to Therapies Targeting the MAPK Pathway in BRAF-Mutant Melanoma

Aberrant extracellular matrix (ECM) deposition and stiffening is a physical hallmark of several solid cancers and is associated with therapy failure. BRAF-mutant melanomas treated with BRAF and MEK inhibitors almost invariably develop resistance that is frequently associated with transcriptional reprogramming and a de-differentiated cell state. Melanoma cells secrete their own ECM proteins, an event that is promoted by oncogenic BRAF inhibition. Yet, the contribution of cancer cell-derived ECM and tumor mechanics to drug adaptation and therapy resistance remains poorly understood. Here, we show that melanoma cells can adapt to targeted therapies through a mechanosignaling loop involving the autocrine remodeling of a drug-protective ECM. Analyses revealed that therapy-resistant cells associated with a mesenchymal dedifferentiated state displayed elevated responsiveness to collagen stiffening and force-mediated ECM remodeling through activation of actin-dependent mechanosensors Yes-associated protein (YAP) and myocardin-related transcription factor (MRTF). Short-term inhibition of MAPK pathway also induced mechanosignaling associated with deposition and remodeling of an aligned fibrillar matrix. This provided a favored ECM reorganization that promoted tolerance to BRAF inhibition in a YAP- and MRTF-dependent manner. Matrix remodeling and tumor stiffening were also observed in vivo upon exposure of BRAF-mutant melanoma cell lines or patient-derived xenograft models to MAPK pathway inhibition. Importantly, pharmacologic targeting of YAP reversed treatment-induced excessive collagen deposition, leading to enhancement of BRAF inhibitor efficacy. We conclude that MAPK pathway targeting therapies mechanically reprogram melanoma cells to confer a drug-protective matrix environment. Preventing melanoma cell mechanical reprogramming might be a promising therapeutic strategy for patients on targeted therapies. SIGNIFICANCE: These findings reveal a biomechanical adaptation of melanoma cells to oncogenic BRAF pathway inhibition, which fuels a YAP/MRTF-dependent feed-forward loop associated with tumor stiffening, mechanosensing, and therapy resistance. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/10/1927/F1.large.jpg.

Cancer cell-derived long pentraxin 3 (PTX3) promotes melanoma migration through a toll-like receptor 4 (TLR4)/NF-κB signaling pathway

Cutaneous melanoma is one of the most aggressive cancers characterized by a high plasticity, a propensity for metastasis, and drug resistance. Melanomas are composed of phenotypically diverse subpopulations of tumor cells with heterogeneous molecular profiles that reflect intrinsic invasive abilities. In an attempt to identify novel factors of the melanoma invasive cell state, we previously investigated the nature of the invasive secretome by using a comparative proteomic approach. Here, we have extended this analysis to show that PTX3, an acute phase inflammatory glycoprotein, is one such factor secreted by invasive melanoma to promote tumor cell invasiveness. Elevated PTX3 production was observed in the population of MITFlow invasive cells but not in the population of MITFhigh differentiated melanoma cells. Consistently, MITF knockdown increased PTX3 expression in MITFhigh proliferative and poorly invasive cells. High levels of PTX3 were found in tissues and blood of metastatic melanoma patients, and in BRAF inhibitor-resistant melanoma cells displaying a mesenchymal invasive MITFlow phenotype. Genetic silencing of PTX3 in invasive melanoma cells dramatically impaired migration and invasion in vitro and in experimental lung extravasation assay in xenografted mice. In contrast, addition of melanoma-derived or recombinant PTX3, or expression of PTX3 enhanced motility of low migratory cells. Mechanistically, autocrine production of PTX3 by melanoma cells triggered an IKK/NFκB signaling pathway that promotes migration, invasion, and expression of the EMT factor TWIST1. Finally, we found that TLR4 and MYD88 knockdown inhibited PTX3-induced melanoma cell migration, suggesting that PTX3 functions through a TLR4-dependent pathway. Our work reveals that tumor-derived PTX3 contributes to melanoma cell invasion via targetable inflammation-related pathways. In addition to providing new insights into the biology of melanoma invasive behavior, this study underscores the notion that secreted PTX3 represents a potential biomarker and therapeutic target in a subpopulation of MITFlow invasive and/or refractory melanoma.

CD99 isoforms regulate CD1a expression in human monocyte-derived DCs through ATF-2/CREB-1 phosphorylation

CD1a expression is considered one of the major characteristics qualifying in vitro human dendritic cells (DCs) during their generation process. Here, we report that CD1A transcription is regulated by a mechanism involving the long and short isoforms of CD99. Using a lentiviral construct encoding for a CD99 short hairpin RNA, we were able to inhibit CD99 expression in human primary DCs. In such cells, CD1a membrane expression increased and CD1A transcripts were much higher in abundance compared to cells expressing CD99 long form (CD99LF). We also show that CD1A transcription is accompanied by a switch in expression from CD99LF to expression at comparable levels of both CD99 isoforms during immature DCs generation in vitro. We demonstrate that CD99LF maintains a lower level of CD1A transcription by up-regulating the phosphorylated form of the ATF-2 transcription factor and that CD99 short form (SF) is required to counteract this regulatory mechanism. Elucidation of the molecular mechanisms related to CD99 alternative splicing will be very helpful to better understand the transcriptional regulatory mechanism of CD1a molecules during DCs differentiation and its involvement in the immune response.

Inhibition of the GTPase Rac1 mediates the antimigratory effects of metformin in prostate cancer cells

Cell migration is a critical step in the progression of prostate cancer to the metastatic state, the lethal form of the disease. The antidiabetic drug metformin has been shown to display antitumoral properties in prostate cancer cell and animal models; however, its role in the formation of metastases remains poorly documented. Here, we show that metformin reduces the formation of metastases to fewer solid organs in an orthotopic metastatic prostate cancer cell model established in nude mice. As predicted, metformin hampers cell motility in PC3 and DU145 prostate cancer cells and triggers a radical reorganization of the cell cytoskeleton. The small GTPase Rac1 is a master regulator of cytoskeleton organization and cell migration. We report that metformin leads to a major inhibition of Rac1 GTPase activity by interfering with some of its multiple upstream signaling pathways, namely P-Rex1 (a Guanine nucleotide exchange factor and activator of Rac1), cAMP, and CXCL12/CXCR4, resulting in decreased migration of prostate cancer cells. Importantly, overexpression of a constitutively active form of Rac1, or P-Rex, as well as the inhibition of the adenylate cyclase, was able to reverse the antimigratory effects of metformin. These results establish a novel mechanism of action for metformin and highlight its potential antimetastatic properties in prostate cancer.

Technical advance: actin CytoFRET, a novel FRET flow cytometry method for detection of actin dynamics in resting and activated T cell

Actin cytoskeleton plays a critical role in regulating T cell motility and activation. However, the lack of a real-time quantitative method to analyze actin assembly has limited the progress toward understanding actin regulation. Here, we describe a novel approach to probe actin dynamics on living T cells using FRET combined with flow cytometry. We have first generated a Jurkat T cell line stably coexpressing EGFP and mOrange FPs fused to actin. The real-time variation of actin monomer assembly or disassembly into filaments was quantified using a ratiometric flow cytometry method measuring changes in the mOrange/EGFP emission ratio. The method was validated on resting T cells by using chemical compounds with known effects on actin filaments and comparison with conventional microscopy imaging. Our method also detected the rapid and transient actin assembly in T cells stimulated by anti-CD3/CD28-coated beads, demonstrating its robustness and high sensitivity. Finally, we provide evidence that lentiviral-mediated transduction of shRNAs in engineered Jurkat cells could be used as a strategy to identify regulators of actin remodeling. In conclusion, the flow cytometric FRET analysis of actin polymerization represents a new technical advance to study the dynamics of actin regulation in intact cells.

CXCR7 receptors facilitate the progression of colon carcinoma within lung not within liver

Background:

Liver and lung metastases are the predominant cause of colorectal cancer (CRC)-related mortality. Chemokine-receptor pairs have a critical role in determining the metastatic progression of tumours. Our hypothesis was that disruption of CXCR7/CXCR7 ligands axis could lead to a decrease in CRC metastases.

Methods:

Primary tumours and metastatic tissues from patients with CRC were tested for the expression of CXCR7 and its ligands. Relevance of CXCR7/CXCR7 ligands for CRC metastasis was then investigated in mice using small pharmacological CXCR7 antagonists and CRC cell lines of human and murine origins, which – injected into mice – enable the development of lung and liver metastases.

Results:

Following injection of CRC cells, mice treated daily with CXCR7 antagonists exhibited a significant reduction in lung metastases. However, CXCR7 antagonists failed to reduce the extent of liver metastasis. Moreover, there were subtle differences in the expression of CXCR7 and its ligands between lung and liver metastases.

Conclusion:

Our study suggests that the activation of CXCR7 on tumour blood vessels by its ligands may facilitate the progression of CRC within lung but not within liver. Moreover, we provide evidence that targeting the CXCR7 axis may be beneficial to limit metastasis from colon cancer within the lungs.

Targeting cancer cell metabolism: the combination of metformin and 2-deoxyglucose induces p53-dependent apoptosis in prostate cancer cells

Targeting cancer cell metabolism is a new promising strategy to fight cancer. Metformin, a widely used antidiabetic agent, exerts antitumoral and antiproliferative action. In this study, the addition of metformin to 2-deoxyglucose (2DG) inhibited mitochondrial respiration and glycolysis in prostate cancer cells leading to a severe depletion in ATP. The combination of the two drugs was much more harmful for cancer cells than the treatment with metformin or 2DG alone, leading to 96% inhibition of cell viability in LNCaP prostate cancer cells. In contrast, a moderate effect on cell viability was observed in normal prostate epithelial cells. At the cellular level, the combination of metformin and 2DG induced p53-dependent apoptosis via the energy sensor pathway AMP kinase, and the reexpression of a functional p53 in p53-deficient prostate cancer cells restored caspase-3 activity. In addition to apoptosis, the combination of metformin and 2DG arrested prostate cancer cells in G(2)-M. This G(2)-M arrest was independent of p53 and correlated with a stronger decrease in cell viability than obtained with either drug. Finally, metformin inhibited 2DG-induced autophagy, decreased beclin 1 expression, and triggered a switch from a survival process to cell death. Our study reinforces the growing interest of metabolic perturbators in cancer therapy and highlights the potential use of the combination of metformin and 2DG as an anticancerous treatment.

Evidence for MPL W515L/K mutations in hematopoietic stem cells in primitive myelofibrosis

The MPL (W515L and W515K) mutations have been detected in granulocytes of patients suffering from certain types of primitive myelofibrosis (PMF). It is still unknown whether this molecular event is also present in lymphoid cells and therefore potentially at the hematopoietic stem cell (HSC) level. Toward this goal, we conducted MPL genotyping of mature myeloid and lymphoid cells and of lymphoid/myeloid progenitors isolated from PMF patients carrying the W515 mutations. We detected both MPL mutations in granulocytes, monocytes, and platelets as well as natural killer (NK) cells but not in T cells. B/NK/myeloid and/or NK/myeloid CD34(+)CD38(-)-derived clones were found to carry the mutations. Long-term reconstitution of MPL W515 CD34(+) cells in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice was successful for as long as 12 weeks after transplantation, indicating that MPL W515 mutations were present in HSCs. Moreover, the 2 MPL mutations induced a spontaneous megakaryocytic growth in culture with an overall normal response to thrombopoietin (TPO). In contrast, erythroid progenitors remained EPO dependent. These results demonstrate that in PMF, the MPL W515L or K mutation induces a spontaneous megakaryocyte (MK) differentiation and occurs in a multipotent HSCs.

The JAK2 617V>F mutation triggers erythropoietin hypersensitivity and terminal erythroid amplification in primary cells from patients with polycythemia vera

The JAK2 617V>F mutation is frequent in polycythemia vera (PV) and essential thrombocythemia (ET). Using quantitative polymerase chain reaction (PCR), we found that high levels of JAK2 617V>F in PV correlate with increased granulocytes and high levels of hemoglobin and endogenous erythroid colony formation. We detected normal progenitors and those that were heterozygous or homozygous for the mutation by genotyping ET and PV clonal immature and committed progenitors. In PV patients, we distinguished homozygous profiles with normal, heterozygous, and homozygous progenitors from heterozygous profiles with only heterozygous and normal progenitors. PV patients with a heterozygous profile had more mutated, committed progenitors than did other PV and ET patients, suggesting a selective amplification of mutated cells in the early phases of hematopoiesis. We demonstrated that mutated erythroid progenitors were more sensitive to erythropoietin than normal progenitors, and that most homozygous erythroid progenitors were erythropoietin independent. Moreover, we observed a greater in vitro erythroid amplification and a selective advantage in vivo for mutated cells in late stages of hematopoiesis. These results suggest that, for PV, erythrocytosis can occur through two mechanisms: terminal erythroid amplification triggered by JAK2 617V>F homozygosity, and a 2-step process including the upstream amplification of heterozygous cells that may involve additional molecular events.

Proplatelet formation is regulated by the Rho/ROCK pathway

Platelets are released by megakaryocytes (MKs) via cytoplasmic extensions called proplatelets, which require profound changes in the microtubule and actin organization. Here, we provide evidence that the Rho/ROCK pathway, a well-known regulator of actin cytoskeleton, acts as a negative regulator of proplatelet formation (PPF). Rho is expressed at a high level during the entire MK differentiation including human CD34(+) cells. Thrombopoietin stimulates its activity but at a higher extent in immature than in mature MKs. Overexpression of a dominant-negative or a spontaneously active RhoA leads to an increase or a decrease in PPF indicating that Rho activation inhibits PPF. This inhibitory effect is mediated through the main Rho effector, Rho kinase (ROCK), the inhibition of which also increases PPF. Furthermore, inhibition of Rho or ROCK in MKs leads to a decrease in myosin light chain 2 (MLC2) phosphorylation, which is required for myosin contractility. Interestingly, inhibition of the MLC kinase also decreases MLC2 phosphorylation while increasing PPF. Taken together, our results suggest that MLC2 phosphorylation is regulated by both ROCK and MLC kinase and plays an important role in platelet biogenesis by controlling PPF and fragmentation.

Thrombopoietin responsiveness reflects the number of doublings undergone by megakaryocyte progenitors

To assess the variation of thrombopoietin (TPO) responsiveness associated with megakaryocyte (MK) progenitor amplification, TPO dose-response curves were obtained for normal human, single-cell plated CD34+CD41+ cells. The number of MKs per well was determined in situ and expressed as number of doublings (NbD). Dose-response curves of the mean frequency of clones of each size versus log TPO concentration showed highly significant differences in the TPO concentration needed for half-maximum generation of clones of different sizes (TPO50): 1.89 ± 0.51 pg/mL for 1 MK clones; 7.75 ± 0.81 pg/mL for 2 to 3 MK clones; 38.5 ± 5.04 pg/mL for 4 to 7 MK clones, and 91.8 ± 16.0 pg/mL for 8 to 15 MK clones. These results were consistent with a prediction of the generation-age model, because the number of previous doublings in vivo was inversely correlated with the number of residual doublings in vitro. TPO responsiveness decreased in vitro by a factor of 3.5 per doubling, reflecting the recruitment of progressively more ancestral progenitors. In support of this hypothesis, the more mature CD34+CD41+CD42+ cell fraction had a lower TPO50 (P < .001), underwent fewer NbD (P < .001), and expressed a 2.8-fold greater median Mpl receptor density (P < .001) than the CD34+CD41+CD42– fraction. Progenitors that have completed their proliferative program have maximum factor responsiveness and are preferentially induced to terminal differentiation.

Multiple signaling pathways are involved in erythropoietin-independent differentiation of erythroid progenitors in polycythemia vera

Polycythemia vera (PV) is a myeloproliferative disorder arising in a multipotent hematopoietic stem cell. The pathogenesis of PV remains poorly understood; however, the biologic hallmark of this disease is the presence of erythropoietin (Epo)-independent colony formation (endogenous erythroid colony [EEC]) and cytokine hypersensitivity. We have developed a simple liquid culture from CD34+ cells to study PV erythroid differentiation. PV erythroid differentiation was characterized in this culture system by two types of abnormalities: 1) an increased proliferation of progenitors in response to cytokines, associated with strict cytokine dependency for preventing apoptosis; and 2) Epo-independent terminal erythroid differentiation in the presence of stem cell factor and interleukin-3 as evidenced by the acquisition of glycophorin A. The level of Epo-independent terminal differentiation correlates in PV patients with the number of EEC. Epo-independent terminal differentiation as well as normal Epo-induced differentiation were repressed by inhibitors of JAK2 (AG490), PI3K (LY294002), and the Src family kinases (PP2). In contrast, an inhibitor of the ERK/MAP kinase pathway (PD98059) had no effect on Epo-independent terminal differentiation. These signaling abnormalities were not mediated by a decreased expression or activity of the membrane tyrosine phosphatase CD45, which dephosphorylates JAK2 and Src family kinases. This study demonstrates that early steps of PV erythroid differentiation are strictly cytokine dependent. In contrast, late erythroid differentiation is an Epo-independent phenomenon that is mediated by signaling pathways identical to those in Epo-induced differentiation.

Spontaneous STAT5 activation induces growth factor independence in idiopathic myelofibrosis: possible relationship with FKBP51 overexpression

Spontaneous growth of megakaryocyte progenitors is one of the biologic hallmarks of idiopathic myelofibrosis (IMF). The molecular mechanisms underlying this hypersensitivity to cytokines are poorly understood. Using a differential display approach, we previously observed FK506 binding protein 51 (FKBP51) overexpression in pathologic megakaryocytes from IMF. Using an FKBP51-overexpressing cell line, we found sustained STAT5 activation associated with JAK2 phosphorylation. We subsequently tested whether this transcription factor was activated in patient samples. We detected a STAT5 nuclear translocation and activation in spontaneously grown megakaryocytes and in circulating CD34(+) cells from the majority of patients studied. The biologic role of this JAK/STAT pathway activation was demonstrated by inhibiting both the anti-apoptotic phenotype mediated by FKBP51 overexpression in UT7 cells and the spontaneous megakaryocytic growth by addition in culture of the JAK2 inhibitor AG490 or overexpression of a STAT5b dominant negative or SOCS-1. These results demonstrate that a constitutive STAT5 activation in IMF is indispensable for spontaneous growth of megakaryocytes. They also suggest that FKBP51 overexpression could be involved in STAT5 activation in IMF cells and in subsequent abnormal growth.

Overexpression of FKBP51 in idiopathic myelofibrosis regulates the growth factor independence of megakaryocyte progenitors

Idiopathic myelofibrosis (IMF) is a chronic myeloproliferative disorder characterized by megakaryocyte hyperplasia and bone marrow fibrosis. Biologically, an autonomous megakaryocyte growth and differentiation is noticed, which contributes to the megakaryocyte accumulation. To better understand the molecular mechanisms involved in this spontaneous growth, we searched for genes differentially expressed between normal megakaryocytes requiring cytokines to grow and IMF spontaneously proliferating megakaryocytes. Using a differential display technique, we found that the immunophilin FKBP51 was 2 to 8 times overexpressed in megakaryocytes derived from patients' CD34(+) cells in comparison to normal megakaryocytes. Overexpression was moderate and confirmed in 8 of 10 patients, both at the mRNA and protein levels. Overexpression of FKBP51 in a UT-7/Mpl cell line and in normal CD34(+) cells induced a resistance to apoptosis mediated by cytokine deprivation with no effect on proliferation. FKBP51 interacts with both calcineurin and heat shock protein (HSP)70/HSP90. However, a mutant FKBP51 deleted in the HSP70/HSP90 binding site kept the antiapoptotic effect, suggesting that the calcineurin pathway was responsible for the FKBP51 effect. Overexpression of FKBP51 in UT-7/Mpl cells induced a marked inhibition of calcineurin activity. Pharmacologic inhibition of calcineurin by cyclosporin A mimicked the effect of FKBP51. The data support the conclusion that FKBP51 inhibits apoptosis through a calcineurin-dependent pathway. In conclusion, FKBP51 is overexpressed in IMF megakaryocytes and this overexpression could be, in part, responsible for the megakaryocytic accumulation observed in this disorder by regulating their apoptotic program.