Lnk adaptor: novel negative regulator of B cell lymphopoiesis

Telomere length and its correlation with gene mutations in chronic lymphocytic leukemia in a Korean population

Telomere length (TL) is a prognostic indicator in Caucasian chronic lymphocytic leukemia (CLL), but its significance in Asian CLL remains unknown. To investigate the prognostic significance of TL and its correlation with cytogenetic aberrations and somatic mutations, we analyzed TL measurements at the cellular level by interphase fluorescence in situ hybridization in patients with CLL in Korea. The present study enrolled 110 patients (41 females and 69 males) diagnosed with CLL according to the World Health Organization criteria (2001-2017). TLs of bone marrow nucleated cells at the single-cell level were measured by quantitative fluorescence in situ hybridization (Q-FISH) in 71 patients. The correlations of TL with clinical characteristics, cytogenetic aberrations, genetic mutations, and overall survival were assessed. The median value of mean TL in CLL patients (T/C ratio 7.46 (range 1.19-18.14) was significantly shorter than that in the normal controls (T/C ratio 15.28 (range 8.59-24.93) (p < 0.001). Shorter TLs were associated with complex karyotypes (p = 0.030), del(11q22) (p = 0.023), presence of deletion and/or mutation in ATM and/or TP53 (p = 0.019), and SH2B3 mutation (p = 0.015). A shorter TL was correlated with lower hemoglobin levels and adverse survival (mean TL < 9.35, p = 0.021). When the proportion of cells with extremely short TLs (< 7.61) was greater than 90%, CLL patients showed poor survival (p = 0.002). Complex karyotypes, TP53 mutation, and the number of mutated genes were determined to be significant adverse variables by multivariable Cox analysis (p = 0.011, p = 0.002, and p = 0.002, respectively). TL was attrited in CLL, and attrited telomeres were correlated with adverse survival and other well-known adverse prognostic factors. We infer that TL is an independent adverse prognostic predictor in Korean CLL.

SH2B3 inactivation through CN-LOH 12q is uniquely associated with B-cell precursor ALL with iAMP21 or other chromosome 21 gain

In more than 30% of B-cell precursor acute lymphoblastic leukaemia (B-ALL), chromosome 21 sequence is overrepresented through aneuploidy or structural rearrangements, exemplified by intrachromosomal amplification of chromosome 21 (iAMP21). Although frequent, the mechanisms by which these abnormalities promote B-ALL remain obscure. Intriguingly, we found copy number neutral loss of heterozygosity (CN-LOH) of 12q was recurrent in iAMP21-ALL, but never observed in B-ALL without some form of chromosome 21 gain. As a consequence of CN-LOH 12q, mutations or deletions of the adaptor protein, SH2B3, were converted to homozygosity. In patients without CN-LOH 12q, bi-allelic abnormalities of SH2B3 occurred, but only in iAMP21-ALL, giving an overall incidence of 18% in this sub-type. Review of published data confirmed a tight association between overrepresentation of chromosome 21 and both CN-LOH 12q and SH2B3 abnormalities in B-ALL. Despite relatively small patient numbers, preliminary analysis linked 12q abnormalities to poor outcome in iAMP21-ALL (p = 0.03). Homology modelling of a leukaemia-associated SH2 domain mutation and in vitro analysis of patient-derived xenograft cells implicated the JAK/STAT pathway as one likely target for SH2B3 tumour suppressor activity in iAMP21-ALL.

Myeloproliferative neoplasms: Current molecular biology and genetics

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by increased production of mature blood cells. Philadelphia chromosome-negative MPNs (Ph-MPNs) consist of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). A number of stem cell derived mutations have been identified in the past 10 years. These findings showed that JAK2V617F, as a diagnostic marker involving JAK2 exon 14 with a high frequency, is the best molecular characterization of Ph-MPNs. Somatic mutations in an endoplasmic reticulum chaperone, named calreticulin (CALR), is the second most common mutation in patients with ET and PMF after JAK2 V617F mutation. Discovery of CALR mutations led to the increased molecular diagnostic of ET and PMF up to 90%. It has been shown that JAK2V617F is not the unique event in disease pathogenesis. Some other genes' location such as TET oncogene family member 2 (TET2), additional sex combs-like 1 (ASXL1), casitas B-lineage lymphoma proto-oncogene (CBL), isocitrate dehydrogenase 1/2 (IDH1/IDH2), IKAROS family zinc finger 1 (IKZF1), DNA methyltransferase 3A (DNMT3A), suppressor of cytokine signaling (SOCS), enhancer of zeste homolog 2 (EZH2), tumor protein p53 (TP53), runt-related transcription factor 1 (RUNX1) and high mobility group AT-hook 2 (HMGA2) have also identified to be involved in MPNs phenotypes. Here, current molecular biology and genetic mechanisms involved in MNPs with a focus on the aforementioned factors is presented.

The genomic landscape of juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a myeloproliferative neoplasm (MPN) of childhood with a poor prognosis. Mutations in NF1, NRAS, KRAS, PTPN11 or CBL occur in 85% of patients, yet there are currently no risk stratification algorithms capable of predicting which patients will be refractory to conventional treatment and could therefore be candidates for experimental therapies. In addition, few molecular pathways aside from the RAS-MAPK pathway have been identified that could serve as the basis for such novel therapeutic strategies. We therefore sought to genomically characterize serial samples from patients at diagnosis through relapse and transformation to acute myeloid leukemia to expand knowledge of the mutational spectrum in JMML. We identified recurrent mutations in genes involved in signal transduction, splicing, Polycomb repressive complex 2 (PRC2) and transcription. Notably, the number of somatic alterations present at diagnosis appears to be the major determinant of outcome.

Disruption of KMT2D perturbs germinal center B cell development and promotes lymphomagenesis

Mutations in the gene encoding the KMT2D (or MLL2) methyltransferase are highly recurrent and occur early during tumorigenesis in diffuse large B cell lymphoma (DLBCL) and follicular lymphoma (FL). However, the functional consequences of these mutations and their role in lymphomagenesis are unknown. Here we show that FL- and DLBCL-associated KMT2D mutations impair KMT2D enzymatic activity, leading to diminished global H3K4 methylation in germinal-center (GC) B cells and DLBCL cells. Conditional deletion of Kmt2d early during B cell development, but not after initiation of the GC reaction, results in an increase in GC B cells and enhances B cell proliferation in mice. Moreover, genetic ablation of Kmt2d in mice overexpressing Bcl2 increases the incidence of GC-derived lymphomas resembling human tumors. These findings suggest that KMT2D acts as a tumor suppressor gene whose early loss facilitates lymphomagenesis by remodeling the epigenetic landscape of the cancer precursor cells. Eradication of KMT2D-deficient cells may thus represent a rational therapeutic approach for targeting early tumorigenic events.

LNK (SH2B3): paradoxical effects in ovarian cancer

LNK (SH2B3) is an adaptor protein studied extensively in normal and malignant hematopoietic cells. In these cells, it downregulates activated tyrosine kinases at the cell surface resulting in an antiproliferative effect. To date, no studies have examined activities of LNK in solid tumors. In this study, we found by in silico analysis and staining tissue arrays that the levels of LNK expression were elevated in high-grade ovarian cancer. To test the functional importance of this observation, LNK was either overexpressed or silenced in several ovarian cancer cell lines. Remarkably, overexpression of LNK rendered the cells resistant to death induced by either serum starvation or nutrient deprivation, and generated larger tumors using a murine xenograft model. In contrast, silencing of LNK decreased ovarian cancer cell growth in vitro and in vivo. Western blot studies indicated that overexpression of LNK upregulated and extended the transduction of the mitogenic signal, whereas silencing of LNK produced the opposite effects. Furthermore, forced expression of LNK reduced cell size, inhibited cell migration and markedly enhanced cell adhesion. Liquid chromatography-mass spectroscopy identified 14-3-3 as one of the LNK-binding partners. Our results suggest that in contrast to the findings in hematologic malignancies, the adaptor protein LNK acts as a positive signal transduction modulator in ovarian cancers.

Genetic loss of SH2B3 in acute lymphoblastic leukemia

The SH2B adaptor protein 3 (SH2B3) gene encodes a negative regulator of cytokine signaling with a critical role in the homeostasis of hematopoietic stem cells and lymphoid progenitors. Here, we report the identification of germline homozygous SH2B3 mutations in 2 siblings affected with developmental delay and autoimmunity, one in whom B-precursor acute lymphoblastic leukemia (ALL) developed. Mechanistically, loss of SH2B3 increases Janus kinase-signal transducer and activator of transcription signaling, promotes lymphoid cell proliferation, and accelerates leukemia development in a mouse model of NOTCH1-induced ALL. Moreover, extended mutation analysis showed homozygous somatic mutations in SH2B3 in 2 of 167 ALLs analyzed. Overall, these results demonstrate a Knudson tumor suppressor role for SH2B3 in the pathogenesis of ALL and highlight a possible link between genetic predisposition factors in the pathogenesis of autoimmunity and leukemogenesis.

When the Brakes are Lost: LNK Dysfunction in Mice, Men, and Myeloproliferative Neoplasms

Aberrant JAK-STAT signaling is a hallmark of myeloproliferative neoplasms (MPNs). These hyperproliferative disorders are classically associated with activating mutations in tyrosine kinases such as JAK2 and the thrombopoietin (TPO) receptor MPL. Activation of JAK-STAT signaling and responses to JAK2 inhibitors have been observed in MPN patients lacking JAK2 or MPL mutations, suggesting that other regulatory elements in the JAK-STAT pathway are altered. However, the molecular basis for this observation has been unclear. Recently, the role of inhibitory regulators of JAK-STAT signaling in MPN pathogenesis has been increasingly recognized. LNK is an adaptor protein that forms a negative feedback loop by binding to MPL and JAK2 and inhibiting downstream STAT activation. Murine models indicate that loss of LNK function can promote the development of a MPN phenotype. Several recent studies have identified novel LNK mutations in MPNs, thus validating this notion in humans. These findings represent a novel genetic paradigm of loss of negative feedback regulation of JAK-STAT activation in MPNs and have implications for the future development of targeted therapies in MPNs.

The Lnk adaptor protein: a key regulator of normal and pathological hematopoiesis

The development and function of blood cells are regulated by specific growth factors/cytokines and their receptors' signaling pathways. In this way, these factors influence cell survival, proliferation and differentiation of hematopoietic cells. Central to this positive and/or negative control are the adaptor proteins. Since their identification 10 years ago, members of the Lnk adaptor protein family have proved to be important activators and/or inhibitors in the hematopoietic, immune and vascular system. In particular, the generation of animal and cellular models for the Lnk and APS proteins has helped establish the physiological role of these molecules through the identification of their specific signaling pathways and the characterization of their binding partners. Moreover, the recent identification of mutations in the LNK gene in myeloproliferative disorders, as well as the correlation of a single nucleotide polymorphism on LNK with hematological, immune and vascular diseases have suggested its involvement in the pathophysiology of these malignancies. The latter findings have thus raised the possibility of addressing Lnk signaling for the treatment of certain human diseases. This review therefore describes the pathophysiological role of this adaptor protein in hematological malignancies and the potential benefits of Lnk therapeutic targeting.

Adaptor protein LNK is a negative regulator of brain neural stem cell proliferation after stroke

Ischemic stroke causes transient increase of neural stem and progenitor cell (NSPC) proliferation in the subventricular zone (SVZ), and migration of newly formed neuroblasts toward the damaged area where they mature to striatal neurons. The molecular mechanisms regulating this plastic response, probably involved in structural reorganization and functional recovery, are poorly understood. The adaptor protein LNK suppresses hematopoietic stem cell self-renewal, but its presence and role in the brain are poorly understood. Here we demonstrate that LNK is expressed in NSPCs in the adult mouse and human SVZ. Lnk(-/-) mice exhibited increased NSPC proliferation after stroke, but not in intact brain or following status epilepticus. Deletion of Lnk caused increased NSPC proliferation while overexpression decreased mitotic activity of these cells in vitro. We found that Lnk expression after stroke increased in SVZ through the transcription factors STAT1/3. LNK attenuated insulin-like growth factor 1 signaling by inhibition of AKT phosphorylation, resulting in reduced NSPC proliferation. Our findings identify LNK as a stroke-specific, endogenous negative regulator of NSPC proliferation, and suggest that LNK signaling is a novel mechanism influencing plastic responses in postischemic brain.

Normal and malignant megakaryopoiesis

Megakaryopoiesis is the process by which bone marrow progenitor cells develop into mature megakaryocytes (MKs), which in turn produce platelets required for normal haemostasis. Over the past decade, molecular mechanisms that contribute to MK development and differentiation have begun to be elucidated. In this review, we provide an overview of megakaryopoiesis and summarise the latest developments in this field. Specially, we focus on polyploidisation, a unique form of the cell cycle that allows MKs to increase their DNA content, and the genes that regulate this process. In addition, because MKs have an important role in the pathogenesis of acute megakaryocytic leukaemia and a subset of myeloproliferative neoplasms, including essential thrombocythemia and primary myelofibrosis, we discuss the biology and genetics of these disorders. We anticipate that an increased understanding of normal MK differentiation will provide new insights into novel therapeutic approaches that will directly benefit patients.

Adaptor protein Lnk binds to PDGF receptor and inhibits PDGF-dependent signaling

OBJECTIVE

Platelet-derived growth factor receptors α and β (PDGFRA, PDGFRB) are frequently expressed on hematopoietic cells and regulate cellular responses such as proliferation, differentiation, survival, and transformation. Stimulation by autocrine loops or activation by chromosomal translocation makes them important factors in development of hematopoietic disorders. Interaction with the ligand PDGF results in activation of the tyrosine kinase domain and phosphorylation of tyrosine residues, thereby creating binding sites for molecules containing Src homology 2 domains. We hypothesized that one such protein may be Lnk, a negative regulator of cytokine receptors, including Mpl, EpoR, c-Kit, and c-Fms.

MATERIALS AND METHODS

Interaction of Lnk with PDGFRA, PDGFRB, or leukemogenic FIP1L1-PDGFRA or TEL-PDGFRB was studied in cotransfected 293T cells. Effects of Lnk on PDGFR signaling were shown in 293T and NIH3T3 cells, whereas its influence on either PDGF-dependent or factor-independent growth was investigated using Ba/F3 or 32D cells expressing wild-type PDGFR, FIP1L1-PDGFRA, or TEL-PDGFRB.

RESULTS

We show that Lnk binds to PDGFR after exposure of cells to PDGF. Furthermore, Lnk can bind the FIP1L1-PDGFRA fusion protein. Mutation or deletion of the Lnk Src homology 2 domain completely abolished binding of Lnk to FIP1L1-PDGFRA, but just partly prevented binding to PDGFRA or PDGFRB. Expression of Lnk inhibited proliferation of PDGF-dependent Ba/F3 cells and diminished phosphorylation of Erk in PDGF-treated NIH3T3. 32D cells transformed by either FIP1L1-PDGFRA or TEL-PDGFRB stopped growing when Lnk was expressed.

CONCLUSIONS

Lnk is a negative regulator of PDGFR signaling. Development of Lnk mimetic drugs might provide a novel therapeutic strategy for myeloproliferative disorders.

ROS-mediated amplification of AKT/mTOR signalling pathway leads to myeloproliferative syndrome in Foxo3(-/-) mice

Reactive oxygen species (ROS) participate in normal intracellular signalling and in many diseases including cancer and aging, although the associated mechanisms are not fully understood. Forkhead Box O (FoxO) 3 transcription factor regulates levels of ROS concentrations, and is essential for maintenance of hematopoietic stem cells. Here, we show that loss of Foxo3 causes a myeloproliferative syndrome with splenomegaly and increased hematopoietic progenitors (HPs) that are hypersensitive to cytokines. These mutant HPs contain increased ROS, overactive intracellular signalling through the AKT/mammalian target of rapamycin signalling pathway and relative deficiency of Lnk, a negative regulator of cytokine receptor signalling. In vivo treatment with ROS scavenger N-acetyl-cysteine corrects these biochemical abnormalities and relieves the myeloproliferation. Moreover, enforced expression of Lnk by retroviral transfer corrects the abnormal expansion of Foxo3(-/-) HPs in vivo. Our combined results show that loss of Foxo3 causes increased ROS accumulation in HPs. In turn, this inhibits Lnk expression that contributes to exaggerated cytokine responses that lead to myeloproliferation. Our findings could explain the mechanisms by which mutations that alter Foxo3 function induce malignancy. More generally, the work illustrates how deregulated ROS may contribute to malignant progression.

Novel mutations in the inhibitory adaptor protein LNK drive JAK-STAT signaling in patients with myeloproliferative neoplasms

Dysregulated Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling due to activation of tyrosine kinases is a common feature of myeloid malignancies. Here we report the first human disease-related mutations in the adaptor protein LNK, a negative regulator of JAK-STAT signaling, in 2 patients with JAK2 V617F-negative myeloproliferative neoplasms (MPNs). One patient exhibited a 5 base-pair deletion and missense mutation leading to a premature stop codon and loss of the pleckstrin homology (PH) and Src homology 2 (SH2) domains. A second patient had a missense mutation (E208Q) in the PH domain. BaF3-MPL cells transduced with these LNK mutants displayed augmented and sustained thrombopoietin-dependent growth and signaling. Primary samples from MPN patients bearing LNK mutations exhibited aberrant JAK-STAT activation, and cytokine-responsive CD34(+) early progenitors were abnormally abundant in both patients. These findings indicate that JAK-STAT activation due to loss of LNK negative feedback regulation is a novel mechanism of MPN pathogenesis.

Lnk constrains myeloproliferative diseases in mice

Hematopoietic stem and progenitor cell (HSPC) expansion is regulated by intrinsic signaling pathways activated by cytokines. The intracellular kinase JAK2 plays an essential role in cytokine signaling, and activating mutations in JAK2 are found in a number of hematologic malignancies. We previously demonstrated that lymphocyte adaptor protein (Lnk, also known as Sh2b3) binds JAK2 and attenuates its activity, thereby limiting HSPC expansion. Here we show that loss of Lnk accelerates and exacerbates oncogenic JAK2-induced myeloproliferative diseases (MPDs) in mice. Specifically, Lnk deficiency enhanced cytokine-independent JAK/STAT signaling and augmented the ability of oncogenic JAK2 to expand myeloid progenitors in vitro and in vivo. An activated form of JAK2, unable to bind Lnk, caused greater myeloid expansion than activated JAK2 alone and accelerated myelofibrosis, indicating that Lnk directly inhibits oncogenic JAK2 in constraining MPD development. In addition, Lnk deficiency cooperated with the BCR/ABL oncogene, the product of which does not directly interact with or depend on JAK2 or Lnk, in chronic myeloid leukemia (CML) development, suggesting that Lnk also acts through endogenous pathways to constrain HSPCs. Consistent with this idea, aged Lnk-/- mice spontaneously developed a CML-like MPD. Taken together, our data establish Lnk as a bona fide suppressor of MPD in mice and raise the possibility that Lnk dysfunction contributes to the development of hematologic malignancies in humans.

Regulation of lifespan, metabolism, and stress responses by the Drosophila SH2B protein, Lnk

Drosophila Lnk is the single ancestral orthologue of a highly conserved family of structurally-related intracellular adaptor proteins, the SH2B proteins. As adaptors, they lack catalytic activity but contain several protein–protein interaction domains, thus playing a critical role in signal transduction from receptor tyrosine kinases to form protein networks. Physiological studies of SH2B function in mammals have produced conflicting data. However, a recent study in Drosophila has shown that Lnk is an important regulator of the insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway during growth, functioning in parallel to the insulin receptor substrate, Chico. As this pathway also has an evolutionary conserved role in the determination of organism lifespan, we investigated whether Lnk is required for normal lifespan in Drosophila. Phenotypic analysis of mutants for Lnk revealed that loss of Lnk function results in increased lifespan and improved survival under conditions of oxidative stress and starvation. Starvation resistance was found to be associated with increased metabolic stores of carbohydrates and lipids indicative of impaired metabolism. Biochemical and genetic data suggest that Lnk functions in both the IIS and Ras/Mitogen activated protein Kinase (MapK) signaling pathways. Microarray studies support this model, showing transcriptional feedback onto genes in both pathways as well as indicating global changes in both lipid and carbohydrate metabolism. Finally, our data also suggest that Lnk itself may be a direct target of the IIS responsive transcription factor, dFoxo, and that dFoxo may repress Lnk expression. We therefore describe novel functions for a member of the SH2B protein family and provide the first evidence for potential mechanisms of SH2B regulation. Our findings suggest that IIS signaling in Drosophila may require the activity of a second intracellular adaptor, thereby yielding fundamental new insights into the functioning and role of the IIS pathway in ageing and metabolism.

Many human populations are experiencing increased life expectancy, and as populations age the incidence of age-related diseases becomes more prevalent. The identification of single gene mutations that extend lifespan in invertebrate model organisms has revealed that several cellular signaling pathways, including the insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway, play a crucial role in modulating the ageing process across multiple species. Thus, studies carried out in yeast, worms, and flies have revealed evolutionarily conserved mechanisms of ageing, which are likely to be relevant to mammals, including humans. A recent study in Drosophila identified the SH2B family adaptor protein, Lnk, as an important regulator of the IIS pathway during organismal growth. In this study, we show that Lnk is also required to determine normal lifespan in Drosophila, as mutations that disrupt Lnk activity result in increased lifespan. In addition, these mutants show improved survival under conditions of stress and metabolic disregulation. Furthermore, we show that the expression of Lnk is regulated by the IIS responsive transcription factor, dFoxo. Our data therefore provide new mechanistic insights into the role of the IIS pathway in ageing.

Expression of the adaptor protein Lnk in leukemia cells

OBJECTIVE

Tyrosine kinases are involved in cytokine signaling and are frequently aberrantly activated in hematological malignancies. Lnk, a negative regulator of cytokine signaling, plays critical nonredundant roles in hematopoiesis. By binding to phosphorylated tyrosine kinases, Lnk inhibits major cytokine receptor signaling, including c-KIT; erythropoietin receptor-Janus kinase 2 (JAK2); and MPL-JAK2. In the present study, we investigated Lnk expression and possible function in transformed hematopoietic cells.

MATERIALS AND METHODS

Coimmunoprecipitations were performed to identify binding between Lnk and mutant tyrosine kinases. Proliferation assays were done to examine the affect of Lnk overexpression on cancer cell growth. Real-time polymerase chain reaction analysis was used to determine Lnk expression in patient samples.

RESULTS

We show that, in parallel to binding wild-type JAK2 and c-KIT, Lnk associates with and is phosphorylated by mutant alleles of JAK2 and c-KIT. In contrast, Lnk does not bind to and is not phosphorylated by BCR-ABL fusion protein. Ectopic expression of Lnk strongly attenuates growth of some leukemia cell lines, while others as well as most solid tumor cancer cell lines are either moderately inhibited or completely insensitive to Lnk. Furthermore, Lnk-mediated growth inhibition is associated with differential downregulation of phosphatidylinositol 3 kinase/Akt/mammalian target of rapamycin and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in leukemia cell lines. Surprisingly, analysis of Lnk in a large panel of myelodysplastic syndrome and acute myeloid leukemia patient samples revealed high levels of Lnk in nearly half of the samples.

CONCLUSION

Although how leukemic cells overcome the antiproliferative effects of Lnk is not yet clear, our data highlight the multifaceted role negative feedback mechanisms play in malignant transformation.

Lnk inhibits myeloproliferative disorder-associated JAK2 mutant, JAK2V617F

The JAK2 mutation JAK2V617F is found frequently in patients with myeloproliferative disorders (MPD) and transforms hematopoietic cells to cytokine-independent proliferation when expressed with specific cytokine receptors. The Src homology 2 (SH2) and pleckstrin homology (PH) domain-containing adaptor protein Lnk (SH2B3) is a negative regulator of hematopoietic cytokine signaling. Here, we show that Lnk is a potent inhibitor of JAK2V617F constitutive activity. Lnk down-regulates JAK2V617F-mediated signaling and transformation in hematopoietic Ba/F3-erythropoietin receptor cells. Furthermore, in CFU assays, Lnk-deficient murine bone marrow cells are significantly more sensitive to transformation by JAK2V617F than wild-type (WT) cells. Lnk, through its SH2 and PH domains, interacts with WT and mutant JAK2 and is phosphorylated by constitutively activated JAK2V617F. Finally, we found that Lnk levels are high in CD34(+) hematopoietic progenitors from MPD patients and that Lnk expression is induced following JAK2 activation. Our data suggest that JAK2V617F is susceptible to endogenous negative-feedback regulation, providing new insights into the molecular pathogenesis of MPD.

Lnk controls mouse hematopoietic stem cell self-renewal and quiescence through direct interactions with JAK2

In addition to its role in megakaryocyte production, signaling initiated by thrombopoietin (TPO) activation of its receptor, myeloproliferative leukemia virus protooncogene (c-Mpl, or Mpl), controls HSC homeostasis and self-renewal. Under steady-state conditions, mice lacking the inhibitory adaptor protein Lnk harbor an expanded HSC pool with enhanced self-renewal. We found that HSCs from Lnk-/- mice have an increased quiescent fraction, decelerated cell cycle kinetics, and enhanced resistance to repeat treatments with cytoablative 5-fluorouracil in vivo compared with WT HSCs. We further provide genetic evidence demonstrating that Lnk controls HSC quiescence and self-renewal, predominantly through Mpl. Consistent with this observation, Lnk-/- HSCs displayed potentiated activation of JAK2 specifically in response to TPO. Biochemical experiments revealed that Lnk directly binds to phosphorylated tyrosine residues in JAK2 following TPO stimulation. Of note, the JAK2 V617F mutant, found at high frequencies in myeloproliferative diseases, retains the ability to bind Lnk. Therefore, we identified Lnk as a physiological negative regulator of JAK2 in stem cells and TPO/Mpl/JAK2/Lnk as a major regulatory pathway in controlling stem cell self-renewal and quiescence.

Enhanced engraftment of hematopoietic stem/progenitor cells by the transient inhibition of an adaptor protein, Lnk

Hematopoietic stem cells (HSCs) are the key elements responsible for maintaining blood-cell production throughout life and for lymphohematopoietic reconstitution following bone marrow (BM) transplantation. Enhancement of the engrafting potential and expansion capabilities of HSCs as well as hematopoietic progenitor cells (HPCs) has been a long-time desire as a means of reducing the risks and difficulties that accompany BM transplantation. The ability of HSCs/HPCs to reconstitute the hematopoietic system of irradiated hosts is negatively regulated by an intracellular adaptor protein, Lnk. Here we have identified the functional domains of Lnk and developed a dominant-negative (DN) Lnk mutant that inhibits the functions of Lnk endogenously expressed in the HSCs/HPCs and thereby potentiates the HSCs/HPCs for engraftment. Importantly, even transient expression of DN-Lnk in HSCs/HPCs facilitated their engraftment under nonmyeloablative conditions and fully reconstituted the lymphoid compartments of immunodeficient host animals. HPCs expressing DN-Lnk were efficiently trapped by immobilized vascular cell adhesion molecule-1 (VCAM-1) in a transwell migration assay, suggesting involvement of Lnk in the regulation of cell mobility or cellular interaction in microenvironments. Transient inhibition of Lnk or Lnk-mediated pathways could be a potent approach to augment engraftment of HSCs/HPCs without obvious side effects.

La protéine adaptatrice Lnk module l'activation des cellules endothéliales

Lnk is an adaptator protein involved in B lymphocytes and platelet differentiation and in T lymphocyte activation. We previously reported on Lnk expression and regulation in endothelial cells (ECs) upon activation. In the present study, the involvement of Lnk in the tumor necrosis factor alpha (TNFalpha) pathway was investigated in vitro through Lnk overexpression in primary cultures of human endothelial cells. Using a recombinant adenovirus encoding human Lnk, we first demonstrated that Lnk overexpression does not induce vascular cell adhesion molecule-1 (VCAM-1) suggesting that Lnk does not promote ECs activation. However, Lnk overexpression significantly reduced TNFalpha-mediated expression of VCAM-1 (at mRNA and protein levels) in activated EC as compared with controls. Western blot analysis showed that Lnk overexpression in HUVEC was associated with phosphorylation of Akt kinase (at Ser 473) with no effect on IkappaBalpha, the specific inhibitor of NFkappaB, indicating that Lnk promotes activation of the phosphatidylinositol 3-kinase (PI3-kinase) pathway in ECs. Altogether, these results suggest that, in ECs, Lnk may participate to a regulatory pathway involving the PI3-kinase and modulating the inflammatory response.

Lnk inhibits erythropoiesis and Epo-dependent JAK2 activation and downstream signaling pathways

Erythropoietin (Epo), along with its receptor EpoR, is the principal regulator of red cell development. Upon Epo addition, the EpoR signaling through the Janus kinase 2 (JAK2) activates multiple pathways including Stat5, phosphoinositide-3 kinase (PI-3K)/Akt, and p42/44 mitogen-activated protein kinase (MAPK). The adaptor protein Lnk is implicated in cytokine receptor signaling. Here, we showed that Lnk-deficient mice have elevated numbers of erythroid progenitors, and that splenic erythroid colony-forming unit (CFU-e) progenitors are hypersensitive to Epo. Lnk(-/-) mice also exhibit superior recovery after erythropoietic stress. In addition, Lnk deficiency resulted in enhanced Epo-induced signaling pathways in splenic erythroid progenitors. Conversely, Lnk overexpression inhibits Epo-induced cell growth in 32D/EpoR cells. In primary culture of fetal liver cells, Lnk overexpression inhibits Epo-dependent erythroblast differentiation and induces apoptosis. Lnk blocks 3 major signaling pathways, Stat5, Akt, and MAPK, induced by Epo in primary erythroblasts. In addition, the Lnk Src homology 2 (SH2) domain is essential for its inhibitory function, whereas the conserved tyrosine near the C-terminus and the pleckstrin homology (PH) domain of Lnk are not critical. Furthermore, wild-type Lnk, but not the Lnk SH2 mutant, becomes tyrosine-phosphorylated following Epo administration and inhibits EpoR phosphorylation and JAK2 activation. Hence, Lnk, through its SH2 domain, negatively modulates EpoR signaling by attenuating JAK2 activation, and regulates Epo-mediated erythropoiesis.

Lnk inhibits Tpo-mpl signaling and Tpo-mediated megakaryocytopoiesis

Thrombopoietin (Tpo) is the primary cytokine regulating megakaryocyte development and platelet production. Tpo signaling through its receptor, c-mpl, activates multiple pathways including signal transducer and activator of transcription (STAT)3, STAT5, phosphoinositide 3-kinase–Akt, and p42/44 mitogen-activated protein kinase (MAPK). The adaptor protein Lnk is implicated in cytokine receptor and immunoreceptor signaling. Here, we show that Lnk overexpression negatively regulates Tpo-mediated cell proliferation and endomitosis in hematopoietic cell lines and primary hematopoietic cells. Lnk attenuates Tpo-induced S-phase progression in 32D cells expressing mpl, and Lnk decreases Tpo-dependent megakaryocyte growth in bone marrow (BM)–derived megakaryocyte culture. Consistent with this result, we found that in both BM and spleen, Lnk-deficient mice exhibited increased numbers of megakaryocytes with increased ploidy compared with wild-type mice. In addition, Lnk-deficient megakaryocytes derived from BM and spleen showed enhanced sensitivity to Tpo during culture. The absence of Lnk caused enhanced and prolonged Tpo induction of STAT3, STAT5, Akt, and MAPK signaling pathways in CD41⁺ megakaryocytes. Furthermore, the Src homology 2 domain of Lnk is essential for Lnk's inhibitory function. In contrast, the conserved tyrosine near the COOH terminus is dispensable and the pleckstrin homology domain of Lnk contributes to, but is not essential for, inhibiting Tpo-dependent 32D cell growth or megakaryocyte development. Thus, Lnk negatively modulates mpl signaling pathways and is important for Tpo-mediated megakaryocytopoiesis in vivo.