Elisabeth A. Pedersen

Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow

HSC homing, quiescence, and self-renewal depend on the bone marrow HSC niche. A large proportion of solid tumor metastases are bone metastases, known to usurp HSC homing pathways to establish footholds in the bone marrow. However, it is not clear whether tumors target the HSC niche during metastasis. Here we have shown in a mouse model of metastasis that human prostate cancer (PCa) cells directly compete with HSCs for occupancy of the mouse HSC niche. Importantly, increasing the niche size promoted metastasis, whereas decreasing the niche size compromised dissemination. Furthermore, disseminated PCa cells could be mobilized out of the niche and back into the circulation using HSC mobilization protocols. Finally, once in the niche, tumor cells reduced HSC numbers by driving their terminal differentiation. These data provide what we believe to be the first evidence that the HSC niche serves as a direct target for PCa during dissemination and plays a central role in bone metastases. Our work may lead to better understanding of the molecular events involved in bone metastases and new therapeutic avenues for an incurable disease.

Annexin II/Annexin II Receptor Axis Regulates Adhesion, Migration, Homing, and Growth of Prostate Cancer

One of the most life‐threatening complications of prostate cancer is skeletal metastasis. In order to develop treatment for metastasis, it is important to understand its molecular mechanisms. Our work in this field has drawn parallels between hematopoietic stem cell and prostate cancer homing to the marrow. Our recent work demonstrated that annexin II expressed by osteoblasts and endothelial cells plays a critical role in niche selection. In this study, we demonstrate that annexin II and its receptor play a crucial role in establishing metastasis of prostate cancer. Prostate cancer cell lines migrate toward annexin II and the adhesion of prostate cancer to osteoblasts and endothelial cells was inhibited by annexin II. By blocking annexin II or its receptor in animal models, short‐term and long‐term localization of prostate cancers are limited. Annexin II may also facilitate the growth of prostate cancer in vitro and in vivo by the MAPK pathway. These data strongly suggest that annexin II and its receptor axis plays a central role in prostate cancer metastasis, and that prostate cancer utilize the hematopoietic stem cell homing mechanisms to gain access to the niche. J. Cell. Biochem. 105: 370–380, 2008.

Erythropoietin Couples Hematopoiesis with Bone Formation

Background It is well established that bleeding activates the hematopoietic system to regenerate the loss of mature blood elements. We have shown that hematopoietic stem cells (HSCs) isolated from animals challenged with an acute bleed regulate osteoblast differentiation from marrow stromal cells. This suggests that HSCs participate in bone formation where the molecular basis for this activity is the production of BMP2 and BMP6 by HSCs. Yet, what stimulates HSCs to produce BMPs is unclear. Methodology/Principal Findings In this study, we demonstrate that erythropoietin (Epo) activates Jak-Stat signaling pathways in HSCs which leads to the production of BMPs. Critically, Epo also directly activates mesenchymal cells to form osteoblasts in vitro, which in vivo leads to bone formation. Importantly, Epo first activates osteoclastogenesis which is later followed by osteoblastogenesis that is induced by either Epo directly or the expression of BMPs by HSCs to form bone. Conclusions/Significance These data for the first time demonstrate that Epo regulates the formation of bone by both direct and indirect pathways, and further demonstrates the exquisite coupling between hematopoesis and osteopoiesis in the marrow.

CD26/dipeptidyl peptidase IV regulates prostate cancer metastasis by degrading SDF-1/CXCL12

Stromal derived factor-1 (SDF-1 or CXCL12) expressed by osteoblasts and endothelial cells, and its receptors CXCR4 and CXCR7/RDC1 are key molecular determinants in prostate cancer (PCa) metastasis. What drives PCa cells into the extravascular marrow space(s) once they make contact with the blood vessel endothelium, however remains unclear. Here, we evaluated whether degradation of CXCL12 facilitates PCa cell entry into the marrow cavity by locally lowering CXCL12 levels intravascularly. To explore this possibility, co-cultured conditioned media from PCa cells and endothelial cells were evaluated for their ability to degrade biotinylated CXCL12 (bCXCL12). Co-culture of PCa cells/endothelial cells resulted in greater digestion of CXCL12 than was achieved by either cell type alone, and this activity regulated invasion in vitro. The ability to degrade CXCL12 was not however observed in PCa and osteoblasts co-cultures. Fractionation and inhibitor studies suggested that the activity was CD26/dipeptidyl peptidase IV (DPPIV) and possibly other cysteine/serine proteases. By inhibiting CD26/DPPIV, invasion and metastasis of PCa cell lines were enhanced in in vitro and in vivo metastasis assays. Together, these data suggest that the degradation of CXCL12 by CD26/DPPIV may be involved in the metastatic cascades of PCa, and suggests that inhibition of CD26/DPPIV may be a trigger of PCa metastasis.

Disseminated prostate cancer cells can instruct hematopoietic stem and progenitor cells to regulate bone phenotype.

Prostate cancer metastases and hematopoietic stem cells (HSC) frequently home to the bone marrow, where they compete to occupy the same HSC niche. We have also shown that under conditions of hematopoietic stress, HSCs secrete the bone morphogenetic proteins (BMP)-2 and BMP-6 that drives osteoblastic differentiation from mesenchymal precursors. As it is not known, we examined whether metastatic prostate cancer cells can alter regulation of normal bone formation by HSCs and hematopoietic progenitor cells (HPC). HSC/HPCs isolated from mice bearing nonmetastatic and metastatic tumor cells were isolated and their ability to influence osteoblastic and osteoclastic differentiation was evaluated. When the animals were inoculated with the LNCaP C4-2B cell line, which produces mixed osteoblastic and osteolytic lesions in bone, HPCs, but not HSCs, were able to induced stromal cells to differentiate down an osteoblastic phenotype. Part of the mechanism responsible for this activity was the production of BMP-2. On the other hand, when the animals were implanted with PC3 cells that exhibits predominantly osteolytic lesions in bone, HSCs derived from these animals were capable of directly differentiating into tartrate-resistant acid phosphatase–positive osteoclasts through an interleukin-6–mediated pathway. These studies for the first time identify HSC/HPCs as novel targets for future therapy involved in the bone abnormalities of prostate cancer. Mol Cancer Res; 10(3); 282–92. ©2012 AACR.

GAS6/Mer axis regulates the homing and survival of the E2A/PBX1-positive B-cell precursor acute lymphoblastic leukemia in the bone marrow niche.

OBJECTIVE Despite improvements in current combinational chemotherapy regimens, the prognosis of the (1;19)(q23;p13) translocation (E2A/PBX1)-positive B-cell precursor acute lymphoblastic leukemia (ALL) is poor in pediatric leukemia patients. MATERIALS AND METHODS In this study, we examined the roles of growth arrest-specific-6 (GAS6)/Mer axis in the interactions between E2A/PBX1-positive B-cell precursor ALL cells and the osteoblastic niche in the bone marrow. RESULTS Data show that primary human osteoblasts secrete GAS6 in response to the Mer-overexpressed E2A/PBX1-positive ALL cells through mitogen-activated protein kinase signaling pathway and that leukemia cells migrate toward GAS6 using pathways activated by Mer. Importantly, GAS6 supports survival and prevents apoptosis from chemotherapy of E2A/PBX1-positive ALL cells by inducing dormancy. CONCLUSIONS These data suggest that GAS6/Mer axis regulates homing and survival of the E2A/PBX1-positive B-cell precursor ALL in the bone marrow niche.

LGR5 is Expressed by Ewing Sarcoma and Potentiates Wnt/β-Catenin Signaling.

Ewing sarcoma (ES) is an aggressive bone and soft tissue tumor of putative stem cell origin that predominantly occurs in children and young adults. Although most patients with localized ES can be cured with intensive therapy, the clinical course is variable and up to one third of patients relapse following initial remission. Unfortunately, little is yet known about the biologic features that distinguish low-risk from high-risk disease or the mechanisms of ES disease progression. Recent reports have suggested that putative cancer stem cells exist in ES and may contribute to an aggressive phenotype. The cell surface receptor leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) is a somatic stem cell marker that functions as an oncogene in several human cancers, most notably colorectal carcinoma. LGR5 is a receptor for the R-spondin (RSPO) family of ligands and RSPO-mediated activation of LGR5 potentiates Wnt/β-catenin signaling, contributing to stem cell proliferation and self-renewal. Given its presumed stem cell origin, we investigated whether LGR5 contributes to ES pathogenesis. We found that LGR5 is expressed by ES and that its expression is relatively increased in cells and tumors that display a more aggressive phenotype. In particular, LGR5 expression was increased in putative cancer stem cells. We also found that neural crest-derived stem cells express LGR5, raising the possibility that expression of LGR5 may be a feature of ES cells of origin. LGR5-high ES cells showed nuclear localization of β-catenin and robust activation of TCF reporter activity when exposed to Wnt ligand and this was potentiated by RSPO. However, modulation of LGR5 or exposure to RSPO had no impact on proliferation confirming that Wnt/β-catenin signaling in ES cells does not recapitulate signaling in epithelial cells. Together these studies show that the RSPO-LGR5-Wnt-β-catenin axis is present and active in ES and may contribute to tumor pathogenesis.

Expression of PGK1 by Prostate Cancer Cells Induces Bone Formation

Prostate cancer (PCa) is one of the solid tumors that metastasize to the bone. Once there, the phenotype of the bone lesions is dependent upon the balance between osteoblastogenesis and osteoclastogenesis. We previously reported that overexpression of phosphoglycerate kinase 1 (PGK1) in PCa cell lines enhanced bone formation at the metastatic site in vivo. Here, the role of PGK1 in the bone formation was further explored. We show that PCa-derived PGK1 induces osteoblastic differentiation of bone marrow stromal cells. We also found that PGK1 secreted by PCa inhibits osteoclastogenesis. Finally, the expression levels of the bone-specific markers in PCa cells were higher in cells overexpressing PGK1 than controls. Together, these data suggest that PGK1 secreted by PCa regulates bone formation at the metastatic site by increasing osteoblastic activity, decreasing osteoclastic function, and expressing an osteoblastic phenotype by PCa cells. (Mol Cancer Res 2009;7(10):1595–604)

Activation of Wnt/β-Catenin in Ewing Sarcoma Cells Antagonizes EWS/ETS Function and Promotes Phenotypic Transition to More Metastatic Cell States

Ewing sarcomas are characterized by the presence of EWS/ETS fusion genes in the absence of other recurrent genetic alterations and mechanisms of tumor heterogeneity that contribute to disease progression remain unclear. Mutations in the Wnt/β-catenin pathway are rare in Ewing sarcoma but the Wnt pathway modulator LGR5 is often highly expressed, suggesting a potential role for the axis in tumor pathogenesis. We evaluated β-catenin and LGR5 expression in Ewing sarcoma cell lines and tumors and noted marked intra- and inter-tumor heterogeneity. Tumors with evidence of active Wnt/β-catenin signaling were associated with increased incidence of tumor relapse and worse overall survival. Paradoxically, RNA sequencing revealed a marked antagonism of EWS/ETS transcriptional activity in Wnt/β-catenin-activated tumor cells. Consistent with this, Wnt/β-catenin-activated cells displayed a phenotype that was reminiscent of Ewing sarcoma cells with partial EWS/ETS loss of function. Specifically, activation of Wnt/β-catenin induced alterations to the actin cytoskeleton, acquisition of a migratory phenotype, and upregulation of EWS/ETS-repressed genes. Notably, activation of Wnt/β-catenin signaling led to marked induction of tenascin C (TNC), an established promoter of cancer metastasis, and an EWS/ETS-repressed target gene. Loss of TNC function in Ewing sarcoma cells profoundly inhibited their migratory and metastatic potential. Our studies reveal that heterogeneous activation of Wnt/β-catenin signaling in subpopulations of tumor cells contributes to phenotypic heterogeneity and disease progression in Ewing sarcoma. Significantly, this is mediated, at least in part, by inhibition of EWS/ETS fusion protein function that results in derepression of metastasis-associated gene programs. Cancer Res; 76(17); 5040-53. ©2016 AACR.

Structure and function of the solid tumor niche.

Although the hematopoietic stem cell (HSC) niche has been an active area of study, the concept of the bone marrow microenvironment (BMM) harboring a niche for solid metastatic tumor cells has only recently been considered. The HSC niche and microenvironment that is thought to constitute the solid tumor niche share many of the same structural and functional components, suggesting the possibility that the HSC and tumor niche are one in the same. The osteoblast is a critical component for each of these niches, and is important for regulating cellular processes such homing and migration, growth and survival, and quiescence and dormancy. Current understanding of the HSC niche may provide more insight to better defining the solid tumor niche. As role of the niche in regulating these processes is better understood, new insights to the role of the BMM in metastatic disease may be gained, and provide more potential targets for therapy.