Leukemia (2005) 19, 2203–2205 & 2005 Nature Publishing Group All rights reserved 0887-6924/05 $30.00 www.nature.com/leu
COMMENTARY Osteopontin, angiogenesis and multiple myeloma V Cheriyath1,2 and MA Hussein2 1
Project Scientist, The Cleveland Clinic Foundation, Taussig Cancer Center, Center for Hematology and Oncology Molecular Therapeutics, Cleveland, OH, USA; and 2The Cleveland Clinic Foundation, Taussig Cancer Center, Multiple Myeloma Research Program, Cleveland, OH, USA
Leukemia (2005) 19, 2203–2205. doi:10.1038/sj.leu.2403978; published online 6 October 2005
The significance of Osteopontin (OPN) and angiogenesis in the pathogenesis and the progression of hematological malignancies is highlighted in a paper by Colla et al.1 In this article, Colla et al, expand their studies on bone marrow (BM) angiogenesis2,3 in an attempt to describe the role of OPN in multiple myeloma (MM). In an in vitro angiogenesis assay system, authors demonstrate that conditioned media from MM cells has a proangiogenic effect and OPN is critical for this outcome (Figure 5). The Runt family of transcription factors (RUNX1/AML1, RUNX2 and RUNX3) can regulate OPN expression.4–6 By comparing the expression profiles and DNA-binding activity of RUNX1/AML1 and RUNX2 in primary and MM cell lines, the authors speculate that RUNX2 is a regulator of OPN expression in MM cells (Figure 2). Using small interfering RNA to silence RUNX2 expression in MM cells authors confirmed that indeed RUNX2 is indispensable for OPN expression (Figure 3). Their preliminary analysis with newly diagnosed MM patients highlights a strong correlation between active RUNX2/Cbfa1 protein expression with OPN production and BM angiogenesis. OPN is a multifunctional Glyco phosphoprotein, highly expressed in bone as well as in various other cell types such as macrophages, endothelial cells, smooth muscles and epithelial cells.7 Initially, OPN was described as a major noncollagenous protein in bone, named bone sialoprotein8 and as a secreted phosphoprotein by transformed malignant epithelial cells.9 OPN also has a role in normal tissue remodeling processes such as bone resorption, angiogenesis, wound healing and tissue injury as well as certain diseases such as vascular restenosis, atherosclerosis and renal diseases.7 Several recent studies sited in this commentary shows OPNs role in various aspects of the formation and progression of MM. MM is a clonal B-cell tumor of slowly proliferating plasma cells within the bone marrow.10 and remains an incurable disease despite aggressive, high-dose therapy and intensive supportive care.11 A better understanding of the pathophysiology of disease progression at molecular level is essential for designing an effective therapy for MM. According to Hanahan and Weinberg12 sustained angiogenesis is one of the six critical steps for tumor progression. The assumption that angiogenesis was critical only for solid tumor progression was challenged by a study in 1994.13 Vacca et al, for the first time reported a high
Correspondence: Dr V Cheriyath, The Cleveland Clinic Foundation, Taussig Cancer Center, R40, Taussig Cancer Center, 9500 Euclid Avenue, Cleveland, OH 44195, USA; Fax: þ 1 216 636 2498; E-mail:
[email protected] Received 22 August 2005; accepted 30 August 2005; published online 6 October 2005
correlation between BM angiogenesis and plasma cell proliferation in MM patients suggesting the role of angiogenesis in the growth and progression of hematological malignancies. In an experimental mouse model for MM (5T2MM), the disease progression was characterized by a preangiogenic stage of slow tumor progression followed by an angiogenic switch and subsequent progressive tumor growth.14 The angiogenesis switch was preceded by an increase in the percentage of Cd-45 MM cells with high levels of VEGF secretion. These findings suggests that MM cells itself can stimulate BM angiogenesis by producing the proangiogenic factors directly and shifting the equilibrium between proangiogenic and antiangiogenic factors in favor of angiogenesis. Human myeloma cell lines and fresh MM cells have been shown to express several proangiogenic factors including VEGF, bFGF, TGFb, IL-8, OPN and Ang-1, but not PIGF and PDGF.15 Recent studies revealed the role of MM-derived VEGF in paracrine and autocrine pathways that affect BM angiogenesis and MM cell proliferation.16–18 However, no significant difference in the plasma cell expression of bFGF and VEGF was observed between MGUS, smoldering MM and active MM19 suggesting a minor role for MM cell-derived VEGF and bFGF in the increased BM angiogenesis. The search for a proangiogenic factor that is critical for increasing BM angiogenesis in MM patients revealed the role of OPN (current study by Colla et al) and Ang-12 in MM progression. The first study on the role of OPN in MM appeared in 2003,20 where the authors examined the direct production of OPN by myeloma cells and plasma OPN levels in MM patients. With RT-PCR and Western blot analysis they demonstrated the direct production of OPN by MM cells. Moreover, plasma OPN levels correlated with both disease progression and bone destruction. These findings suggest that increased OPN production by MM cells might be associated with enhanced osteoclasts (OCs) resorption in MM patients. The clinical symptoms of MM are often manifested by devastating bone destruction by OCs recruited around malignant plasma cells. Contradictory reports exist on the role of OPN in osteoclastogenesis and maintaining the equilibrium between OCs and osteoblasts in nonpathological conditions.21–23 However, in mouse models for postmenopausal osteoporosis24 and mechanical stress,25 OPN was found to be indispensable for normal function, suggesting the influences of OPN on bone homeostasis during pathological conditions. OPN can exist in two forms: a soluble and in immobilized forms. Soluble, OPN has a prosurvival or proliferative function whereas an immobilized form OPN functions as an extracellular matrix protein and protects cells from undergoing apoptosis.26 Compared to stromal cells, OCs derived from peripheral blood mononuclear cells enhanced growth and survival of primary MM cells and cell lines and protected them from apoptosis
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induced by serum depletion or Doxorubicin.27 These findings can partly explain OPN’s role in MM biology, however, its role in cell adhesion and cell migration may also be important in tumor progression and metastasis. MM cell lines INA-6 and ANBL-6 adhere better to OPN than to BSA28 and the increased plasma and serum OPN levels in MM patients suggests a role for elevated stromal OPN in retaining MM cells in bone marrow. The importance of OPN in MM is further demonstrated by the current report of Colla et al,1 showing its role in angiogenesis in MM. However, OPN’s contribution to myelomagenesis in vivo from the aspect of angiogenesis remains to be clarified. A multifunctional factor such as OPN is a more desirable candidate as a prognosticator when compared with VEGF overexpression which is noted in more than 90% of MM patients. VEGF can induce extravasation of the extrinsic coagulation pathway to generate thrombin from prothrombin. Active thrombin cleaves OPN29 to enhance its adhesive and cell-migration activity. Compared to intact OPN, the aminoterminal cleaved OPN is a better enhancer of VEGF-stimulated endothelial cell migration.30 These findings suggest the possibility of crosstalk between VEGF and OPN signaling pathways during MM progression. Further investigations are needed to assess the activation of OPN by thrombin cleavage in MM formation or progression and to determine whether VEGF has any role in regulating this process. OPN is also a ligand for the CD44 family of receptors that mediate both cell–matrix and cell–cell interaction.31 Compared to the Standard CD44 (CD44 s), CD44 splice variants, CD44v6 and v7 are better receptors for OPN to induce cell survival, chemotaxis, homing and adhesion pathways enhancing the metastatic behavior in tumors.32–35 Although Colla et al,1 show that CD44 is expressed in MM cell lines, it will be interesting to see whether MM cells also express the splice variants of CD44 which promote tumor progression. Elevated expression of OPN in tumor cells suggests that regulators of OPN expression play a critical role in tumor biology. Wide varieties of stimuli and transcription factors have been shown to regulate OPN expression. Of these RUNX family transcription factors generate special interest for investigators in MM biology because targeted deletion of RUNX2 gene in mice impaired osteoblast differentiation.36–38 RUNX belongs to Runt related family of genes and is composed of three family members, RUNX1/AML1, RUNX2 and RUNX3. They are essential regulators of cell fate during development and depending on the context they can function either as tumor suppressor genes or dominant oncogenes in cancer.39 From the current study of Colla et al, RUNX2 and not RUNX1/AML1 proteins is the major factor that appears to regulate OPN expression in MM cells. The strong correlation between the expression of RUNX2 and OPN expression in MM patients suggests an oncogenic role for RUNX2 in MM. However, it is not clear from the current study whether overexpression of RUNX2 is sufficient to deregulate OPN expression in MM cells. In summary, the current study by Colla et al adds to the mounting evidence that the pathophysiology of BM angiogenesis is a crucial step in MM progression and the multifunctional protein OPN might play a key role in this process. Indeed, there is growing evidence for the role of OPN in various steps or processes of MM formation and progression. An intriguing aspect of these studies is why only a percentage MM patients express a multifunctional protein like OPN. At which stage of the disease and by which mechanisms is OPN expression deregulated? Additionally, the nature of post-transcriptional regulation of OPN in MM is still unknown. The involvement of RUNX2 highlights the complex nature of OPN regulation in
MM. RUNX family members can function either as oncogenes or as tumor suppressors in a context dependent manner. Thus, one can envision the studies of OPN in MM possibly resulting in improved prognostic and therapeutic strategies. It will be of substantial interest to watch the emerging definitions of the role of these interesting genes in MM.
Acknowledgements We thank Dr Ernest C Borden and Barbara Jacobs for their critical reading of this commentary and valuable suggestions. This work is partially supported by an ACS pilot grant to VC.
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