Bone marrow angiogenesis in multiple myeloma: closing in on the loop.

Angiogenesis refers to the process of new blood vessel formation that occurs during embryonal growth, wound healing, the menstrual cycle, and in certain diseases of the eye. Angiogenesis is also recognized as being critical for tumor growth, invasion and metastasis.1,2 The various steps of angiogenesis, such as basement membrane disruption, endothelial cell migration and proliferation, and tube formation, occur in response to angiogenic triggers mediated by cytokines such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). VEGF (also referred to as VEGF-A) is one of the major inducers of angiogenesis. The protein is structurally related to PIGF (placenta growth factor), VEGF-B, VEGF-C, VEGF-D and Orf virus-derived VEGF (also called VEGF-E).3 VEGF is a survival and a proliferation factor for human microvascular endothelial cells. There are 5 different isoforms formed by mRNA splicing: VEGF121, VEGF145, VEGF165, VEGF189, and VEGF206. VEGF plays a critical role during embryo development; the loss of just one allele in knockout mouse models results in embryonic death.4,5 To date, there are two known receptor tyrosine kinases that bind VEGF: VEGFR-1 (also called Fms-like tyrosine kinase, Flt-1) and VEGFR-2 (also called kinase domain region, KDR).6 VEGFR-3 (also called FLT 4) is the receptor tyrosine kinase that mediates lymphangiogenesis; VEGF-C and VEGF-D serve as ligands for VEGFR-3. VEGF mRNA and protein are upregulated by hypoxia and VEGF mRNA is often elevated near areas of tumor necrosis. Tumor hypoxia and locally increased VEGF concentrations also upregulate VEGFR-1 and VEGFR-2. Stimulation of VEGF receptors in endothelial cells by VEGF leads to activation of the MAP kinase and JAKSTAT signaling pathways.6 The role of angiogenesis and VEGF in multiple myeloma has been the subject of intense investigation in recent years. In 1994, Vacca et al. first determined that bone marrow angiogenesis was markedly increased in myeloma compared to in its premalignant state, monoclonal gammopathy of undetermined significance (MGUS).7 Further they showed that the increase in angiogenesis was correlated to plasma cell proliferative rate. Subsequent studies by our group have confirmed these pivotal observations.8-10 Further confirmation came from more recent work by Vacca et al., who demonstrated that 76% of purified myeloma samples from patients are angiogenic whereas only 20% of MGUS samples are so in the in vitro chick embryo chorioallantoic membrane (CAM) angiogenesis assay.11 Bone marrow angiogenesis has since been shown to have prognostic value in myeloma,9,12 and in some studies appears to persist even after conventional dose or high dose chemotherapy.8,13 In fact, microvessels in the marrow appear to persist even after therapy with thalidomide, an agent with known anti-angiogenic properties;14 however the lack of resolution of microvessels may not be an accurate way to measure the effect of anti-angiogenic therapy.15 Overall, these studies suggest that induction of angiogenesis is a feature of, and possibly important in the transformation of MGUS to myeloma, and in the progression of early stage myeloma to advanced, refractory disease. Increased angiogenesis in myeloma, as in other tumors, is likely mediated by an alteration in balance between proand anti-angiogenic cytokines. Several studies show overexpression of VEGF by clonal plasma cells.16-20 bFGF also appears to be important. Sezer et al. found increased levels of serum bFGF in myeloma and that these levels decreased with effective chemotherapy.21 Vacca et al. have shown that antibodies to bFGF cause a significant inhibition (>50%) of the angiogenesis induced by myeloma cells in the CAM assay.11 Besides VEGF and bFGF, aquaporin 1 and matrix metalloproteinase-2 may be important, and their expression appears to correlate with the increased angiogenesis seen in myeloma.11,22 Angiogenesis may contribute to the pathogenesis and progression of myeloma in two ways: (i) by ensuring an adequate tumor oxygen and nutrient supply and (ii) by paracrine stimulation of tumor cell migration and proliferation. Likewise, in addition to stimulating angiogenesis, VEGF may also have paracrine or even autocrine effects in myeloma. The paracrine role of VEGF in myeloma was first proposed by Dankbar et al., who found that stimulation of myeloma cell lines with interleukin-6 (IL-6) results in an increase in VEGF secretion.17 Similarly, stimulation of endothelial cells and bone marrow stromal cells with VEGF induced a significant increase in IL-6 secretion in a dose-dependent manner. Starting with their pioneering observations in 1994, Vacca et al. have contributed immensely to our understanding of the role of angiogenesis and angiogenic cytokines in multiple myeloma. In this issue of Haematologica, they add to their vital contributions in an important paper that presents further evidence for the role of VEGF in myeloma.23 In a series of well conducted experiments, Vacca et al. show that VEGF is expressed and secreted by myeloma cells and that it stimulates proliferation and chemotaxis in both endothelial cells (VEGFR-2 signaling) and stromal cells (VEGFR-1 signaling). Their data provide further support for the presence of a paracrine loop for tumor growth and angiogenesis in multiple myeloma. The study adds to prior data about the role of VEGF during myeloma progression and provides further rationale for considering the Editorials, Comments and Views