Myeloma-derived Dickkopf-1 disrupts Wnt-regulated osteoprotegerin and RANKL production by osteoblasts: a potential mechanism underlying osteolytic bone lesions in multiple myeloma

Abstract

Multiple myeloma (MM) is characterized by osteolytic bone lesions (OBL) that arise as a consequence of osteoblast inactivation and osteoclast activation adjacent to tumor foci within bone. Wnt signaling in osteoblasts regulates osteoclastogenesis through the differential activation and inactivation of Receptor Activator of Nuclear factor Kappa B Ligand (RANKL) and osteoprotegerin (OPG), positive and negative regulators of osteoclast differentiation, respectively. We demonstrate here that MM cell-derived DKK1, a soluble inhibitor of canonical Wnt signaling, disrupted Wnt3a-regulated OPG and RANKL expression in osteoblasts. Confirmed in multiple independent assays, we show that pretreatment with rDKK1 completely abolished Wnt3a-induced OPG mRNA and protein production by mouse and human osteoblasts. In addition, we show that Wnt3a-induced OPG expression was diminished in osteoblasts cocultured with a DKK1-expressing MM cell line or primary MM cells. Finally, we show that bone marrow sera from 21 MM patients significantly suppressed Wnt3a-induced OPG expression and enhanced RANKL expression in osteoblasts in a DKK1-dependent manner. These results suggest that DKK1 may play a key role in the development of MM-associated OBL by directly interrupting Wnt-regulated differentiation of osteoblasts and indirectly increasing osteoclastogenesis via a DKK1-mediated increase in RANKL-to-OPG ratios. (Blood. 2008;112:196-207) IntroductionBone destruction, a cardinal feature of multiple myeloma (MM), results from an uncoupling of osteoclast and osteoblast activities adjacent to intramedullary tumor foci. [1][2][3] Osteoclasts are activated by binding of the receptor activator of nuclear factor kappa B ligand (RANKL) 416 to its cognate receptor, RANK, whereas osteoprotegerin (OPG), 7 a soluble member of the tumor necrosis receptor super-family, acts as a naturally occurring decoy receptor that competes with RANK for binding of RANKL. 8 The balance of these 2 molecules plays a critical role in the control of osteoclastogenesis. MM cells likely stimulate expression of RANKL and suppress expression of OPG by osteoblasts or their progenitors. 9,10 Increased serum levels of RANKL and decreased levels of OPG have been associated with a poor prognosis in MM. 11 Restoring the RANKL/OPG imbalance by RANKL antagonist or recombinant OPG not only reduces MM-associated bone lesions but also halts disease progression in animal models. 10,[12][13][14] Mechanistically, regulation of osteoclastogenesis by osteoblast-derived OPG [15][16][17]18,19 involves Wnt signaling, a pathway that is regulated by a large number of antagonists, including the Dickkopf family, 20 secreted frizzled-related proteins (sRFPs), 21,22 and sclerostin. 23 DKK1 blocks maturation of osteoblasts and formation of mineralized matrix by antagonizing the canonical Wnt pathway through it binding to LRP5/6 and Kremen. [24][25][26][27][28] Germ line inactivating mutations in the Wnt coreceptor LRP5 causes the osteoporosis-pseudoglioma syndrome (OPPG), 29 whereas a high bone m...

Bone Anabolic Agents for the Treatment of Multiple Myeloma

“…OBs secrete receptor activator of NF-kB ligand (RANKL), a critical growth factor for OCs, and osteoprotegerin (OPG), a soluble RANKL inhibitor. The balance between RANKL and OPG modulates osteoclastogenesis and MM cells typically increase the RANKL/OPG ratio [17,18] promoting OC development.…”

“…Most likely, the effects are mediated by the non-canonical Wnt signaling pathway since DKK1 levels are independent from β-catenin activation status [31,32]. In addition, MMderived DKK1 inhibits OPG expression in OBs and upregulates RANKL secretion [18], therefore stimulating osteoclastogenesis. Neutralizing antibodies against DKK1 promote OB differentiation [30] and limit MM cell proliferation when co-cultured with BMSC [33].…”

Mechanisms of multiple myeloma bone disease

“…35 -38 Myeloma cells do induce production of high levels of RANKL by marrow stromal cells through adhesive interactions between 4 1 integrin on myeloma cells and vascular cell adhesion molecule-1 (VCAM-1) on marrow stromal cells, 39 as well as by soluble factors produced by myeloma cells, such as TNF- 28,40 and Dickkopf1 (DKK1). 41 Further enhancing the effects of increased RANKL in myeloma is the decreased production of OPG by marrow stromal cells via inhibition of osteoblast differentiation, decreased transcription of OPG and degradation of OPG by myeloma cell endocytosis of the OPG bound to CD138 (syndecan-1). 42,43 Thus, the ratio of RANKL to OPG in myeloma patients is markedly increased and drives osteoclastogenesis.…”

“…86 DKK1 can enhance RANKL production as well as suppress OPG expression, further driving the bone-destructive process. 41 Preclinical studies with blocking antibodies to DKK1 (BHQ880) have clearly shown that they can enhance bone formation and block tumor growth in murine models of myeloma bone disease. 87 Anti-DKK1 does not have direct anti-tumor effects, although anti-DKK1 may suppress tumor growth via its effects on osteoblast differentiation.…”

Serum Dickkopf-1 is increased and correlates with reduced bone mineral density in patients with thalassemia-induced osteoporosis. Reduction post-zoledronic acid administration

“…This phenomenon has been already described in myeloma where Dkk1 produced by myeloma cells reduces osteoblast function but also increases the production of RANKL and decreases the production of OPG by stromal cells, and thus leads to increased bone resorption. 15 Furthermore, in patients with rheumatoid arthritis, Dkk1 directly impaired new bone formation through the reduction of osteoblast function and the decreased production of OPG, which shifted the RANKL/OPG ratio in favor of bone resorption. 16 Thus it seems that Dkk1 inhibits bone formation while promoting bone resorption in different bone disorders and supports the notion that Dkk1 is a key molecule in bone biology.…”