The impact of myostatin on breast cancer and multiple myeloma bone metastases

骨骼中癌症的生长与对骨骼的严重影响有关，并导致相当大的发病率和死亡率。乳腺癌（BC）和多发性骨髓瘤（MM）的骨转移瘤都是溶骨性的，其发生是由成熟破骨细胞的形成和激活介导的，肿瘤细胞和骨微环境之间存在复杂的相互作用，为癌细胞和骨细胞因子提供多种生长因子。在这方面，TGF β家族的两个成员TGF β或激活素A的抑制抑制MM和BC小鼠模型中的骨细胞骨吸收，表明这些因子在骨转移形成中的重要作用。强烈增强破骨细胞发育，且肌生长抑制素缺乏或抑制可高度改善关节炎小鼠的疾病严重程度，特别是骨侵蚀。有趣的是，初步实验揭示了肌抑素和作用A之间的高度相似性。我们可以证明，肌肉生长抑制素和激活素A增强RANKL诱导的破骨细胞形成的程度相同，表明这两种因子是破骨细胞发育的等效促进剂。此外，这两种因子通过类似的Smad途径（Smad 2/3）起作用，表明破骨细胞分化中的共同机制。除了这种相似性，其他证据表明肌生长抑制素在骨转移中的作用：1。肌生长抑制素在乳腺癌细胞中表达，2. ActRII是激活素和肌肉生长抑制素共有的受体类型，由乳腺癌和骨髓瘤细胞表达。人BC骨转移酶中存在较高水平的磷酸化SMAD 2。我们假设，肌生成抑制素参与肿瘤细胞与骨微环境的相互作用，促进破骨细胞发育，从而促进骨病变，抑制肌生成抑制素可减少骨病变。为此，我们将用抗肌生长抑制素抗体治疗BC和MM的两个同基因和两个异种移植模型，并评估骨转移的形成。此外，将研究肌生长抑制素在细胞-细胞相互作用中的作用，并测试肿瘤细胞对破骨细胞分化和活性的直接/间接影响。此外，假设肌肉生长抑制素和激活素A都参与骨转移，同时抑制肌肉生长抑制素和激活素A可以为治疗BC和MM中的骨转移提供有力的工具。能够结合和拮抗肌肉生长抑制素以及激活素的蛋白质是称为“卵泡抑素相关基因”（FLRG）的天然存在的拮抗剂。在初步实验中，我们可以证明FLRG完全消除RANKL诱导的破骨细胞形成，无论是用肌肉生长抑制素和激活素刺激。因此，我们建议在疾病过程中也用重组FLRG治疗BC和MM小鼠，并如上所述进行分析。

The growth of cancers in the skeleton is associated with severe effects on bone and results in considerable morbidity and mortality. Both, breast cancer (BC) as well as multiple multiple (MM)-derived bone metastases are osteolytic and generation of metastases is mediated by the formation and activation of mature osteoclasts.There is a complex crosstalk between tumor cells and the bone microenvironment providing a variety of growth factors for cancer cells and osteoclastic factors. In this regard inhibition of TGFß or activin A, both members of the TGFß familiy, suppress osteoclastic bone resorption in mouse models of MM and BC, indicating an important role of these factors in the formation of bone metastases.In this context, we have recently discovered that myostatin, another member of the TGFß family, strongly enhances osteoclast development and that deficiency or inhibition of myostatin highly ameliorates disease severity and in particular bone erosion in arthritic mice. Interestingly, preliminary experiments revealed a high similarity between mystatin and action A. We could show that myostatin and activin A enhance RANKL-induced osteoclast formation to the same extent, indicating that both factors are equivalent promoters of osteoclast development. Moreover, both factors act via similar Smad-pathways (Smad2/3), pointing to a common mechanism in osteoclast differentiation. Beside this similarity, additional lines of evidence suggest a role for myostatin in bone metastases: 1. Myostatin is expressed in breast cancer cells, 2. ActRII, the receptor type shared by activin and myostatin, is expressed by breast cancer and myeloma cells, 3. Higher levels of phosphorylated SMAD2 are present in human BC bone metastatases.We hypothesize, that myostatin participates in the interactions of tumor cells with the bone microenvironment, promoting osteoclast development and thereby bone lesions and that inhibition of myostatin reduce bone metastases.For this purpose, we will treat two syngeneic and two xenograft models for both BC and MM with anti-myostatin antibody and assess the formation of bone metastases. Moreover, the role of myostatin in cell-cell interaction will be investigated and direct/ indirect effects of tumor cells on osteoclast differentiation and activity will be tested. Furthermore, assuming that both myostatin and activin A are involved in bone metastases, inhibiting myostatin and activin A simultaneously may provide a powerful tool for the treatment of bone metastases in BC and MM. A protein which is able to bind and antagonize myostatin as well as activin is a natural occuring antagonist called „Follistatin-related gene“ (FLRG). In preliminary experiments, we could demonstrate that FLRG completely abrogates RANKL-induced osteoclast formation either stimulated with myostatin and activin. We therefore proposed to treat BC and MM mice also with recombinant FLRG in the course of the disease and analysed as described above.