Suppression of cancer-associated bone loss through dynamic mechanical loading

Abstract

Patients afflicted with or being treated for cancer constitute a distinct and alarming subpopulation who exhibit elevated fracture risk and heightened susceptibility to developing secondary osteoporosis. Cancer cells uncouple the regulatory processes central for the adequate regulation of musculoskeletal tissue. Systemically taxing treatments to target tumors or disrupt the molecular elements driving tumor growth place considerable strain on recovery efforts. Skeletal tissue is inherently sensitive to mechanical forces, therefore attention to exercise and mechanical loading as non-pharmacological means to preserve bone during treatment and in post-treatment rehabilitative efforts have been topics of recent focus. This review discusses the dysregulation that cancers and the ensuing metabolic dysfunction that confer adverse effects on musculoskeletal tissues. Additionally, we describe foundational mechanotransduction pathways and the mechanisms by which they influence both musculoskeletal and cancerous cells. Functional and biological implications of mechanical loading at the tissue and cellular levels will be discussed, highlighting the current understanding in the field. Herein, in vitro, translational, and clinical data are summarized to consider the positive impact of exercise and low magnitude mechanical loading on tumor-bearing skeletal tissue.

Keywords: Bone remodeling; Breast cancer bone metastases; Cancer-associated bone disease; Low intensity vibrations; Low magnitude mechanical signals; Mechanical loading; Multiple myeloma; Osteolytic lesions.

Fig. 1.

The quantity and quality of bone can be maintained through daily activity and improved through exercise. Exercise has been clearly demonstrated to improve quality of life by providing adequate mechanical stimuli to maintain steady bone remodeling cues. Conversely, the absence of mechanical signals leads to significant and swift interruption of the bone remodeling pathway in favor of resorption, which highlights the reliance of musculoskeletal tissue on mechanical signals. Clinical settings that lack sufficient mechanical loading, which in turn elicit a catabolic response in bone, consist of chronic bedrest, disuse, sedentary behavior. Said conditions also arise subsequent to altered metabolism as a byproduct of diseases ranging from type-II diabetes and high-fat diet to cancer-associated bone loss. The subsequent degradation bone quantity and quality undermines its mechanical integrity by elevating fracture risk and systemic adiposity, thereby decreasing quality-of-life. To counter these effects, the introduction of dynamic mechanical stimuli, whether tolerated through high-impact exercises or through low magnitude, high-frequency mechanical signals, can translate to improved bone indices, suppression of elevated bone resorption, and reduced accrual of adipose tissue. At the level of the cell, actin fiber assemblies connect to focal adhesions at the cell membrane and to the nuclear membrane via LINC complex proteins, creating a more rigid cytoskeletal architecture that enables increased transmission of mechanical signals into the nucleus. As a result, mesenchymal stem cells undergo osteogenic differentiation and metastatic breast cancer cells exhibit decreased migratory and proliferative capacity.

Fig. 2.

Evidence derived from both in vivo and in vitro studies demonstrate the efficacy of LIV in improving bone and reducing tumor burden at both the animal and cellular level. (A.) A xenograft murine model harboring multiple myeloma cells (U266) throughout the bone marrow demonstrated significant loss of bone by 8w post-disease induction via μCT analysis relative to sham-mice. Administration of low magnitude mechanical signals in the form of low intensity vibrations (LIV) for the 8w significantly reduced porosities across the femoral cortex and preserved trabecular microarchitecture in the same region-of-interest relative to mock-LIV-treated, myeloma-bearing mice. Indications of bone resorption were elevated in diseased mice relative to sham-mice, but decreased serum-TRAP5b and eroded surface (ES/TS) via static histomorphometry indicated that LIV reduced bone resorption. (B.) A reduction in tumor burden was measured via histological quantification of tumor area and flow cytometry (CD138⁺ cells), coinciding with measured retention of bone in myeloma-bearing mice. These data suggest the effects of LIV suppress the release of bone matrix factors that fuel tumor progression and osteoclast-mediated bone resorption. (C.) In vitro assessment of metastatic breast cancer cells (MDA-MB-231) demonstrated a reduction in osteoclastogenesis via Nfatc1, Cathepsin-K, and TRAP expression as well as in the number of nuclei per osteoclast (RAW267) were when cultured in media from LIV-treated breast cancer cells. (D.) LIV-treated breast cancer cells exhibited increased Sun 1 and 2 and Nespirin 1 and 2 proteins relative to their controls, even when challenged with TGF-β. Further, twice-daily administration of LIV increased expression of Fas relative to mock-LIV-treated cultures, rendering the cells more susceptible to apoptosis-mediated cell death. Data and figures presented above are reprinted with permission from Pagnotti GM, et al.; Bone; PMID: 27262776, PMCID: PMC4970889, DOI: https://doi.org/10.1016/j.bone.2016.05.014; 2016 and Thompson WR, et al.; Bone Research; PMID: 33298883, PMCID: PMC7673025, DOI: https://doi.org/10.1038/s41413-020-00111-3; 2020.

Fig. 3.

(Left) Mesenchymal stem cells are essential to sustaining the bone marrow progenitor pool. Uncommitted progenitor cells in the marrow subject to dynamic mechanical loads are driven towards osteogenesis through increased focal adhesions, which are driven by upregulated FAK expression. Increased adherence to the substrate enables further transmission of mechanical signals across cytoskeletal actin assemblies that are anchored to the nuclear membrane. Transmission of the mechanical signal into the nucleus is achieved through the LINC protein complex, facilitating the translocation of β-catenin into the nucleus to initiate Wnt pathways central to bone formation. (Right) LINC proteins are not as prevalent in metastatic cancer cells. An intact LINC complex tethering cytoskeletal proteins to the bone ensures effective reception of mechanical stimuli, resulting in downregulation of inflammatory cytokines (Parathyroid hormone-like protein: PTHLH; IL-11; and RANKL) that fuel tumor activity and osteoclast-mediated bone resorption. As a result, administration of low magnitude mechanical loads slows metastatic cancer cell migratory capacity and proliferation.

Fig. 4.

Contrasting the impact of metastatic cancer cells and mechanical signals on the bone marrow microenvironment. A) Mechanosensitive osteocytes help initiate healthy bone remodeling. Osteoclast recruitment functions to resorb weak or diseased tissue. Activated osteoblasts follow, inducing bone apposition and mineralization at the site of previously resorbed bone. The cyclical remodeling results in a net zero balance in bone. B) Myeloma or metastatic cancer cells invade the marrow microenvironment, uncoupling the tight bone remodeling signaling pathway to favor bone resorption. Osteocytes secrete elevated SOST, suppressing the formation of new bone while RANKL and matrix-metalloproteinases that induce osteoclastogenesis are unchanged. Heightened bone resorption releases matrix-bound growth factors that perpetuate tumor growth and drive increased osteoclast activity. Alternatively, metastatic prostate cancer stimulates woven bone formation, which negatively impacts the structural integrity of the bone. In either osteolytic or osteoblastic bone metastases, factors within the bone matrix drive proliferation of tumor cells that further bone loss, increasing vascularization to the tumor via VEGF expression, and upregulation in SOST which further suppresses bone formation. C) Mechanically-stimulating bone elicit multiple positive effects within the bone-tumor microenvironment. Decreased expression of osteoclast markers slow bone formation, osteocytes decrease secretion of SOST to engage osteoblasts, which then increase their expression of bone formation markers. Osteolytic cancer cells are both directly and indirectly affected by the influence of mechanical loads: reduced release of bone matrix factors limit tumor growth and an intrinsic response to mechanical signals increases Fas, rendering the tumor more susceptible to apoptosis.