Mechanobiological origins of osteolysis during bone metastasis
Verbruggen, Anneke
Verbruggen, Anneke
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Publication Date
2023-03-28
Type
Thesis
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Abstract
Metastasis is the final, lethal stage of cancer where cells migrate from a primary tumour site to colonise a secondary organ and is the primary cause of mortality in cancer patients. Breast cancer is the leading cause of cancer death in women in the world, with a recorded 627,000 deaths in 2018, and projected to reach 800,000 by 2030. In advanced breast cancer patients, cancer cells favour metastasis to bone tissue 70-80% of the time, primarily leading to osteolysis (bone destruction) and sometimes unwanted tissue formation. The evolving mechanical environment during tumour invasion might play an important role in these processes, as the activity of both bone and cancer cells is regulated by mechanical cues. However, it is not yet known how specific bone tissue composition is associated with tumour invasion. In particular, how compositional and nano-mechanical properties of bone tissue evolve during metastasis, where in the bone they arise, and how they influence the overall aggressiveness of tumour invasion, are not well understood. The first study of this thesis sought to develop an advanced understanding of temporal and spatial changes in nano-mechanical properties and composition of bone tissue during metastasis. Primary mammary tumours were induced by inoculation of immune-competent BALB/c mice with 4T1 breast cancer cells, and microcomputed tomography and nanoindentation were conducted to quantify cortical and trabecular bone matrix mineralisation and nano-mechanical properties, respectively. Spatial analysis was performed in proximal and distal femur regions of tumour-adjacent (ipsilateral) and contralateral femurs after 3 weeks and 6 weeks of tumour and metastasis development. By 3 weeks post-inoculation there was no significant difference in bone volume fraction or nano-mechanical properties of bone tissue between the metastatic femora and healthy controls. However, early osteolysis was indicated by trabecular thinning in the distal and proximal trabecular compartment of tumour-bearing femora. Moreover, cortical thickness was significantly increased in the distal region, and the mean mineral density was significantly higher in cortical and trabecular bone tissue in both proximal and distal regions, of ipsilateral (tumour bearing) femurs compared to healthy controls. By 6 weeks post-inoculation, overt osteolysis, decreased bone volume fraction, cortical area, cortical and trabecular thickness were reported in metastatic femora. Trabecular bone tissue stiffness in the proximal femur decreased in the ipsilateral metastatic femurs compared to contralateral and control sites. This study uncovered changes in bone tissue composition prior to and following overt metastatic osteolysis, local and distant from the primary tumour site. On the basis of these findings, it was proposed that changes in tissue composition may alter the mechanical environment of both the bone and tumour cells, and thereby perpetuate the cancer vicious cycle during breast cancer metastasis to bone tissue. The objective of the second study was to quantify changes in the mechanical environment within bone tissue, during bone metastasis and osteolytic resorption. This study used finite element (FE) models reconstructed from micro-CT images obtained during the first study of this thesis. In particular, the time-dependent changes in the mechanical environment, local to and distant from an invading tumour mass, were quantified to investigate putative mechanobiological cues for osteolysis during bone metastasis. This study reported a decrease in strain distribution within the proximal femur trabecular and distal cortical bone tissue in early metastasis (3 weeks after tumour inoculation). These changes in the mechanical environment preceded extensive osteolytic destruction, but coincided with the onset of early trabecular thinning, cortical thickening and mineralisation of proximal and distal femur bone, which were reported in the first study of this thesis. From these findings, it was proposed that early changes in the mechanical environment within bone tissue may activate resorption by osteoclast cells and thereby contribute to the extensive osteolytic bone loss at later stage (6 weeks) bone metastasis. To investigate this proposed adaptation of bone tissue upon breast cancer metastatic invasion, the third and final study of this thesis sought to apply the mechanoregulation theory, which predicts tissue adaptation on the basis of changes within the mechanical environment. This was performed using a bone remodelling algorithm driven by changes in mechanical strain. A user-defined field (USDFLD) subroutine was applied to murine proximal femur models, with material properties obtained from the first study, such that each individual element within an FE model adapted material density and stiffness according to pre-defined strain stimuli criteria. In this way, this model generated an iterative mechanoregulatory response to changes in strain distribution throughout the bone mechanical environment, over a period of 3 weeks. This study predicted that bone tissue would undergo resorption in regions which corresponded to those in the first study of this thesis upon overt osteolysis by 6 weeks of bone metastasis. These findings further support the proposal that mechanobiology may play a role in breast cancer bone osteolysis. Together, the studies in this thesis report, for the first time, changes in bone mineral content and mechanical properties prior to overt osteolytic destruction in an in vivo animal model of breast cancer metastasis. Computational analysis revealed decreased strain distribution at this early time point, prior to osteolysis, and on this basis it was proposed that a mechanoregulatory response may contribute to subsequent osteolytic destruction.
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NUI Galway