Molecular regulation of adipogenesis in secondary lymphoedema: a common complication of cancer therapies

Abstract

Secondary lymphoedema is a common, chronic disease caused by inadequate drainage of interstitial tissue fluid from a limb due to damaged lymphatic vessels. It may develop after radiation therapy or cancer surgery involving lymph node dissection, in particularly for breast cancer, as well as a variety of other conditions. The resulting accumulation of interstitial fluid promotes pathological changes including oedema, expansion of fat and dermal fibrosis, which contribute to extensive chronic tissue swelling, typically in the upper or lower limbs. There is no curative treatment or molecular-based therapy for secondary lymphoedema, and current treatments for this condition have limited efficacy so it is an important unmet clinical need in medicine with an estimated 300,000 patients in Australia. A pharmacological intervention to restrict or reduce expansion of fat tissue and associated tissue swelling would be highly beneficial for patients however, the molecular mechanisms driving the development of fat in secondary lymphoedema are unknown. Here, a surgical mouse tail model of secondary lymphoedema is employed to investigate molecular pathways that drive expansion of fat tissue in this condition, and to explore a potential pharmacological approach for restricting this process.

The mouse model of secondary lymphoedema employed exhibited key pathophysiological features of clinical secondary lymphoedema such as tissue swelling, subcutaneous oedema, dermal fibrosis and excess formation of fat. With the expansion of fat occurring during the early phase of lymphoedema, and elevated levels of fat persisting in the chronic phase, this mouse lymphoedema model was well suited for studying molecular mechanisms driving the initial expansion of fat as well as the persistence of excess fat tissue in the chronic setting. Whole-genome microarray analysis was used to study the mouse lymphoedema model which revealed that mRNA for insulin-like growth factor binding protein 5 (IGFBP5), an inhibitor of the insulin-like growth factor 1 receptor (IGF1R) adipogenic signalling pathway, was down-regulated in the model compared to controls. Further analyses by immunohistochemistry revealed the presence of IGF1 and IGF1R on adipocytes in the model and in a clinical sample of secondary lymphoedema, and activated IGF1R in the clinical sample. Moreover, the levels of IGF1 (an activating ligand for IGF1R) associated with adipocytes, were elevated in the mouse lymphoedema model. Thus, the IGF1R signalling axis could be active and promote expansion of fat tissue in secondary lymphoedema. Importantly, pharmacological targeting of this pathway in the mouse lymphoedema model with linsitinib, a small molecule tyrosine kinase inhibitor of IGF1R, led to significant reductions in tissue swelling and fat expansion which was associated with decreases in both the number and size of adipocytes. Furthermore, linsitinib also restricted the degree of dermal fibrosis in the mouse lymphoedema model.

The work presented in this thesis demonstrated the role of the IGF1R signalling pathway in promoting adipogenesis in a mouse model of secondary lymphoedema. Hence, pharmacologically targeting IGF1R might be a viable therapeutic approach for restricting expansion of fat and tissue swelling in secondary lymphoedema patients.