ROLE OF FERROPORTIN-MEDIATED IRON RELEASE FROM MACROPHAGES IN TISSUE HOMEOSTASIS AND REPAIR

Macrophages are essential in the inflammatory response and also have a critical function in body iron homeostasis. Moreover, it is known that iron metabolism is important in the context of inflammation. Indeed, it has been demonstrated that, in line with their functions, distinct macrophage populations, such as M1 and M2 polarized cells, differ in the expression of genes involved in iron homeostasis as well as in the expression of immunoregulatory genes. The functional significance of these differencies are not completely understood: we hypotized that iron released by M2 macrophages could promote cell proliferation and extracellular matrix deposition in the resolution phase of inflammation. Moreover, in line with the increasing awareness that macrophages have an important trophic role in addition to their immunological function, macrophages may play a role of “local iron redistributors” in tissues, where they would manage iron availability for neighbouring cells. Therefore, the major aim of this project was to exploit a mouse model of impaired iron release from macrophages, caused by deletion of Ferroportin (Fpn), to understand the functions of macrophage iron in two situations of tissue repair: cutaneous wound healing and liver fibrosis. The characterization of mice with loss of macrophage Fpn showed that they are affected by transient alopecia caused by impaired hair follicle growth. The local impairment of iron distribution due to macrophage Fpn inactivation was accompanied by cellular iron deprivation and decreased proliferation in adjacent epithelial cells. By exposing mice to an iron-restricted diet we concluded that hair loss was not related to hypoferremia/anemia. Taken together, these results suggest that iron retention in resident macrophages has detrimental effects on tissue homeostasis by inhibiting the proliferation of hair follicle cells. We observed a considerable delay in the closure of excisional skin wounds of Fpnfl/flLysCre/- mice compared to controls, with defective granulation tissue formation and diminished fibroplasia. Moreover, the development of both lymphatic and blood vascular network was impaired. Conversely, inactivation of Fpn in macrophages had no impact on inflammatory processes accompanying wound healing, such as production of inflammatory molecules, content of leukocyte subsets and macrophage polarization. Altogether, these results indicate that, though it does not interfere with immune cells recruitment and local activation, Fpn deletion in macrophages impairs blood vessels formation and stromal cells proliferation, leading to delayed skin repair. Fpn inactivation in macrophages had no impact on inflammation, steatosis and fibrosis associated with exposition to the MCD diet, a model of non-alcoholic steatohepatitis. Levels of inflammatory and fibrogenic markers did not show significant differences between Fpnfl/flLysCre/- mice and controls. Interestingly, the levels of transaminases were significantly lower in mice with Fpn inactivation in macrophages, suggesting a different susceptibility to liver damage. These data suggest that, in this model, Fpn deletion in macrophages does not affect the inflammatory response to liver damage and fibrogenesis. The different susceptibility to liver damage and the different results we observed in the cutaneous wound healing and in the process of hepatic fibrosis should be further explored, perhaps using another model of fibrosis. In conclusion, the results reported in this thesis indicate that the macrophage trophic function in skin homeostasis and healing is iron and Fpn-dependent.