GM-CSF AND MMP9 ARE KEY REGULATORS OF THE EFFECT OF ADIPOSE PROGENITORS OVER BREAST CANCER ONSET AND METASTATIC PROGRESSION

Recent epidemiological and clinical data underlined the critical role of obesity in breast cancer (BC) progression. Among several white adipose tissue (WAT) cells, which may promote a permissive tumor microenvironment, a population with progenitor-like phenotype (CD45-CD34+) was reported to support local and metastatic BC. This population is composed by distinct WAT progenitors: adipose-derived stem cells (ASCs) and endothelial progenitor cells (EPCs), displaying complementary role in BC progression in preclinical models. However, molecular mechanisms involved in this interaction have been so far elusive and need to be clarified. An extensive screening of candidate molecules revealed two proteins being significantly up-regulated in WAT-derived progenitors after being co-cultured with several BC cells: Granulocyte-macrophage colony-stimulating factor (GM-CSF) and Matrix metallopeptidase 9 (MMP9). Both factors were detected over-expressed in orthotopic xenograft models, when co-injected with BC and human WAT progenitors. ASC and EPCs displayed similar ability to induce GM-CSF/MMP9, suggesting a complementary role in their release. GM-CSF neutralization in WAT progenitors inhibited MMP9 secretion, which was also reduced by IL-1β neutralization. GM-CSF displayed an additional positive feedback regulation on its own release. The inhibition of GM-CSF in diet-induced obese (DIO) syngeneic mice led to reduced intratumor vascularization and strong impairment of immunosuppressive microenvironment, targeting mainly tumor-associated macrophages (TAMs), myeloid derived suppressor cells (MDSCs) and T-regulatory (T-regs) cells. This resulted in a significant impairment of local BC growth and a slower metastatic progression. Conversely, MMP9 inhibition reduced neoplastic angiogenesis and significantly decreased local and metastatic tumor growth. The combined GM-CSF/MMP9 inhibition synergically impaired tumor angiogenesis, local and metastatic BC growth. Metformin was reported to significantly affect tumor progression and neoplastic angiogenesis, targeting both BC and WAT cells. In the present study, Metformin inhibited GM-CSF and MMP9 release from WAT progenitors in vitro and in xenograft models. Metformin had similar effects of GM-CSF/MMP9 specific inhibitions in DIO syngeneic mice, but was more effective in reducing tumor angiogenesis and targeted different immune cells. Collectively, these results indicate GM-CSF and MMP9 as new potential targets to prevent the pro-tumorigenic effect of WAT progenitors on BC. Furthermore, Metformin ability to reduce GM-CSF and MMP9 supports Metformin administration in clinical studies on BC, especially in a setting of obesity and/or insulin resistance.