Control of metastatic niche formation by targeting APBA3/Mint3 in inflammatory monocytes.

Cancer metastasis is intricately orchestrated by both cancer and normal cells, such as endothelial cells and macrophages. Monocytes/macrophages, which are often co-opted by cancer cells and promote tumor malignancy, acquire more than half of their energy from glycolysis even during normoxic conditions. This glycolytic activity is maintained during normoxia by the functions of hypoxia inducible factor 1 (HIF-1) and its activator APBA3. The mechanism by which APBA3 inhibition partially suppresses macrophage function and affects cancer metastasis is of interest in view of avoidance of the adverse effects of complete suppression of macrophage function during therapy. Here, we report that APBA3-deficient mice show reduced metastasis, with no apparent effect on primary tumor growth. APBA3 deficiency in inflammatory monocytes, which strongly express the chemokine receptor CCR2 and are recruited toward chemokine CCL2 from metastatic sites, hampers glycolysis-dependent chemotaxis of cells toward metastatic sites and inhibits VEGFA expression, similar to the effects observed with HIF-1 deficiency. Host APBA3 induces VEGFA-mediated E-selectin expression in the endothelial cells of target organs, thereby promoting extravasation of cancer cells and micrometastasis formation. Administration of E-selectin-neutralizing antibody also abolished host APBA3-mediated metastatic formation. Thus, targeting APBA3 is useful for controlling metastatic niche formation by inflammatory monocytes.

Genetic dissection of proteolytic and non-proteolytic contributions of MT1-MMP to macrophage invasion.

MT1-MMP/MMP-14 is a major invasion-promoting membrane protease expressed in macrophages. In addition to its proteolytic activity that degrades the extracellular matrix, MT1-MMP also boosts ATP production in cells in a manner independent of its proteolytic activity. It remains unclear to what extent the proteolytic and energy-boosting activities of MT1-MMP contribute to macrophage invasion. Recently, we demonstrated that the cytoplasmic tail of MT1-MMP makes use of APBA3/Mint3 to activate HIF-1 and thereby boosts glycolysis for ATP production. Here, we used Apba3(-/-) macrophages to dissect the contribution of the proteolytic and the energy-boosting activities of MT1-MMP. The proteolytic activity of MT1-MMP was not affected by the lack of APBA3 in macrophages. Apba3(-/-) and Mmp14(-/-) macrophages exhibited a 55% reduction of ATP levels compared to wild-type (WT) cells and the rate of motility of the mutant cells was accordingly reduced. In contrast, matrigel invasion by Mmp14(-/-) and Apba3(-/-) macrophages was reduced to 24% and 55.4%, respectively, of the level observed in WT cells. These results represent the first attempt to dissect the contribution of the two invasion-promoting activities of MT1-MMP to macrophage invasion.

Deletion of the Mint3/Apba3 gene in mice abrogates macrophage functions and increases resistance to lipopolysaccharide-induced septic shock.

Two major metabolic systems are usually used to generate ATP: oxidative phosphorylation (OXPHOS) in the mitochondria and glycolysis. Most types of cells employ OXPHOS for ATP production during normoxia but then shift energy production from OXPHOS to glycolysis when exposed to hypoxia. Hypoxia-inducible factor-1 (HIF-1) is the master transcription factor regulating this metabolic shift. On the other hand, macrophages are unique in making use of glycolysis for ATP generation constitutively even during normoxia. We recently proposed that in macrophages, Mint3/APBA3 inhibits factor inhibiting HIF-1 (FIH-1) during normoxia, which in turn releases the suppression of HIF-1 activity by FIH-1. To demonstrate the physiological function of APBA3 in macrophages, we established Apba3(-/-) mice. The mutant mice presented no apparent gross phenotype but exhibited significant resistance against LPS-induced septic shock. The level of ATP in macrophages obtained from the mutant mice was reduced to 60% of the level observed in wild type cells, which in turn led to reduced ATP-dependent activities such as glycolysis, cytokine production, and motility. We also generated mutant mice with the Apba3 gene deleted specifically from cells of the myeloid lineage and confirmed that LPS-induced septic shock is mitigated significantly. Thus, we show cell type-specific regulation of energy production by APBA3 in macrophages using genetically manipulated mice. The specific function of APBA3 in macrophages might allow us to develop therapeutics to regulate aberrant macrophage function during infection and diseases.