Extracellular Matrix Protein Tenascin C Increases Phagocytosis Mediated by CD47 Loss of Function in Glioblastoma.

Glioblastomas (GBM) are highly infiltrated by myeloid-derived innate immune cells that contribute to the immunosuppressive nature of the brain tumor microenvironment (TME). CD47 has been shown to mediate immune evasion, as the CD47-SIRPα axis prevents phagocytosis of tumor cells by macrophages and other myeloid cells. In this study, we established CD47 homozygous deletion (CD47-/-) in human and mouse GBM cells and investigated the impact of eliminating the "don't eat me" signal on tumor growth and tumor-TME interactions. CD47 knockout (KO) did not significantly alter tumor cell proliferation in vitro but significantly increased phagocytosis of tumor cells by macrophages in cocultures. Compared with CD47 wild-type xenografts, orthotopic xenografts derived from CD47-/- tumor cells grew significantly slower with enhanced tumor cell phagocytosis and increased recruitment of M2-like tumor-associated microglia/macrophages (TAM). CD47 KO increased tumor-associated extracellular matrix protein tenascin C (TNC) in xenografts, which was further examined in vitro. CD47 loss of function upregulated TNC expression in tumor cells via a Notch pathway-mediated mechanism. Depletion of TNC in tumor cells enhanced the growth of CD47-/- xenografts in vivo and decreased the number of TAM. TNC knockdown also inhibited phagocytosis of CD47-/- tumor cells in cocultures. Furthermore, TNC stimulated release of proinflammatory factors including TNFα via a Toll-like receptor 4 and STAT3-dependent mechanism in human macrophage cells. These results reveal a vital role for TNC in immunomodulation in brain tumor biology and demonstrate the prominence of the TME extracellular matrix in affecting the antitumor function of brain innate immune cells. SIGNIFICANCE: These findings link TNC to CD47-driven phagocytosis and demonstrate that TNC affects the antitumor function of brain TAM, facilitating the development of novel innate immune system-based therapies for brain tumors

The effect of the mTOR inhibitor rapamycin on glucoCEST signal in a preclinical model of glioblastoma

Purpose: The mammalian target of rapamycin is an enzyme that regulates cell metabolism and proliferation. It is up-regulated in aggressive tumors, such as glioblastoma, leading to increased glucose uptake and consumption. It has been suggested that glucose CEST signals reflect the delivery and tumor uptake of glucose. The inhibitor rapamycin (sirolimus) has been applied as a glucose deprivation treatment; thus, glucose CEST MRI could potentially be useful for monitoring the tumor responses to inhibitor treatment. Methods: A human U87-EGFRvIII xenograft model in mice was studied. The mice were treated with a mammalian target of Rapamycin inhibitor, rapamycin. The effect of the treatment was evaluated in vivo with dynamic glucose CEST MRI. Results: Rapamycin treatment led to significant increases (P < 0.001) in dynamic glucose-enhanced signal in both the tumor and contralateral brain as compared to the no-treatment group, namely a maximum enhancement of 3.7% ± 2.3% (tumor, treatment) versus 1.9% ± 0.4% (tumor, no-treatment), 1.7% ± 1.1% (contralateral, treatment), and 1.0% ± 0.4% (contralateral, no treatment). Dynamic glucose-enhanced contrast remained consistently higher in treatment versus no-treatment groups for the duration of the experiment (17 min). This was confirmed with area-under-curve analysis. Conclusion: Increased glucose CEST signal was found after mammalian target of Rapamycin inhibition treatment, indicating potential for dynamic glucose-enhanced MRI to study tumor response to glucose deprivation treatment