Autophagy in the Immunosuppressive Perivascular Microenvironment of Glioblastoma

Abstract

Glioblastoma (GB) has been shown to up-regulate autophagy with anti- or pro-oncogenic effects. Recently, our group has shown how GB cells aberrantly up-regulate chaperone-mediated autophagy (CMA) in pericytes of peritumoral areas to modulate their immune function through cell-cell interaction and in the tumor's own benefit. Thus, to understand GB progression, the effect that GB cells could have on autophagy of immune cells that surround the tumor needs to be deeply explored. In this review, we summarize all the latest evidence of several molecular and cellular immunosuppressive mechanisms in the perivascular tumor microenvironment. This immunosuppression has been reported to facilitate GB progression and may be differently modulated by several types of autophagy as a critical point to be considered for therapeutic interventions.

Keywords: autophagy; autophagy inhibitors; chaperone-mediated autophagy; glioblastoma; immunosuppressive; pericytes; perivascular cells; tumor; tumor immune tolerance.

Figure 1

Types of autophagy in mammals and their anti- and pro-oncogenic roles. Macroautophagy, is a process of lysosomal degradation and recycling of cellular components, including damaged organelles, intracellular bacterial pathogens, and aggregated proteins. Macroautophagy sequesters cytosolic contents in de novo-generated double membrane vesicles called autophagosomes that finally fuse with lysosomes. Macroautophagy can be either in bulk or selective for a kind of cargo, and selectivity is mediated by selective autophagy receptors (SAR), or Hsc70 chaperone for protein aggregates in the case of Chaperone-Assisted Selective Autophagy (CASA). Selective macroautophagy binds cargo to the phagophore through lipidated LC3 proteins anchored to the latter’s membrane. Endosomal microautophagy, termed microautophagy in mammalian cells, caters on the degradation of cytosolic regions by late endosomes recognizing the substrates to be degraded, both by bulk degradation of proteins present in cytosol trapped in vesicles forming at the late endosome membrane, and by a selective degradation after binding to Hsc70 chaperone though KFERQ-like motifs. Latter are sorted into intraluminal vesicles in a manner dependent on the endosomal sorting complex required for transport III (ESCRT-III). CMA is a selective process of degradation of soluble, amenable to unfolding, cytosolic proteins presenting KFERQ-like motifs. The chaperone Hsc70 recognizes the motifs and transports the protein substrates to the lysosome where they bind to LAMP2A. LAMP2A multimerizes being stabilized by a glial fibrillary acidic protein (GFAP) and upon unfolding, the substrate is translocated through the lysosomal membrane with the assistance of a lysosomal resident Hsc70, and degraded in the lysosomal lumen. Both macroautophagy and CMA have anti- and pro-oncogenic roles, which depend on the cellular context and are summarized here.

Figure 2

Autophagy function in the immune responses of peritumoral cells during GB progression. Macroautophagy and CMA activation in different immune or brain-resident cells, represents a key point of regulation to favors progression of tumor cells (green arrows) or to promotes its anti-tumor activity (red arrows), respectively. Macroautophagy and CMA up-regulation support tumor progression by increasing phagocytosis and by inhibiting inflammasome-mediated responses of TAMs and microglial cells, and by stimulating differentiation of monocytes into anti-inflammatory M2 macrophages. However, macroautophagy promotion hinders polarization of monocyte into pro-inflammatory M1 macrophages, which may represent an indirect mechanism to advantage tumor progression. Astrocytes have direct physical contact with tumor cells whereas macroautophagy/CMA activity in this cell type contributes to its anti-inflammatory phenotype. Neutrophils require macroautophagy to exert its anti-tumor activity. Regarding the adaptive immune responses, T cells has been shown to require macroautophagy and CMA to develop its anti-tumor activity by regulation of several immune checkpoints (i.e., increasing cytokine release, proliferation, energy store mobilization, and degradation of negative regulators of T cell activation or by prevention of T cell anergy). Macroautophagy and CMA are also necessary for maintaining B cell-specific functions such as antigen presentation. However, macroautophagy promotion favors tumor tolerance by stimulation of FoxP3 T regulatory cell function. GB-induced CMA modulates pericytes immune function through cell-cell stable interactions promoting GB survival and progression. GB-conditioned pericytes display an aberrant up-regulation of CMA that lead to secretion of anti-inflammatory cytokines, angiogenic molecules, pro-regenerative extracellular vesicles, and prevention of anti-tumor proteins secretion that benefits tumor growth. Furthermore, GB-induced CMA in PC down-regulates expression of co-stimulatory molecules, prevents pro-inflammatory cytokine secretion and fails to promote anti-tumor T cell responses, enhancing Treg responses, which contributes to the immunosuppressive peritumoral niche of GB. Ig: immunoglobulins; EVs: extracellular vesicles; EC: endothelial cells.