EGFR‐Induced and c‐Src‐Mediated CD47 Phosphorylation Inhibits TRIM21‐Dependent Polyubiquitylation and Degradation of CD47 to Promote Tumor Immune Evasion

Abstract

Tumor cells often overexpress immune checkpoint proteins, including CD47, for immune evasion. However, whether or how oncogenic activation of receptor tyrosine kinases, which are crucial drivers in tumor development, regulates CD47 expression is unknown. Here, it is demonstrated that epidermal growth factor receptor (EGFR) activation induces CD47 expression by increasing the binding of c‐Src to CD47, leading to c‐Src‐mediated CD47 Y288 phosphorylation. This phosphorylation inhibits the interaction between the ubiquitin E3 ligase TRIM21 and CD47, thereby abrogating TRIM21‐mediated CD47 K99/102 polyubiquitylation and CD47 degradation. Knock‐in expression of CD47 Y288F reduces CD47 expression, increases macrophage phagocytosis of tumor cells, and inhibits brain tumor growth in mice. In contrast, knock‐in expression of CD47 K99/102R elicits the opposite effects compared to CD47 Y288F expression. Importantly, CD47‐SIRPα blockade with an anti‐CD47 antibody treatment significantly enhances EGFR‐targeted cancer therapy. In addition, CD47 expression levels in human glioblastoma (GBM) specimens correlate with EGFR and c‐Src activation and aggravation of human GBM. These findings elucidate a novel mechanism underlying CD47 upregulation in EGFR‐activated tumor cells and underscore the role of the EGFR‐c‐Src‐TRIM21‐CD47 signaling axis in tumor evasion and the potential to improve the current cancer therapy with a combination of CD47 blockade with EGFR‐targeted remedy.

Glioblastoma Phagocytic Cell Death: Balancing the Opportunities for Therapeutic Manipulation

“…Although anti-CD47 showed promising results in preclinical studies, their therapeutic effects could be limited by the pattern of CD47 expression in glioma patients, especially newly diagnosed subjects. Contrary to clonotypic homogeneous glioma cell lines where CD47 is consistently highly expressed, newly diagnosed glioblastoma specimens exhibited heterogeneous and very low expression of CD47 (Figure 2) [32]. Assessing CD47 expression of 75 human glioma specimens of different grade with immunohistochemical staining revealed that less than 30% tumor cells are CD47-positive in grade 4 gliomas, and the percentage is even lower in low-grade gliomas [32].…”

“…Contrary to clonotypic homogeneous glioma cell lines where CD47 is consistently highly expressed, newly diagnosed glioblastoma specimens exhibited heterogeneous and very low expression of CD47 (Figure 2) [32]. Assessing CD47 expression of 75 human glioma specimens of different grade with immunohistochemical staining revealed that less than 30% tumor cells are CD47-positive in grade 4 gliomas, and the percentage is even lower in low-grade gliomas [32]. As such, the experimental models likely overestimate the therapeutic effect.…”

Structural–functional diversity of CD47 proteoforms

“…Nevertheless, further research is required to verify these modifications and their functions. A recent study identified two ubiquitylation sites on CD47 that are distinct from the putative sites mentioned above and showed that in human glioma cells, activated EGFR enhanced Src-mediated CD47 Y288 phosphorylation, which blocked binding of the E3 ubiquitin ligase TRIM21 to CD47 and prevented TRIM21-mediated CD47 K99/102 polyubiquitylation and CD47 degradation, resulting in increased CD47 protein levels, which led to inhibition of phagocytosis by tumor-infiltrating macrophages and ultimately facilitated tumor immune escape ( 48 ).…”

“…In addition to the traditional methods of blocking CD47 with antibodies or SIRPα or genetically downregulating CD47 expression to enhance antitumor immune responses, QPCTL inhibitors, combined with anti-CD47 antibodies, can enhance the antitumor effect by blocking the post-translational pGlu modification catalyzed by QPCTL, which is critical for CD47–SIRPα interaction ( 24 , 25 ). In EGFR-activated tumor cells, CD47 phosphorylation inhibits ubiquitylation and degradation of CD47, leading to upregulation of CD47, indicating the role of these PTMs in tumor evasion and the potential to improve the current cancer therapy ( 48 ) ( Figure 2 ). Elevated CD47 levels on aged MuSCs with impaired regenerative capacity resulted from increased U1 snRNA expression, which disrupted alternative polyadenylation, generating a long 3’-UTR of CD47 mRNA ( 75 ).…”

“…Notably, different molecular weights have been reported for CD47 proteins by independent studies ( 2 , 23 , 47 ), suggesting that the degree of N-glycosylation and glycosaminoglycan modification (or other unidentified PTMs) varies across cells. Inhibition of pGlu formation by glutaminyl cyclase isoenzyme (QPCTL), another PTM, affects the CD47–SIRPα interaction and enhances the antitumor effect ( 24 , 25 ); epidermal growth factor receptor (EGFR) -induced and c-Src-mediated CD47 phosphorylation inhibits TRIM21-dependent ubiquitylation and degradation of CD47, promote tumor immune evasion ( 48 ); suggesting that PTMs play an important role in the function and clinical application of CD47.…”