Ribosomal protein S3A (RPS3A), as a transcription regulator of colony-stimulating factor 1 (CSF1), promotes glioma progression through regulating the recruitment and autophagy-mediated M2 polarization of tumor-associated macrophages.

Dysregulated expression of ribosomal protein S3A (RPS3A) is associated with the tissue infiltration of immune-related cells in a variety of cancers. However, the role of RPS3A in immune cell infiltration in glioma remains unclear. This study aimed to explore the role of RPS3A in the glioma immune microenvironment. RPS3A expression was detected in tumor tissues from patients with glioma. U251 cells were transfected with RPS3A shRNA (sh-RPS3A) and overexpression vector (pcDNA-RPS3A) and then co-cultured with PMA-induced THP-1 cells. Cell viability, invasion, and apoptosis were detected by Edu staining, Transwell, and flow cytometry, respectively. The expression of tumor-associated macrophage (TAM) M1 and M2 markers was detected with RT-qPCR. Next, the interaction between RPS3A and E4 transcription factor 1 (E4F1) was verified by Co-IP analysis, and the binding of E4F1 to colony-stimulating factor 1 (CSF1) promoter was verified by ChIP analysis. Overexpression vectors of CSF1 and E4F1 were used to treat sh-RPS3A-transfected U251 cells for reversal experiments. Finally, U251 cells transfected with sh-RPS3A adenovirus vectors were subcutaneously injected into nude mice to construct a xenograft tumor model, and the growth and metastasis of glioma in vivo were monitored. RPS3A was significantly upregulated in glioma tissues. Overexpression of RPS3A promoted glioma cell proliferation and invasion and inhibited apoptosis. Moreover, overexpression of RPS3A promoted TAM proliferation, invasion, and M2 polarization. Silencing RPS3A had the opposite effect. Silencing RPS3A inhibited autophagy in U251 cells, whereas rapamycin, an activator of autophagy, reversed the inhibitory effect of RPS3A silencing on TAM M2 polarization. Meanwhile, RPS3A promoted its expression by interacting with E4F1, and E4F1 promoted CSF1 transcriptional activation. Overexpression of CSF1 promoted the proliferation and invasion of U251 cells and reversed the inhibitory effect of RPS3A silencing on TAM proliferation and invasion, but had no effect on TAM M2 polarization. The results of in vivo experiments showed that knockdown of RPS3A significantly inhibited glioma tumor growth and metastasis in vivo. This study revealed that RPS3A recruited TAMs by upregulating E4F1-mediated transcription activation of CSF1, and promoted the M2 polarization of TAMs through autophagy, promoting glioma cell malignant growth and tumor progression.