SIRP Alpha Protein Downregulates in Human Astrocytoma: Presumptive Involvement of Hsa-miR-520d-5p and Hsa-miR-520d-3p

Astrocytomas are the most common brain tumors with poor survival in malignant forms. Signal regulatory protein alpha (SIRP alpha) is a transmembrane protein expressed on immune cells and macrophages and is reported to modulate tumor cell phagocytosis. In the present study, we investigated the involvement of miR-520d-5p and miR-520d-3p in regulation of SIRP alpha expression. Here, we report mRNA and protein expression profile of SIRP alpha in 39 surgically resected human astrocytoma tissue samples and 14 control brain tissue samples. Transcript expression pattern was studied by real-time PCR while Western blotting and immunohistochemistry were used to evaluate protein expression. Expression profile of miR-520d-5p and miR-520d-3p was studied by real-time PCR. Computational prediction was employed to analyze the binding of miR-520d-5p and miR-520d-3p for SIRP alpha mRNA. It is evident from preliminary investigation that SIRP alpha transcripts are expressed in control brain tissues, increased in low-grade (grade II) tumor tissues, and decreased with further grade progression (P < 0.05). SIRP alpha protein was moderately expressed in control brain tissues but under-expressed in low- and high-grade tissue samples (P < 0.05). Immunohistochemistry results further confirmed Western blot outcomes. Computational prediction supplemented with 3' and 5'UTR targeting analysis and correlation studies reveals that hsa-miR-520d-5p (P = 0.028, R (2) = 0.94) (95 % CI 0.15 to 0.99) and hsa-miR-520d-3p (P = 0.027, R (2) = 0.94) (95% CI 0.17 to 0.99) may be the putative microRNAs involved in regulation of SIRP alpha protein expression. Real-time PCR expression profile depicts that mature form of both miRNAs is significantly overexpressed in low-grade (GII) tumor tissue samples compared to control and high-grade (GIII and GIV) tissue samples. MiR-520d-5p and miR-520d-3p were found with expression pattern similar to SIRP alpha transcripts. We show that SIRP alpha protein is under-expressed in low and high grades of astrocytoma patients' tissue samples. Control brain tissues were found to be positive with SIRP alpha protein expression. Real-time PCR expression analysis confirms that miR-520d-5p and miR-520d-3p expression levels were significantly correlated with SIRP alpha transcripts in control, low-grade, and high-grade tissue samples. Computational prediction further evidenced for binding sites of these miRNAs on 3' and 5'UTR of SIRP alpha transcripts. Taken together, we predict that miR-520d-5p and miR-520d-3p may be having role in the regulation of under-expressed SIRP alpha protein expression.