Poly(rC) Binding Protein 1 Represses the Translation of STAT3 through 5' UTR

Background: Signal transducer and activator of transcription 3 (STAT3) is an oncogene and frequently overexpressed in cancers. However, the regulatory mechanisms of STAT3 expression are not fully understood. Poly(rC)-binding protein1 (PCBP1) is an RNA-binding protein that regulates mRNA stability, splicing, and translation. PCBP1 is a tumor suppressor and can inhibit the translation of several oncogenic genes. Objective: We aimed to understand the regulatory mechanisms of STAT3 expression. Methods: The 5' UTR or 3’ UTR regions of the human STAT3 gene were inserted upstream or downstream of the green fluorescent gene (GFP), respectively, which were used as reporter systems to analyze the inhibitory effects of PCBP1 on the STAT3 gene expression. The deletion and point mutation in 5' UTR were used to search the essential regulatory sequences of the translation inhibition. The mutations of PCBP1 protein were analyzed in the cBioPortal online service. The effects of mutated PCBP1 proteins on STAT3 expression, cancer cell proliferation, and colony formation were analyzed in oral squamous cell carcinoma (OSCC) cell lines. Results: PCBP1 inhibits mRNA translation through a motif in the 5' UTR of STAT3. Moreover, we found two leucine residues (Leu100 and Leu102) of PCBP1 protein frequently mutated in cancers. These mutations abolished the inhibition function of PCBP1 on STAT3 translation. Surprisingly, in contrast to wild-type PCBP1 protein, these mutations can promote the growth and colony formation of cancer cells. Conclusion: Overall, we demonstrate that PCBP1 can inhibit the expression of STAT3 through its 5' UTR, and two leucine residues of PCBP1 protein are essential for its functions.