Abstract
Background: Very few proteins encoded by the presumed non-coding RNA transcripts have been identified. Their cellular functions remain largely unknown. This study identifies the tumor-suppressor function of a novel microprotein encoded by the precursor of miR-34a. It consists of 133 amino acid residues, thereby named as miPEP133 (pri-microRNA encoded peptide 133). Methods: We overexpressed miPEP133 in nasopharyngeal carcinoma (NPC), ovarian cancer and cervical cancer cell lines to determine its effects on cell growth, apoptosis, migration, or invasion. Its impact on tumor growth was evaluated in a xenograft NPC model. Its prognostic value was analyzed using NPC clinical samples. We also conducted western blot, immunoprecipitation, mass spectrometry, confocal microscopy and flow cytometry to determine the underlying mechanisms of miPEP133 function and regulation. Results: miPEP133 was expressed in normal human colon, stomach, ovary, uterus and pharynx. It was downregulated in cancer cell lines and tumors. miPEP133 overexpression induced apoptosis in cancer cells and inhibited their migration and invasion. miPEP133 inhibited tumor growth in vivo. Low miPEP133 expression was an unfavorable prognostic marker associated with advanced metastatic NPC. Wild-type p53 but not mutant p53 induced miPEP133 expression. miPEP133 enhanced p53 transcriptional activation and miR-34a expression. miPEP133 localized in the mitochondria to interact with mitochondrial heat shock protein 70kD (HSPA9) and prevent HSPA9 from interacting with its binding partners, leading to the decrease of mitochondrial membrane potential and mitochondrial mass. Conclusion: miPEP133 is a tumor suppressor localized in the mitochondria. It is a potential prognostic marker and therapeutic target for multiple types of cancers.