microRNAs Regulate Cellular Magnesium by Tuning Expression of the Plasma Membrane Protein CNNM4

Cyclin M4 (CNNM4) is a plasma membrane Mg²⁺ transporter that is important for the regulation of cellular and organismal Mg²⁺ homeostasis. CNNM4 is overexpressed in liver diseases, including non-alcoholic steatohepatitis and acetaminophen-induced liver injury, leading to aberrant levels of Mg²⁺. The regulatory mechanisms underlying the overexpression of this protein, however, remain unknown. Here, we demonstrate regulation of CNNM4 and cellular magnesium levels by microRNA. Using a high-throughput assay, we established the complete miRNA binding profile of the 3'UTR of the gene. Both upregulation and downregulation of CNNM4 were observed, though upregulatory activity appears more prominent. We demonstrate that in both cases, the effects arise from direct binding of the miRNAs to the CNNM4-3'UTR, and we provide direct evidence that they result in changes of cellular concentration of the metal in hepatocytes. The results reveal a potential role of microRNAs in the alteration of magnesium homeostasis in the liver through CNNM4 and, based on the miRNA proxy hypothesis, predict a broader role of CNNM4 and aberrant magnesium levels in breast cancer and other pathologies.