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.