Mitochondrial calcium uniporter, MiRNA and cancer: Live and let die.

Mitochondria receive calcium (Ca(2+)) signals from endoplasmic reticulum (ER) and decode them into pro-apoptotic inputs, which lead to cell death. Therefore, mitochondrial Ca(2+) overload is considered a fundamental trigger of the apoptotic process, and several oncogenes and tumor suppressors modify the activity of protein involved in Ca(2+) homeostasis to control apoptosis. The identification of the channel responsible for mitochondrial Ca(2+) entry, the Mitochondrial Ca(2+)Uniporter (MCU), together with its regulatory components, MICU1 and MCUR1, provides new molecular tools to investigate this process. Recent data have also shown that miR-25 decreases mitochondrial Ca(2+) uptake through selective MCU downregulation, conferring resistance to apoptotic challenges. MCU appears to be downregulated in human colon cancer samples, and accordingly, miR-25 is aberrantly expressed, indicating the importance of mitochondrial Ca(2+) regulation in cancer cell survival.

In the last two years, the discovery of the pore-forming subunit of the mitochondrial Ca2+ uptake channel (Mitochondrial Calcium Uniporter, MCU), and its regulatory subunits, termed MICU1 (mitochondrial calcium uptake 1) and MCUR1 (mitochondrial calcium uniporter regulator 1), opened a new era for the study of mitochondrial Ca2+ regulation and its key role in a variety of processes, including cell death. In the presence of an apoptotic stimulus, mitochondria receive Ca2+-mediated inputs that induce the release of a number of pro-apoptotic factors from the mitochondria., Several oncogenes and tumor suppressors manipulate Ca2+ to exert their anti/pro-apoptotic activities. For example, Akt and Bcl-2 regulate ER Ca2+ flux to avoid mitochondrial Ca2+ overload and apoptosis;, in contrast, pro-apoptotic genes, such as Fhit and PML, act at mitochondrial and ER levels, respectively, to promote mitochondrial Ca2+ accumulation., Although the connection between mitochondrial Ca2+ increase and apoptosis is widely accepted, the mechanistic role of mitochondrial Ca2+ homeostasis in tumorigenesis is not fully understood. MicroRNAs (miRNAs) are a class of naturally occurring small noncoding RNAs that are capable of regulating the expression of protein-coding genes at the posttranscriptional level, which consequently leads to a decrease in target protein abundance. Dysregulation of miRNA expression could lead to a variety of human disorders, including cancer. Thus, miRNAs may function as oncogenes or tumor suppressors. Among the oncogenic miRNAs, miR-25 is one of the most studied and well described. miR-25 is 22 nucleotides in length, hosted by the minichromosome maintenance protein-7 (MCM7) gene, and transcribed as part of the mir-106b~25 polycistron; it is overexpressed in several human cancers, including pediatric brain tumors, gastric adenocarcinoma, epidermal growth factor receptor-positive lung adenocarcinoma and prostate carcinoma and has been reported to target different regulators of the apoptotic pathway, such as BIM, PTEN and TRAIL. Mir-25 also affects Ca2+ homeostasis through the specific downregulation of MCU, causing a strong decrease in mitochondrial Ca2+ uptake and, importantly, conferring resistance to Ca2+-dependent apoptotic challenges. Prostate cancer cell lines, which exhibit high levels of miR-25, display very low amounts of MCU, and this inverse correlation (high miR-25/low MCU) is also maintained in colon cancer cell lines. Expression of miR-25 inhibitor in HCT116 cells increases mitochondrial Ca2+ levels and re-sensitizes cells to apoptosis. A cancer link has been established through the detection of high miR-25 levels in stage II and III colonic adenocarcinoma samples, whereas MCU is virtually undetectable by immunohistochemistry. Other members of the miR-25 family, such as miR-92a and miR-363, have the same effect on MCU expression and Ca2+ signaling as miR-25. These observations not only highlight the deep involvement of the whole family in the regulation of Ca2+ homeostasis, but also suggest how cancer cell survival, which is favored by MCU downregulation, might be ascribed to the upregulation of all miR-25 family members or strong expression of a singular miR, different from miR-25. Thus, both miR-25-5p, which is the different mature miR that originates from the opposite arm of the same pre-miRNA, and members of the same miRNA cluster, i.e., miR-106b, were predicted to target MCU mRNA, although their activity has not yet been tested. Therefore, the miR-106b~25 cluster might also play an important role in the control of MCU levels.

Regulation of intracellular Ca2+ levels by miRNAs might be considered a fundamental aspect in several physio-pathological conditions. In the Ca2+-dependent apoptosis context, which is characterized by sustained Ca2+ release from the ER and consequent accumulation at the mitochondrial level, the specific expression of miR-targeting mitochondrial Ca2+ effectors, such as miR-25, may be considered one of the most rapid intracellular mechanisms to prevent mitochondrial Ca2+ overload and avoid cell death. This process appears to be aberrantly overexpressed in tumors, especially in colon and prostate cancer cells.

Interplay between the modulation of Ca2+ levels and miRNAs has also been highlighted in other pathological scenarios. For example, in cardiomyocytes, loss of miR-133a–mediated IP3R II (inositol 1,4,5 trisphosphate receptor, the calcium channel within the membranes of sarco/endoplasmic Ca2+ stores) repression generates a positive feedback loop to drive the hypertrophic response, a process that is primarily Ca2+ dependent. In the same cellular setting, miR-214 protects the mouse heart from ischemic injury by controlling Ca2+ overload and cell death through the repression of the mRNA encoding sodium/calcium exchanger 1 (Ncx1), a key regulator of Ca2+ influx. Moreover, miR-708, which is transcriptionally repressed in metastatic breast cancer, targets the ER protein neuronatin to decrease intracellular calcium levels, resulting in decreased cell migration and impaired metastases.

In conclusion, the interplay between intracellular Ca2+ and miRNAs might be a key aspect in several pathological conditions. Specifically, the suppression of mitochondrial Ca2+ entry by cancer-related miR-25 represents the first study of the control of the mitochondrial uniporter by miRNAand offers initial clues to the relevance of this pathway in human cancers.