miRNAs and Cholesterol
Controlling cholesterol/lipid metabolism is another new avenue being pursued for the therapeutic targeting of miRNAs. Anders M. Näär, Ph.D., associate professor of cell biology at Massachusetts General Hospital and Harvard Medical School, described his team’s approach at the meeting.
“Coordination of the biosynthesis, trafficking, and degradation of cholesterol and lipids is essential for proper health. We discovered a new miRNA-mediated pathway that regulates the levels of high-density lipoprotein (HDL), or good cholesterol.”
“We found that miR-33a and miR-33b are respectively embedded in introns of the sterol regulatory element-binding proteins SREBP-2 and -1. SREBPs are key transcription factors that are involved in cholesterol/lipid biosynthesis and uptake. Both miR-33a and miR-33b target several key regulators of both cholesterol trafficking and fatty acid/triglyceride homeostasis for post-transcriptional repression. These include the ABC transporter A1 (ABCA1) that is important for regulation of cholesterol efflux and HDL synthesis.”
The researchers performed studies in mice and cultured human cells that demonstrated that antisense inhibition of miR-33a/b caused increased expression of ABCA1. “We now have strong evidence that miR-33a/b not only regulate the ABCA1 cholesterol transporter and HDL biosynthesis and trafficking, but also have effects on other key regulators of metabolic homeostasis such as fatty acid beta-oxidation enzymes, the fasting/stress-response regulator AMP kinase, and the sirtuin and glucose/lipid regulator SIRT6.”
Dr. Näär and colleagues are now embarking on nonhuman primate studies to gain additional insights into the in vivo functions of miR-33a/b and potential therapeutic targeting.
“Because mice lack miR-33b, which is present in the insulin-regulated SREBP-1 gene in humans and primates, they are poor models for further investigation of therapeutic feasibility, so we are embarking on collaborative nonhuman primate (African green monkey) studies. We are testing the efficacy of targeting miR-33a/b using antisense oligonucleotides for raising circulating HDL and lowering triglycerides in animals with high-fat diet-induced insulin resistance/metabolic syndrome.”
Dr. Näär’s take-home message is that miRNAs represent new therapeutic targets to control cholesterol/lipid homeostasis and could be useful tools to ameliorate cardiometabolic diseases.
Erythrocytes continuously replenish themselves in adults. This process is sensitive to cytokine signaling and oxidative stress. miRNAs are key regulators of this vital process. David M. Patrick, an M.D./Ph.D. student in the department of molecular biology at the University of Texas Southwestern Medical Center, and colleagues found that one erythroid-enriched miRNA, miR-451, is an important regulator of this pathway and potential therapeutic target.
“To investigate the functions of miR-451 in vivo, we created mice that lacked it. We found that when miR-451 is deleted, there is a corresponding reduction in hematocrit, defective erythroid differentiation, and suboptimal erythropoiesis. When we used antagomirs of miR-451 in normal mice, we could recapitulate this phenotype.”
According to Patrick, he and colleagues are collaborating with MiRagen Therapeutics to develop inhibitors of miR-451 to evaluate in mouse models of human erythroid diseases. “Inhibiting miR-451 in a mouse model of the disease polycythemia vera, a neoplasm characterized by excessive production of erythrocytes, reduces hematocrit and improves outcome.”
“We next characterized the mechanism for this and found that miR-451 can repress a chaperone protein called 14-3-3ζ that is an intracellular regulator of cytokine signaling and erythropoiesis. Mice lacking miR-451 show an upregulation of 14-3-3ζ in their erythroblasts. However, inhibition of 14-3-3ζ rescues the differentiation defect.
“Overall, our studies demonstrate the therapeutic potential of inhibiting miR-451 for the treatment of polycythemia vera. We are now engaged in completing efficacy studies.”
The growth and regeneration of skeletal muscles in adults is controlled largely by specialized satellite cells present in the tissue. These adult muscle stem cells drive two important processes: proliferation and differentiation. miRNAs help control this circuit in both cardiac and skeletal muscles. Dysregulation of this circuit may lead to diseases such as muscular dystrophy and heart failure.
Da-Zhi Wang, Ph.D., associate professor, Children’s Hospital Boston, Harvard Medical School, is studying how miRNAs impact skeletal muscle tissue and how targeting their dysfunctions may lead to therapeutics.
“As stem cells in muscle differentiate, they turn on lineage-specific genes and turn off stem cell maintenance genes in response to biological cues. How this process works is largely unknown. We found that two particular miRNAs, miR-1 and miR-206, were highly induced when satellite cells and satellite cell-derived primary myoblasts differentiate. We also determined that they downregulate proliferation potential of satellite cells.”
According to Dr. Wang, both miRNAs function, at least in part, by repressing a key protein called paired-box transcription factor Pax7. “Pax7 is highly expressed in quiescent and proliferating satellite cells but is rapidly downregulated in satellite cell-derived myogenic progenitor cells.”
“Both miR-1 and miR-206 impact this circuit. They are sharply upregulated during satellite cell differentiation and downregulated after muscle injury. Both also facilitate satellite cell differentiation by restricting their ability to proliferate. When you inhibit miR-1 and miR-206, satellite cell proliferation is enhanced, leading to increases in the level of Pax7 protein. Conversely, sustained Pax7 expression due to the loss of miR-1 and miR-206 repression elements at its 3´UTR significantly inhibits myoblast differentiation. Thus, these miRNAs are key players in this regulatory circuit that allows rapid gene program transitions from proliferation to differentiation.”
Dr. Wang’s group will look for other targets that may be impacted by miR-1 and -206. “Understanding the specificity and molecular mechanisms of miRNAs are key factors that will help in the more specific design of therapeutics such as for muscular dystrophy.”
Many feel miRNA therapeutics will soon make the leap to the clinic. The exciting speed at which progress is being made has also prompted some scientists to caution that it would be wise to take a step back and learn more about how miRNAs function before humans are dosed. One thing is clear—the miRNA market is poised for growth, with a predicted market size of nearly $100 million by 2015.