A few years ago, Vikas Sukhatme began noticing a dull pain in his legs and back. Although the sensation never became debilitating, he said, “it felt like somebody had put lead on my legs.” Like many people around the world, Sukhatme, the Victor J. Aresty professor of medicine at Beth Israel Deaconess Medical Center, was taking a statin to help control his blood cholesterol levels. Knowing that muscle pain can be a side effect of statins, he decided to stop his treatment. It took nine months, but the pain finally went away. When he tried resuming treatment, the pain returned.
Though known as a renal expert, Sukhatme was spurred by this personal experience to begin investigating the mechanism by which statins might lead to muscle weakness. He and Stewart Lecker, HMS assistant professor of medicine at BID, report on a potential culprit in the December issue of the Journal of Clinical Investigation. Drawing from experiments in humans, zebrafish, and cultured mouse cells, they provide evidence that atrogin-1, a protein involved in muscle atrophy, is at least partly responsible for the effects that statins have on muscles. This finding gives researchers a target for tracking statins’ effects and for developing therapies that avoid this potentially debilitating consequence.
Muscle WasterStatins have quickly become one of the most widely prescribed drugs in developed countries. Because they are so widely used, many clinicians and researchers are in the role of patient, and Lecker and Sukhatme said that they frequently hear about side effects from their colleagues. It has long been known that in rare cases, certain statins can lead to a life-threatening breakdown of muscle, called rhabdomyolysis. But physicians have also found that patients experience a wide range of muscle-related symptoms, from mild weakness to pain, and it is not clear how widespread these milder side effects are. Sukhatme’s case was unusual, since most people develop symptoms when they begin statin treatment whereas his muscle pain did not appear until years after he began.
Although blood tests can detect more severe rhabdomyolysis, there is no indicator of less severe symptoms. Given that millions of people are now taking statins, even these small effects may add up. Sukhatme notes that they may keep many people who could benefit from statins from taking them.
Lecker had helped to discover the atrogin-1 gene several years ago while participating in a postdoctoral fellowship in the HMS lab of Alfred Goldberg. At the time, he was interested in studying the process of muscle wasting, which is a problem for many people with kidney disease. Using gene expression studies, Goldberg’s team identified about a hundred genes involved in muscle atrophy, which they termed “atrogenes,” the most prominent one being atrogin-1. Since then, studies on atrogin-1 show that it is a necessary player in the process of atrophy in skeletal muscle, smooth muscle of the uterus, and the heart. It seems to work by targeting other proteins for destruction. Lecker believed that atrogin-1 probably was involved in other forms of muscle damage, and after he was approached by Sukhatme, he said, “There was some reason to speculate that statins might turn on this gene.”
Animal EvidenceThe team, led by postdoc Peirang Cao and Jun-ichi Hanai, an HMS instructor in medicine at BID, began looking at how statins affect cultured muscle cells. When grown in vitro, these cells fuse together into structures called myotubes, akin to muscle fibers in the body. When the team added lovastatin, a widely prescribed statin that was the first to be approved by the Food and Drug Administration, these tube structures became thinner. At the same time, levels of atrogen-1 increased in the cells. The researchers were also able to find evidence for an association in humans. Working with Paul Phillips at Scripps Mercy Hospital in San Diego, they tested biopsy samples from the muscles of patients with and without muscle symptoms and found increased expression of atrogin-1 in patients taking statins compared to control samples.
Further studies showed that atrogin-1 activity was a cause of muscle toxicity by statins, not just a byproduct. Muscle cells cultured from mice that lack atrogin-1 were resistant to the thinning effect of lovastatin. Because a mouse model of muscle damage by statins was not available, the researchers turned to zebrafish to test the relationship in a living animal. Adding lovastatin to the water of developing zebra-fish embryos caused notable muscle fiber damage—the appearance of gaps and irregularities in the fibers under lower doses and more severe muscle defects at higher doses. Fish whose atrogin-1 gene was suppressed, however, were less affected by lovastatin. The team also found that the effect was not specific to one statin. When they knocked down the target of statins, HMG-CoA reductase, they saw the same muscle damage, but when both HMG-CoA reductase and atrogin-1 were knocked down, they did not. Sukhatme said the results show that “atrogin-1 is critical for the damage that ensues” with statin treatment.
Alfred Goldberg, HMS professor of cell biology, said the study shows that “this group of atrophy genes is involved in broader pathology.” Though muscle atrophy is a different process from statin toxicity, there may be some common links. The fact that atrogin-1 is turned on in muscle, Goldberg said, suggests that proteins are being broken down in the cell under statin treatment. The researchers found that statins may target atrogin-1 by switching off the IGF-1 signaling pathway, which also has a role in protecting muscle from atrophy. These connections raise the question of whether the muscle pain that patients experience with statins is an indication of damage to the muscles.
“This study gives a molecular explanation to the muscle toxicity of statins,” Lecker said, “and it puts atrogin-1 in the center of it.” With a target identified, further research can explore ways to dampen atrogin-1 or counteract its effects in muscles in patients taking statins. The primary candidates are compounds that boost the number or function of mitochondria in cells, an approach that also helps prevent muscle atrophy. The researchers found that expressing PGC-1 alpha, a protein that boosts the function and number of mitochondria, helped to prevent the muscle damage caused by lovastatin in zebrafish and cultured cells. Lecker and Sukhatme hope that understanding the atrogin-1 pathway could eventually help people receive the benefits of statins without their negative consequences.
