Inducible nitric oxide synthase (iNOS) in muscle wasting syndrome, sarcopenia, and cachexia

Abstract

Muscle atrophy-also known as muscle wasting-is a debilitating syndrome that slowly develops with age (sarcopenia) or rapidly appears at the late stages of deadly diseases such as cancer, AIDS, and sepsis (cachexia). Despite the prevalence and the drastic detrimental effects of these two syndromes, there are currently no widely used, effective treatment options for those suffering from muscle wasting. In an attempt to identify potential therapeutic targets, the molecular mechanisms of sarcopenia and cachexia have begun to be elucidated. Growing evidence suggests that inflammatory cytokines may play an important role in the pathology of both syndromes. As one of the key cytokines involved in both sarcopenic and cachectic muscle wasting, tumor necrosis factor α (TNFα) and its downstream effectors provide an enticing target for pharmacological intervention. However, to date, no drugs targeting the TNFα signaling pathway have been successful as a remedial option for the treatment of muscle wasting. Thus, there is a need to identify new effectors in this important pathway that might prove to be more efficacious targets. Inducible nitric oxide synthase (iNOS) has recently been shown to be an important mediator of TNFα-induced cachectic muscle loss, and studies suggest that it may also play a role in sarcopenia. In addition, investigations into the mechanism of iNOS-mediated muscle loss have begun to reveal potential therapeutic strategies. In this review, we will highlight the potential for targeting the iNOS/NO pathway in the treatment of muscle loss and discuss its functional relevance in sarcopenia and cachexia.

Figure 1. Inflammatory-induced sarcopenia vs. cachexia

Sarcopenia and cachexia represent two distinct diseased states, though both can result from an imbalance in the body's inflammatory mechanisms. Whereas sarcopenia (blue) results from chronic inflammation associated with age, cachexia (orange) results from inflammation associated with a primary disease (e.g. cancer, AIDS, and sepsis). Although resulting from different overlying conditions, both sarcopenia and cachexia result in muscle atrophy and loss. The dramatic loss of skeletal muscle tissue, occurring gradually in sarcopenia and acutely in cachexia, leads to disability and increased mortality.

Figure 2. Underlying mechanisms involved in muscle wasting diseases

Despite the fact that both diseases result in muscle wasting, the underlying causes of sarcopenia (blue) and cachexia (orange) are distinct. Sarcopenia arises from a multitude of factors, including [1] decreased amino acid intake, [2] diminished physical activity, [3] loss of motor neurons with age, and [4] a decline in anabolic stimulating hormones. Cachexia, in contrast, results from the physiological changes that occur during the progression of other chronic inflammatory illnesses. In cancer, the REE (Resting Energy Expenditure) is known to increase, pushing the overall energy state towards a negative energy balance. This effect is further exacerbated (dashed arrow) by anorexia, which, although not a direct cause of cachectic muscle loss, often accompanies cachexia and contributes towards the overall negative energy balance. Finally, several host humoral factors, such as glucocorticoids and angiotensin II, are known to induce muscle wasting, affecting the overall metabolic state by either by augmenting catabolism, decreasing anabolism, or both. Furthermore, the tumor factor PIF (Proteolysis Inducing Factor) has also been implicated in murine models of cancer cachexia, though its role in human cachexia has yet to be confirmed. In addition to the above factors, inflammatory cytokines are believed to play a key role in the pathology of both sarcopenia and cachexia. As a uniquely common cause of both diseased states, inflammatory cytokines represent an enticing target for the development of drug therapies.

Figure 3. The mechanism of iNOS-induced muscle wasting

TNFα, a key proinflammatory cytokine in the induction of muscle wasting, binds to its receptor, activating a signaling pathway that culminates in the activation of NF-κB. NF-κB then enhances the transcription of the iNOS transcript, which is subsequently bound by HuR at an ARE in the 3'-UTR and stabilized. This results in a dramatic increase in iNOS mRNA levels, resulting in enhanced translation of the iNOS protein. iNOS converts L-arginine into citrulline, releasing NO in the process. Several NO-dependent pathways may be responsible for the induction of muscle wasting. First, NO diffuses out of the cell where it combines with superoxide (O₂^-) to form peroxynitrite (ONOO^-). Peroxynitrite then diffuses back into the cell, selectively inhibiting MyoD, an important myogenic transcription factor, at the post-transcriptional level. Loss of MyoD leads to a reduction in MyHC expression, compromising the integrity of the myofibrillar protein complex. Second, NO-production leads to the oxidative modification of Jun-D, which, together with myogenin, regulates several key skeletal muscle-specific proteins, like CKM. Finally, NO-production may inhibit protein synthesis by inhibiting mTOR signaling and by increasing eIF2α and eEF2 phosphorylation, though the mechanism by which this occurs is uncertain. It is also unclear whether NO causes these last two effects directly, or through the formation of peroxynitrite.