Pathogenesis of light chain-induced dysfunction in cardiac amyloidosis
Snyder, Christina AnnaMarie
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Although a rare disease, light chain (LC) amyloidosis (AL) is the most common systemic amyloidosis in developed countries. It is caused by an overproduction of immunoglobulin LC proteins in bone marrow plasma cells. In AL amyloidosis, LCs that are prone to misfolding and insolubility will aggregate, form fibrils, and deposit themselves in various tissues, thereby causing organ dysfunction. The most fatal manifestation of AL amyloidosis is associated with cardiac involvement, defined by the presence of extracellular AL amyloid deposits within the heart. Cardiac amyloid infiltration typically leads to diastolic dysfunction followed by heart failure and has a median survival of approximately 6 months from the time of diagnosis if untreated. Clinical observation suggests that a reduction in circulating LCs results in an improvement in heart failure symptoms despite minimal changes in amyloid deposition. This has led to the concept that LCs themselves are cytotoxic to cardiomyocytes. Recent studies indicate that AL LCs induce oxidative stress, cellular dysfunction, and apoptosis (programmed cell death) in cardiomyocytes via a p38α mitogen-activated protein kinase (MAPK) mechanism. They may therefore be a target for amyloidosis therapy. By understanding how LCs cause cardiac dysfunction, we can target this process with therapies and utilize downstream measures of LC activity as diagnostic and prognostic tools. The objective of this study was to determine the role of autophagy in AL amyloidosis. Autophagy is the intracellular process of degrading aging or dysfunctional cellular components. Autophagy can be beneficial by preventing proteotoxicity and providing nutrients, amino acids, and other necessities during times of cellular stress. On the other hand, increased autophagy, like apoptosis, may mediate cellular death depending on the type of stimulus and its duration. Autophagy is induced by a variety of stimuli, including oxidative stress. AL has been demonstrated to increase reactive oxygen species (ROS), and it is unknown if autophagy mediates cardiomyocyte dysfunction in AL cardiac amyloidosis. We thus sought to determine if it is a factor in amyloid cardiotoxicity. We explored the ERK1/2, p38, and JNK MAPK pathways in particular, since MAPK signaling cascades regulate several transcription factors involved in the cell cycle and p38α has been implicated in ROS-induced cardiac AL amyloidosis. Adult rat ventricular myocytes (ARVM) were harvested from healthy adult male rats and exposed to a variety of experimental conditions in vitro. ARVM were treated with vehicle control, human LC obtained from a patient without cardiac involvement, a positive control (aldosterone), and human AL light chains obtained from a patient with AL cardiac amyloidosis in the presence or absence of UO126, SB203580, or SP600125 (specific inhibitors of ERK1/2, p38, and JNK, respectively). The resulting protein expression levels of autophagy indicators LC3II and ATG4B in cardiomyocytes were analyzed by Western blotting. The ratio of phosphorylated to total ERK1/2 protein expression was also explored. We found that AL light chains did not contribute to autophagy via the ERK1/2, p38, or JNK pathways. In contrast to our previous unpublished findings, the protein levels of autophagy indicators in AL-treated ARVM did not differ from vehicle control levels, suggesting that AL did not activate autophagy. However non-cardiomyopathic light chains (LC) did increase LC3II expression in ARVM, despite their human source exhibiting no clinical indications of cardiac involvement. This implies that autophagy induced by non-cardiomyopathic LCs may be beneficial and protect against the development of the cardiotoxicity seen in AL cardiac amyloidosis. Further studies are necessary to understand the effect of autophagy in the heart and its role in cardiac amyloidosis. Continuing to explore the underlying mechanisms of AL light chain toxicity will contribute to the development of diagnostic, prognostic, and treatment strategies for AL amyloidosis.