Abstract
The skeletal muscle mass has been shown to be affected by catecholamines, such as epinephrine (Epi), norepinephrine (NE), and isoproterenol (ISO). On the other hand, lipopolysaccharide (LPS), one of the causative substances of sepsis, induces muscle wasting via toll-like receptors expressed in skeletal muscle. Although catecholamines are frequently administered to critically ill patients, it is still incompletely understood how these drugs affect skeletal muscle during critical illness, including sepsis. Herein, we examined the direct effects of catecholamines on LPS-induced skeletal muscle wasting using the C2C12 myoblast cell line. Muscle wasting induced by catecholamines and/or LPS was analyzed by the use of the differentiated C2C12 myotubes, and its underlying mechanism was explored by immunoblotting analysis, quantitative reverse transcription polymerase chain reaction (qRT-PCR), enzyme-linked immunosorbent assay (ELISA), and the TransAM kit for p-65 NF-κB. Epi augmented myosin heavy chain (MHC) protein loss and reduction of the myotube diameter induced by LPS. LPS induced C/EBPδ protein, Atrogin-1 and inteleukin-6 (IL-6), and these responses were potentiated by Epi. An IL-6 inhibitor, LMT28, suppressed the potentiating effect of Epi on the LPS-induced responses. NF-κB activity was induced by LPS, but was not affected by Epi and recombinant IL-6, and the NF-κB inhibitor, Bay 11-7082, abolished Atrogin-1 mRNA expression induced by LPS with or without Epi. NE and ISO also potentiated LPS-induced IL-6 and Atroign-1 mRNA expression. Carvedilol, a nonselective β-adrenergic receptor antagonist, suppressed the facilitating effects of Epi on the Atrogin-1 mRNA induction by LPS, and abolished the effects of Epi on the MHC protein loss in the presence of LPS. It was concluded that Epi activates the β-adrenergic receptors in C2C12 myotubes and the IL-6-STAT3 pathway, leading to the augmentation of LPS-induced activation of the NF-κB- C/EBPδ-Atrogin-1 pathway and to the exacerbation of myotube wasting.
Highlights
Catecholamines, such as epinephrine (Epi), norepinephrine (NE), and isoproterenol (ISO), produce cellular effects through the adrenergic receptors expressed in a variety of tissues, including blood vessels, heart, adipose tissue, and skeletal muscle
There is abundant experimental evidence that proinflammatory cytokines released by innate immune cells and skeletal muscle cells in response to various stresses [15], such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in plasma levels which are often elevated in septic patients [16, 17], are involved in muscle wasting [18]
In view of the report that LPS induces C2C12 myotube wasting via activation of ubiquitin proteasomal pathway (UPP) [10], we analyzed the expression of Atrogin-1 and MuRF1, essential regulators of UPP in skeletal muscle, with Quantitative reverse transcription polymerase chain reaction (qRT-PCR)
Summary
Catecholamines, such as epinephrine (Epi), norepinephrine (NE), and isoproterenol (ISO), produce cellular effects through the adrenergic receptors expressed in a variety of tissues, including blood vessels, heart, adipose tissue, and skeletal muscle. A recent clinical study reported that catecholamine may be one of the risk factors in the development of muscle wasting [5], which constitutes a serious clinical consequence associated with critical illness that results in significant morbidity and mortality [6, 7]. It remains to be clarified whether catecholamines can affect skeletal muscle mass in systemic inflammation that constitutes a common problem in critically ill patients. There is abundant experimental evidence that proinflammatory cytokines released by innate immune cells and skeletal muscle cells in response to various stresses [15], such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in plasma levels which are often elevated in septic patients [16, 17], are involved in muscle wasting [18]
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