Mechanisms by which hydrogen sulfide attenuates muscle function following ischemia–reperfusion injury: effects on Akt signaling, mitochondrial function, and apoptosis

摘要

BackgroundIschemic-reperfusion injury (IR) occurs when there is a restriction of blood flow to tissue, followed by massive reperfusion caused by sudden blood flow to the affected area. Deprived of oxygen cells rely on anaerobic metabolism during IR, resulting in decreases in pH, followed by reduction of available ATP and calcium overload in cells. This is accompanied by opening of the mitochondrial permeability transition pore (mPTP), disrupting mitochondrial membrane potential and electron transport chain [1]. Lack of oxygen can also lead to capillary dysfunction and breakdown of cell membranes, contributing to tissue necrosis [1,2,3]. IR can affect many tissues, including brain, intestine, kidney, heart, and skeletal muscle. It is also associated with impaired healing of chronic wounds, organ transplant complications, and tourniquet application [3,4,5]. IR can be a result of different types of injuries that include compartment syndrome, crush injuries, and vascular injuries [3]. In addition to loss of blood flow and nutrients to affected tissues IR is exasperated by increased inflammation and reactive oxygen species (ROS) release, which cause further damage to cells and can initiate apoptosis by mPTP opening and caspase activation [3, 6, 7].Muscle, particularly skeletal muscle, is one of the primary tissues affected by IR, which is marked by changes in microvasculature, muscle volume, loss of function, and increased inflammation [3, 8, 9]. Different tissues have specific critical times before onset of serious injury; for muscle this is approximately 4?h [8]. Beyond this time unrepairable tissue necrosis and tissue loss occurs due to mitochondrial loss and apoptotic activation, which can necessitate amputation of the affected limb [10,11,12]. Different types of muscle display differing response to ischemia based on their mitochondrial content. Highly oxidative muscles such as the soleus displayed less severe damage in response to IR than glycolytic muscles such as the gastrocnemius, likely due to increased anti-oxidant presence in oxidative muscles [9]. Additionally, IR can affect organs beyond the affected limb by increases of inflammatory cytokines. For example, kidney and heart cells are extremely vulnerable to restrictions of blood flow, and introduction of free radical scavengers can improve total body function in ischemic animal models by reduction of inflammatory cytokines such as interleukins (IL) and tumor necrosis factor alpha (TNFα) [13,14,15,16,17,18,19].Hydrogen sulfide (H2S) is a gasotransmitter, along with nitric oxide (NO) and carbon monoxide (CO) that initiates a variety of signaling pathways within cells. Hydrogen sulfide has traditionally been thought of as a poisonous gas emitting a rotten egg smell, but recent evidence suggests that in micromolar amounts H2S can alter various signaling pathways involved in vasodilation, metabolism, apoptosis, and mitochondrial electron transport chain (ETC) [20,21,22,23]. In addition to environmental H2S that is absorbed across cell membranes via diffusion cells are also able to produce small amounts of endogenous H2S by reverse transsulfuration of dietary L-homocysteine [24]. This process is mainly carried out by the cytosolic enzymes cystathionine β-synthase (CBS; mostly found in nerves) and cystathionine γ-lyase (CSE; mostly found in muscle), which utilize cystathione to convert homocysteine to cysteine, with H2S as a by-product [24, 25]. Additionally, H2S can also be generated by mitochondrial mercaptopyruvate sulphur transferase (3-MST), which utilizes mercaptopyruvate to form a persulfide intermediate by cysteine transanimation of α-ketoglutarate and l-cysteine. Presence of a reducing agent such as thioredoxin then releases H2S and pyruvate [25, 26]. Once released from cells H2S has a short half-life of up to 12?min in vivo (in contrast to aerosol half-life of up to 37?h), making continuous endogenous production of H2S critical to its activity [27, 28]. Interestingly, it has been shown that the three major endogenous hydrogen sulfide producing enzymes (CBS, CSE, 3-MST), as well as total hydrogen sulfide are reduced in muscle and kidney following ischemia, which can be attenuated through introduction of H2S donors [29,30,31], suggesting that H2S can be useful in reducing IR complications. Once released H2S can modify proteins by sulfurhydration to augment preservation by cryoprotection, alter ion channel activity (K+, Ca2+, KATP), regulate apoptosis by affecting Akt (also known as protein kinase B) and phosphoinosiol kinase (PI3K)-mammalian target of rapamycin (mTOR), reduce inflammation, act as a free radical scavenger, and alter mitochondrial electron transport chain activity by alteration of KATP pore formation and regulation of cyclic AMP (cAMP) activity [21, 24, 26, 32,33,34]. H2S uses the KATP pump as a second messenger system, and is also proposed to cross-talk with the other gasotransmitters by regulation of endothelial nitric oxide synth