Discovery and investigation of a novel role for mitochondrial prohibitin in mitigating acute heart failure in endotoxic shock
Mattox, Taylor A.
Sepsis results in more than 200,000 deaths annually and is the 10th leading cause of death in the United States (US). In spite of significant advances in medical care the mortality rate for sepsis has continued to rise. Sepsis and the diseases along its continuum (septic shock and multiple organ dysfunction syndrome, MODS) are characterized by excessive production of inflammatory mediators via a feed-forward mechanism that results in a condition commonly referred to as the `cytokine storm.' This maladaptive inflammatory response, and the subsequent mitochondrial dysfunction that results from it, are thought to underlie the impairment of cardiac function that occurs in the progression from sepsis to MODS. Prohibitin (PHB) is a ubiquitously expressed mitochondrial localized protein, which recent evidence has suggested has a wide variety of roles from transcriptional regulator to a mediator of inflammatory and oxidative signaling. Here, using a comprehensive series of in vitro and in vivo experiments, we tested the hypothesis that PHB confers cardiac protection during endotoxic shock (i.e., cytokine storm induced by LPS) through 1) preservation of mitochondrial integrity 2) attenuated inflammatory signaling, primarily through NFêB and 3) augmented Nrf2 signaling and antioxidant capacity. As expected, LPS and cytokines disrupted mitochondrial function and integrity in both our models. In rats, LPS injection reduced PHB expression in whole heart while simultaneously concentrating the remainder in the nucleus. Interestingly, serum levels of PHB were transiently elevated 3-fold by LPS, but were restored to normal levels within 24 hours. Similarly, whole cell PHB expression was reduced and nuclear accumulation of PHB was increased in cardiomyocytes following TNFá/IL1â treatment in vitro. Overexpression of PHB (oPHB) and treatment with recombinant PHB (rPHB) protected cardiomyocytes from TNFá/IL1â-induced toxicity by preserving mitochondrial function, suppressing oxidative stress and ultimately mitigating cytokine-induced cytotoxicity. We further observed that oPHB and rPHB attenuated the TNFá/IL1â-induced transcription of pro-inflammatory genes in cardiomyocytes, while augmenting Nrf2 nuclear transactivation and up-regulation of Nrf2-mediated genes. Next, using wild-type (WT) and Nrf2-/- mice we tested the hypothesis that cytokine generation and inflammatory signaling induced by LPS challenge would be suppressed by rPHB treatment in a Nrf2-dependent manner, leading to protection of cardiac mitochondria and recovery of cardiac function. Following LPS challenge, endogenous levels of PHB transiently increased dramatically in WT but not Nrf2-/- mice. Treatment with rPHB following LPS challenge suppressed circulating levels of IL6 and TNFá in WT and Nrf2-/- mice, resulting in decreased NFêB and STAT3 activation in heart and complete attenuation of proinflammatory cytokines and iNOS in this organ. Additionally, rPHB treatment reversed the LPS-induced decrease in mitochondrial ATP generation in heart, simultaneously leading to rapid recovery of cardiac function within ~12 hours. Interestingly, rPHB treatment mitigated the LPS-induced inflammatory response and cardiac dysfunction to similar extent in both WT and Nrf2-/- mice, suggesting the effects of rPHB are independent of Nrf2. Collectively, these findings suggest that PHB is a mitochondrial inner-membrane protein that acts as a mobile signal transducer, capable of moving from mitochondria to nucleus and also between cell and tissue compartments, to suppress inflammation and cytotoxicity during severe inflammatory stress. They further suggest that PHB may be critical to alleviating the maladapative host response to infection and represents a novel therapeutic target for the treatment of sepsis-associated cardiac dysfunction. Future studies directed at exploiting the pleiotropic functions of PHB to mitigate inflammation and oxidative stress in other cardio-metabolic disease models will be important to building on the knowledge that the present studies have established.
Mattox, Taylor A.. (January 2014). Discovery and investigation of a novel role for mitochondrial prohibitin in mitigating acute heart failure in endotoxic shock (Doctoral Dissertation, East Carolina University). Retrieved from the Scholarship. (http://hdl.handle.net/10342/4428.)
Mattox, Taylor A.. Discovery and investigation of a novel role for mitochondrial prohibitin in mitigating acute heart failure in endotoxic shock. Doctoral Dissertation. East Carolina University, January 2014. The Scholarship. http://hdl.handle.net/10342/4428. July 07, 2020.
Mattox, Taylor A., “Discovery and investigation of a novel role for mitochondrial prohibitin in mitigating acute heart failure in endotoxic shock” (Doctoral Dissertation., East Carolina University, January 2014).
Mattox, Taylor A.. Discovery and investigation of a novel role for mitochondrial prohibitin in mitigating acute heart failure in endotoxic shock [Doctoral Dissertation]. Greenville, NC: East Carolina University; January 2014.
East Carolina University