Ph.D., State University of New York at Buffalo, 2003
Alcohol-induced liver injury, inflammation and innate immunity, transcriptional control of inflammatory gene expression, wound-healing response, hepatic fibrosis, hepatic stellate cell function
Prolonged alcohol abuse promotes a step-wise progression of increasingly more severe liver injury; this progression begins with lipid accumulation (steatosis) in hepatocytes. Twenty percent of steatotic patients will progress to hepatitis and of those, 50% will further progress to fibrosis, cirrhosis and, in some cases, hepatocellular carcinoma. Despite the many advances in our understanding of the mechanisms which promote fibrosis, efficacious therapies remain elusive. Indeed, the only cure for advanced stages of liver disease is liver transplantation. Due to the burgeoning population of patients requiring liver transplantation in the Western population, organ availability is becoming increasingly limited. Therefore, there is an urgent need to identity novel approaches to halt the progression of alcoholic liver disease and facilitate hepatic healing to reduce the requirement for organ transplantation.
We have targeted our efforts on hepatic fibrosis as it is viewed as the last reversible stage of liver disease. Once patients progress to cirrhosis, the liver can no longer repair itself sufficiently to reinstate normal liver function; this necessities liver transplant. The overall goal of our research program is to discover new mechanisms which contribute to ethanol-induced hepatic fibrogenesis from which we can design novel therapeutic strategies. We have two projects which approach this problem from different directions:
Liver fibrosis and ethanol: Role of the transcription factor, Egr-1
Early growth response (Egr)-1 is a redox-sensitive transcription factor that regulates a broad array of genes involved in the inflammatory, anti-oxidant and wound-healing responses. Egr-1 is a positive regulator of ethanol-induced fatty liver injury and acute hepatic inflammation in mice. Indeed, mice that are deficient in Egr-1 have reduced liver injury in these models associated with reduced hepatic inflammatory signatures. Paradoxically, in a mouse model of carbon tetrachloride-induced liver injury and fibrosis, Egr-1 is protective. Collectively, these data illustrate the importance of differential gene regulation in the hepatic injury vs wound healing responses.
The reactive byproducts of ethanol metabolism cause oxidative injury to the liver. Because we cannot address the effect of these ethanol metabolites on liver injury and repair processes using carbon tetrachloride exposure alone, we have recently implemented a new mouse model in which we combine moderate ethanol feeding to mice with carbon tetrachloride exposure. Indeed, moderate ethanol exacerbates carbon tetrachloride-induced profibrotic changes in the liver in wild-type mice in the absence of increased liver injury. Interestingly, this effect is worsened if the mice are deficient in Egr-1 and is associated with reduced expression of hepatic anti-oxidant genes. Using a combination of genetic and therapeutic approaches, we are testing the hypothesis that ethanol and/or ethanol metabolites exacerbate carbon tetrachloride-induced hepatic fibrogenesis through reduced, Egr-1 dependent, antioxidant defenses.
Hyaluronan and hepatic fibrosis
Tissue injury results in the release of normally sequestered, intracellular molecules to the extracellular environment as well as the processing of large extracellular matrix molecules into smaller matrix fragments. These molecules promote sterile, i.e. in the absence of pathogen invasion, inflammatory responses. Considerable attention is given these 'danger-associated molecular patterns' as they signal through many of the same receptors that pathogen associated molecular patterns utilize to induce inflammatory responses and appear to play a large role in many animal models of disease.
Hyaluronan (HA), an extracellular matrix glycosaminoglycan produced by activated fibroblasts, is increased in the plasma of patients with liver disease; HA plasma concentration directly correlates with the severity of liver disease and is greater in plasma from patients with alcoholic liver disease compared to patients with non-alcoholic liver disease. These data suggest that ethanol, or products of ethanol metabolism, have unique pro-fibrotic properties not present in liver disease of other etiologies. While low molecular weight HA fragments produced through HA metabolism in mouse models of lung disease promote pulmonary inflammation and fibrosis, the role that low molecular weight HA fragments play in hepatic fibrogenesis is unknown. In our current studies, we are investigating the potential roles for HA in sterile inflammation and resolution of inflammation in the liver using a model of ethanol-exacerbated, carbon tetrachloride-induced fibrosis. Specifically, we are testing the hypothesis that ethanol exposure enhances carbon tetrachloride-induced fibrogenesis and activation of hepatic stellate cells, the cell type predominantly responsible for hepatic fibrosis. Further, we predict that these effects are due, at least in part, to ethanol-dependent increases in HA production, its selective metabolism to low molecular weight fragments and signaling events downstream of receptor ligation by those low molecular weight HA fragments. Conversely, we hypothesize that maintenance or induction of high molecular weight HA will act as an 'all clear' signal, promote resolution of inflammation and attenuate fibrogenesis.
In summary, it is the goal of these two projects to elucidate novel mechanisms which contribute to alcohol-induced hepatic fibrosis and use those discoveries to design new therapeutic approaches to halt the progression and accelerate reversal of hepatic fibrosis.
(* indicates manuscripts for which I am corresponding author.)
* M.T. Pritchard, R.N. Malinak and L.E. Nagy: Early growth response (Egr)-1 is required for timely cell cycle entry and progression in hepatocytes after acute carbon tetrachloride exposure in mice. Am. J. Physiol.- Gastr. Liver Physiol., 300(6):G1124-31, 2011.
* M.T. Pritchard, J.I Cohen, S. Roychowdhury, B.T. Pratt and L.E. Nagy. Egr-1promotes hepatoprotection and attenuates carbon tetrachloride-induced liver injury in mice. J Hepatol. 53(4):655-662, 2010.
*M.T. Pritchard and L.E. Nagy. Hepatic fibrosis is enhanced and accompanied by robust oval cell activation in Egr-1-deficient mice after chronic carbon tetrachloride administration. Am J Pathol, 176(6): 2743 - 2752, 2010.
S. Roychowdhury, M.R. McMullen, M.T. Pritchard, W. Lei, R.G. Solomon and L.E. Nagy. Formation of -ketoaldehyde-protein adducts during ethanol-induced liver injury in mice. Free Rad. Biol. Med. 47:1526-1538, 2009. PMCID: PMC2783279
S. Roychowdhury, M.R. McMullen, M.T. Pritchard, M.E. Medof, A.B. Stavitsky and L.E. Nagy. An early complement dependent and TLR4 independent phase in the pathogenesis of ethanol-induced liver injury. Hepatology, 49:1326-1334, 2009. PMCID: PMC2666108
M.T. Pritchard, M.R. McMullen, M.E. Medof, A.B. Stavitsky and L.E. Nagy. Role of complement in ethanol-induced liver injury. Invited book chapter in Current Topics on Complement, Volume II, John D. Lambris, Ph.D. Editor. Adv in Exp Med Biol., 632:175-186, 2008.
* M.T. Pritchard, S. Roychowdhury, M.R. McMullen, L. Guo, G.E. Arteel and L.E. Nagy. Early growth response-1 contributes to galactosamine/lipopolysaccharide-induced acute liver injury in mice. Am. J. Physiol.- Gastr. Liver Biol., 293:G1124-G1133, 2007.
M.T. Pritchard, M.R. McMullen, A.B. Stavitsky, J.I. Cohen, F. Lin, M.E. Medof, L.E. Nagy. Differential contributions of C3, C5 and decay accelerating factor to ethanol-induced fatty liver in mice. Gastroenterology, 132(3):1117-1126, 2007. PMCID: PMC1838572
M.T. Pritchard and L. E. Nagy. Ethanol-induced liver injury: potential roles for Egr-1. Invited review. Alcohol. Clin. Exp. Res., 29:146S-150S, 2005.
M.R. McMullen, M.T. Pritchard, Q. Wang and L.E. Nagy. Early growth response-1 transcription factor is essential in the development of ethanol-induced fatty liver injury in mice. Gastroenterology, 128:2066-2076, 2005.