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Volume 18, Number 1, March 2007
Crews Lab Discovers Inflammatory Mechanisms Underlying
Alcohol-Induced Brain Damage
Crews Lab: Jian Zou, PhD, Richard Hanes, Jun He, PhD, Fulton Crews, PhD, Liya Qin, PhD, Tonya Hurst, Olivera Pluzarev, PhD.
“I don’t drink these days. I am allergic to alcohol.....
I break out in handcuffs.”
-Robert Downey, Jr.
While Robert Downey, Jr.’s statement was made in jest, it encapsulates an earnest opinion often expressed by alcoholics: that alcohol provokes an “allergic reaction” characterized by a unique physiological and psychological sensitivity to alcohol’s effects. The idea that an allergic reaction to alcohol underlies alcoholism was first introduced into the medical literature by the New York physician W. D. Silkworth. In his pioneering 1937 paper, “Alcoholism as a Manifestation of Allergy,” Silkworth argued that alcoholism is a physical malady with a development and course analogous in many respects to allergies such as hay fever. His conceptualization of alcoholism as an allergic state ran counter to the prevailing view of alcoholism as a moral failing and continues to resonate among today’s alcoholics and recovering alcoholics, who still widely cite and discuss Silkworth’s work.
In the 1930s, before the advent of neurobiological research on alcohol effects, Silkworth had no means of garnering biological support for his hypothesis of alcoholism as an allergic reaction. Subsequent advances in research technology have made it possible to study biological correlates of alcoholism. Dr. Fulton Crews, alcohol researcher and Director of the University of North Carolina’s Skipper Bowles Center for Alcohol Studies, is at the forefront of efforts to elucidate the neurobiology of alcoholism. Crews says that, while Silkworth’s concept of alcoholism as a classical allergic reaction has not been fully supported, remarkable parallels between allergic responses and the body’s response to alcohol have been revealed. In short, both allergic responses and the body’s response to alcohol involve inflammatory processes in which cytokines play key roles. Cytokines constitute a diverse group of proteins that regulate immune responses, inflammation, hormones, wound healing, and communication among cells throughout the body. Only recently have they been studied in the brain. The action of pro-inflammatory cytokines is crucial for defending the body against infections. Anti-inflammatory cytokines and growth factors also contribute to the diverse actions of this large family of mediators.
Crews’ interest in cytokines was sparked by his Bowles Center colleagues’ research demonstrating that tumor necrosis factor-a (TNFa), a proinflam-matory cytokine released by specific liver cells, contributes to liver disease in alcoholism. Chronic alcohol use causes a leaky gut, with bacteria and ethanol combining to stimulate the liver to make TNFa and other cytokines as part of an inflammatory reaction. Present in excess, TNFa and other cytokines cause cellular toxicity and over time contribute to alcoholic liver disease. Crews, whose primary research focus is alcohol-induced brain degeneration, speculated that cytokines might also contribute to neurodegeneration in alcoholism. Cytokines are produced in the brain by microglia, which are the brain’s immune cells. In a study in rats, Crews and his colleagues discovered that a binge alcohol treatment regimen modeling the drinking pattern of human alcoholics in fact caused microglial activation accompanied by persistent increases in the proinflammatory cytokines TNFa, interleukin-1 beta (IL-1b), and monocyte chemotactic protein-1 (MCP-1). These cytokine changes were accompanied by binge-induced brain damage. These findings are consistent with Crews’ hypothesis that uncontrolled activation of microglia directly releases cytokines that are toxic to neurons.
Knowing that liver cells, like the brain’s microglia, overproduce cytokines when exposed to alcohol, Crews hypothesized that the brain might be also exposed to cytokines originating from the liver in individuals who consume excessive alcohol. Liver-originating cytokines such as TNFa are released into the bloodstream to cause inflammation throughout the body. To explore the hypothesis that peripheral TNFa affects inflammation in brain, Crews and his collaborators used the bacterial endotoxin lipopolysaccharide (LPS) to stimulate TNFa production in mice. They found that elevations in TNFa levels in the liver, serum, and brain coincided shortly after LPS administration. However, whereas TNFa levels declined to normal in liver and in serum within a short period of time, TNFa levels remained elevated in the brain for up to 10 months. In another experiment, Crews and his colleagues systemically administered LPS or TNFa to normal mice and to a group of mice bred to genetically lack the TNFa receptor, a receptor previously shown to transport TNFa from serum to brain. In the normal mice, but not in mice lacking the TNFa receptor, systemic administration of LPS or TNFa was associated with activation of microglia; increased levels of pro-inflammatory cytokines, specifically TNFa, MCP-1, and IL-1b, in the brain; and loss of neurons in the substantia nigra, a brain region known to have lots of microglia. These experiments illustrate how inflammation originating outside the brain can trigger persistent brain inflammation that is associated with brain damage. Elevated TNFa in the periphery induced brain TNFa, activated microglia, and caused delayed neurodegeneration. “These results suggest that when TNFa is elevated in serum, the brain is also in trouble,” says Crews. “One fascinating aspect of this discovery is that serum cytokine levels return to normal, whereas the brain, once primed by serum TNFa, has elevated proinflammatory cytokine levels for long periods, perhaps forever.”
One of the myriad pro-inflammatory actions of TNFa is activation of nuclear factor kB (NF-kB), a transcription factor that regulates genes’ production of proteins by binding to particular sites on DNA. NF-kB is a pro-inflammatory transcription factor that causes genes to produce potentially cell-damaging oxidative enzymes such as nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) and cyclooxygenase-2 (COX2), as well as more TNFa and other proinflammatory cytokines. Crews showed that alcohol increased NF-kB DNA-binding activity in brain. Alcohol, proinflammatory cytokines, NF-kB DNA-binding, and oxidative stress were found to interact to promote neurodegeneration contributing to alcohol-associated brain damage as well as other neurodegenerative conditions including Alzheimer’s disease and Parkinson’s disease. An antioxidant (butylated hydroxytoluene) reduced NF-kB DNA-binding activity and alcohol-induced neurotoxicity both in vitro and in vivo using the binge model of alcohol neurodegeneration. Binge alcohol treatment was associated with microglial activation, increased NF-kB DNA-binding activity, and brain damage. The brain damage was reversed by antioxidant treatment. The results suggest a crucial role of NF-kB in neurotoxicity caused by oxidative stress, including that associated with alcohol, and support the hypothesis that neuroinflammation contributes to alcohol-induced brain damage.
Crews and his colleagues have recently extended their work to humans. In a study with human postmortem brains supplied by the Australia Brain Donor Program, they observed neuroinflammation and increased morphological signs of microglial activation. Furthermore, they found that levels of the proinflammatory cytokine MCP-1 were significantly higher in the brains of alcoholics than the brains of control, moderate-drinking individuals.
Considered in aggregate, Crews’ findings suggest that excessive alcohol consumption sets off a spreading cytokine process (Figure). Alcohol disrupts cytokines both within organs and between organs. For example, alcohol perpetuates inflammation within the brain when it induces NF-kB DNA-binding activity, which activates inflammatory mediators that stimulate further increases in NF-kB DNA-binding activity and trigger various inflammatory cascades. Alcohol perpetuates inflammation between organs as when TNFa produced in the liver affects inflammatory responses in the brain (Figure). Crews found that the liver makes anti-inflammatory cytokines in response to alcohol that in time suppress proinflammatory responses, whereas ethanol suppresses proinflammatory cytokines in brain, an effect that might contribute to the previously observed pro-longed increases in brain TNFa. Although more research needs to be done, it is possible that increased anti-inflammatory cytokines could contribute to the health benefits associated with low levels of drinking.
“Many physiological and psychological aspects of alcoholism could be secondary to the cytokine changes we have discovered,” notes Crews. “Understanding cytokines could provide new diagnostic and therapeutic approaches to many chronic diseases, particularly those known to be influenced by alcohol.”
Figure: Ethanol increases systemic and brain cytokines. Alcohol (ethanol-ETOH) is consumed orally, enters the gut and makes the gut leaky allowing endotoxin (lipopolysaccharide-LPS) to enter the circulation. LPS and ethanol activate the liver to produce cytokines such as TNFa. Ethanol and TNFa enter the brain and increase brain cytokine synthesis. Within brain, microglia, astrocytes and neurons respond by altering gene expression that contributes to alcoholic neurodegeneration.