MedicalResearch.com Interview with:
Prof. Daniel Klessig
Professor at Boyce Thompson Institute
and Cornell University
Medical Research: What is the background for this study?
Acetyl salicylic acid, commonly called aspirin, has been the most widely used drug worldwide for more than a century. Currently, 80 million pounds of aspirin are produced worldwide every year and almost 30 billion tablets are consumed annually in the US alone. Long before German pharmacologist Johann Buchner identified the salicylic acid derivative salicin in 1828 as the ingredient in willow bark that is responsible for its therapeutic effects, different cultures throughout the world were, and many still are, using a variety of plants rich in salicylic acid derivatives, such as willow, wintergreen, and meadowsweet, to treat pain, fever, swelling, and other maladies. Aspirin also is used to reduce the risk of heart attack, stroke, and certain cancers.
One might expect that aspirin’s mechanisms of action would be well understood, given its extraordinarily widespread use and the fact that it was first synthesized by the Bayer chemist Felix Hoffmann over 100 years ago. The prevailing view in the biochemical community has been that aspirin works primarily, if not exclusively, by irreversibly inhibiting the enzymatic activities of cyclooxygenases 1 and 2 (COX1 and COX2), thereby disrupting the synthesis of inflammation-inducing prostaglandins. However, this assumption ignores two important facts
First, aspirin is rapidly converted to salicylic acid (SA) in the body. Indeed, almost all aspirin is metabolized to SA within an hour after ingestion.
Second, SA and many of its natural plant derivatives are rather poor inhibitors of COX1 and COX2 as compared to aspirin, yet SA and aspirin have nearly the same beneficial pharmacological effects. Thus, there must be additional targets through which aspirin/SA exerts its many effects.
Over the past two decades, a number of proteins whose activities are altered by aspirin/SA have been identified; however, their relevance as aspirin/SA targets has been called into question due to the very high, non-physiological levels of aspirin/SA required to alter their activities.
In light of our unexpected discovery that SA mediates its physiological effects in plants via many targets, and given that SA is a key hormone produced by all plants, we hypothesized that there might be multiple targets through which SA acts in animals, regardless of whether it is obtained in low to moderate levels via the diet or in moderate to high doses through herbal-based medicines or aspirin usage.
Medical Research: What are the main findings?
To investigate whether aspirin/SA exerts its pharmacological activities by targeting proteins besides the cyclooxygenases in humans, we used high-throughput approaches developed to identify proteins that mediate SA signaling in plant immunity. This strategy identified several proteins that bind SA and as a result they exhibit altered activity, including High Mobility Group Box1 (HMGB1). In the body, HMGB1 is normally found inside the cell’s nucleus where it helps package the DNA so that it fits in the nucleus. However, HMGB1 also can be released outside of cells following tissue injury or secretion by certain immune or cancer cells. Once in this extracellular location, HMGB1 triggers inflammation by recruiting immune cells involved in fighting infection and repairing damaged tissue. HMGB1 also stimulates these recruited immune cells to express genes that code for pro-inflammatory signaling proteins called cytokines. These pro-inflammatory activities of HMGB1 are associated with many prevalent and devastating diseases, including rheumatoid arthritis, lupus, heart disease, sepsis, and colorectal and mesothelioma cancers.
In collaboration with Marco Bianchi’s group at San Raffaele University and Research Institute in Milan, Italy and Gaetano Montelione’s group at Rutgers University in New Jersey, we have discovered that SA binds to HMGB1, thereby blocking its pro-inflammatory activities. It does so at concentrations that are far lower than those required to suppress the enzymatic activity of COX1 and COX2. Notably, we have discovered that HMGB1 also induces the expression of the gene encoding COX2, and that low levels of SA suppress this induction. Thus, SA does act, in part, through its effect on cyclooxygenase, but it does so by inhibiting the production
rather than the activity of this enzyme. The discovery that HMGB1’s various pro-inflammatory activities are inhibited by low levels of SA provides at least one likely explanation for the protective effects of low-dose aspirin usage.
Importantly, we also have identified several natural and synthetic derivatives of SA that bind to HMGB1 more tightly than aspirin/SA and inhibit its pro-inflammatory activities much more effectively (40 -1000 fold). Interestingly, these natural derivatives are produced by an herb used in traditional Chinese medicine, while the synthetic derivative was designed based on both the structure of the herbal derivative and our studies of the binding activities of other SA-like compounds with HMGB1 and other SA-binding proteins from plants and humans.
In conclusion, the identification of HMGB1 as a novel pharmacological target of SA/aspirin provides new insights into the mechanisms of action through which this widely used drug reduces inflammation and inflammation-associated diseases. Moreover, the existence of natural and synthetic SA derivatives that are even more potent than aspirin/SA at suppressing HMGB1’s pro-inflammatory activities argues that there is tremendous potential for developing SA-based drugs that retain all of the beneficial properties of SA but lack its deleterious side effects.