Buck Institute researchers have discovered a novel biomarker, detectable in blood and urine, which could form the basis of a noninvasive test to measure and track the performance of senolytics, a class of drugs that selectively eliminate senescent cells. The oxylipin biomarker, 15-deoxy-delta-12, 14-prostaglandin J2 (dihomo-15d-PGJ2), is a signaling lipid metabolite oxylipin molecule that normally accumulates inside cells, but is released when senescent cells die, and can be detected in urine and blood. The results of the team’s newly reported studies in cultured cells and in mice could aid the development of senolytic drugs against a range of chronic age-related conditions, such as arthritis, lung disease, Alzheimer’s disease, and glaucoma. And with a growing list of such treatments in development, detecting the dihomo-15d-PGJ2 oxylipin via a companion diagnostic could verify the performance of senolytic candidates.

“The list of age-related diseases definitively linked to cellular senescence keeps growing, as does the number of biotech companies racing to develop drugs to eliminate senescent cells,” said Judith Campisi, PhD, professor at the Buck Institute and the study’s senior scientist. “While the field has never been more promising, the lack of a simple biomarker to measure and track efficacy of these treatments has been a hindrance to progress. We are excited to bring this new biomarker to the field and look forward to it being used in the clinic.”

Campisi and colleagues reported on their findings in Cell Metabolism, in a paper titled, “Oxylipin biosynthesis reinforces cellular senescence and allows detection of senolysis.”

During cellular senescence, stressed or damaged cells permanently stop dividing, a putative mechanism to safeguard against cancer. Senescent cells are not dead, however, and they release a stew of bioactive molecules that promote wound healing and chronic inflammation, the latter playing a major role in many age-related diseases as the cells accumulate over time. This bioactive stew is known as the senescence-associated secretory phenotype (SASP), and its protein composition, and deleterious effects have been well studied. “This senescence-associated secretory phenotype has been characterized largely for secreted proteins that participate in embryogenesis, wound healing, inflammation, and many age-related pathologies,” the authors wrote. In contrast, the lipid components of SASP have been less well studied.

Through their newly reported studies, the Buck Institute team showed that senescent cells also synthesize a large array of oxylipins, which are bioactive metabolites derived from the oxygenation of polyunsaturated fatty acids. “Lipid components of the SASP have been vastly understudied,” said lead scientist Christopher Wiley, PhD, a former assistant research professor at the Buck, now at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. Yet, as the researchers noted, “Oxylipins have diverse physiological effects, including inflammation, fever, vasoconstriction and vasodilation, pain, hair loss, asthma, and fibrosis.”

Wiley and Campisi had previously shown that senescence and SASP are associated with significant changes in metabolism, including lipid metabolism. Fatty acid metabolism is changed in such a way that free polyunsaturated fatty acids accumulate inside the arrested cells, where they are used to manufacture oxylipins. The team’s lipid profiling studies in cultured senescent cells highlighted some of these changes. “ … certain subsets of lipids significantly increased or decreased upon senescence,” they wrote. “Most notable were striking elevations in the relative abundance of oxylipins: a class of potent signaling lipids derived from 20-and 22-carbon fatty acids such as arachidonic (AA) and adrenic acid (AdA).” Wiley further explained, “The biosynthesis of these signaling lipids promotes segments of the SASP and reinforces the permanent growth arrest of senescent cells.

The researchers identified one of these fatty acids, dihomo-15d-PGJ2, as unique to senescent cells. They found that this oxylipin accumulates inside senescent cells, and is released when the cells die. “Notably, senescent cells synthesize and accumulate an unstudied intracellular prostaglandin, 1a,1b-dihomo-15-deoxy-delta-12,14-prostaglandin,” they wrote. A series of in vivo studies was then carried out in mice that were given chemotherapy—which induces widespread senescence—followed by a senolytic drug. The results confirmed that the dihomo-15d-PGJ2 biomarker was only detected in the blood and urine of animals treated with both chemotherapy and the senolytic drug, and not with either treatment on its own, confirming specificity for senolysis.

Oxylipin biosynthesis reinforces cellular senescence and allows detection of senolysi. [Christopher Wiley, PhD]

The researchers also showed that dihomo-15d-PGJ2 had a functional role in senescence. Inhibiting its synthesis allowed a subset of cells to escape senescence and continue dividing, and demonstrate a less inflammatory SASP profile. Conversely, addition of dihomo-15d-PGJ2 to non-senescent cells drove them into senescence by activating RAS, a cancer-promoting gene that is also known to cause senescence. “Given that prostaglandin synthesis is required for senescence, the data indicate that prostaglandin-mediated RAS activation is a common and necessary feature of at least some forms of cellular senescence,” the team concluded.

“ … our finding that released dihomo-15d-PGJ2 can be used as a biomarker for senolysis has several potential applications,” the team concluded. “Senolytic drugs are increasingly being used in aging and related research and—importantly—have entered early clinical trials. Determining that senolysis is taking place is essential for evaluating these compounds as therapeutic agents. Detection of dihomo-15d-PGJ2 in biological fluids may allow rapid evaluation of the efficacy of these compounds.”

Campisi commented, “We hope that identifying and including these bioactive lipids as part of the SASP will encourage researchers working in a broad range of fields to take a new look at cellular senescence. The fact that one of these lipids ends up being a simple non-invasive biomarker for tracking the efficacy of treatments is a huge plus for those of us working to stem the ravages of age-related disease.”

Wiley added, “This work provides a new way of understanding and studying senescence-driven pathology,” he said. Oxylipins are implicated in many inflammatory conditions including cardiovascular disease and pain response. Many commonly used drugs, such as aspirin and ibuprofen, act by preventing oxylipin synthesis.