UCLA Scientists Identify 'Zombie Cells' Driving Fatty Liver Disease and Aging—With a Potential Cure

The Discovery of Senescent Macrophages

UCLA researchers have identified a destructive population of immune cells that accumulate in aging tissues and fatty livers, triggering chronic inflammation and organ damage. Published in *Nature Aging*, the study focuses on cellular senescence—a biological state where cells stop dividing but remain metabolically active, continuously releasing inflammatory signals that harm surrounding tissue. These dysfunctional cells, colloquially known as "zombie cells," are particularly abundant in people with fatty liver disease and aging individuals.

"Senescent cells are fairly rare, but think of them like a broken-down car on the 405. Just one stalled car can back up traffic for miles. Now imagine five or ten of them slowly accumulating."

The key innovation was solving a longstanding scientific puzzle: distinguishing truly senescent macrophages from healthy ones. Senior author Anthony Covarrubias and his team identified a precise molecular signature—the combination of proteins p21 and TREM2—that reliably marks dysfunctional macrophages, even though healthy macrophages share some molecular similarities with senescent cells.

The Aging Connection and Cholesterol's Role

The research reveals a dramatic age-related shift in macrophage dysfunction. In young mice, only about 5% of liver macrophages were senescent. By contrast, older mice showed senescence rates between 60-80%—directly correlating with the chronic liver inflammation observed in aging. But age isn't the only culprit: excess cholesterol appears equally culpable. When healthy macrophages were exposed to high levels of LDL cholesterol in laboratory conditions, they entered a senescent state, stopped dividing, and began releasing inflammatory proteins.

"In a chronic state, it's pathological. When you look at fatty liver disease, which is driven by overnutrition and too much cholesterol in the blood, that excess cholesterol appears to be a major driver of the senescent macrophage population."

This discovery has sobering implications for modern diets. High-fat, high-cholesterol eating patterns may be accelerating biological aging by promoting macrophage senescence not just in the liver, but potentially across multiple organs—the brain, heart, and fat tissue—creating a shared mechanism linking poor nutrition to age-related disease.

Reversing Liver Damage by Eliminating Zombie Cells

The therapeutic potential became apparent when researchers treated mice with ABT-263, a drug designed to selectively eliminate senescent cells. The results were striking: even without dietary changes, liver size dropped from roughly 7% of body weight to a healthier 4-5%, while overall body weight declined approximately 25% (from 40 grams to 30 grams). Treated livers appeared visibly healthier—displaying normal coloring instead of the enlarged, yellowish appearance seen in untreated animals with fatty liver disease.

First author Ivan Salladay-Perez expressed astonishment at the magnitude of improvement: the removal of senescent cells didn't merely slow fatty liver progression—it actually reversed the disease. This represents a fundamental shift from managing symptoms to addressing the underlying pathology, achieved without requiring patients to change their eating habits.

When the team examined genomic data from human liver biopsies, they found that the same senescent macrophage signature was significantly elevated in diseased livers compared to healthy tissue, suggesting the findings may translate to human disease. This timing is critical: in Los Angeles, an estimated 30-40% of residents suffer from fatty liver disease, with disproportionately higher rates in Latino communities. As Covarrubias noted, "We're seeing fatty liver disease in younger and younger people," making this a mounting public health emergency.

From Mouse Model to Human Medicine

While ABT-263 proved effective in animals, it's too toxic for widespread human use. The research team is now screening for safer compounds that can selectively target senescent macrophages without causing collateral damage. Beyond fatty liver disease, investigators are exploring whether similar senescent cell accumulation drives other age-related conditions—particularly in the brain, where microglia (the central nervous system's macrophages) may become senescent during Alzheimer's disease as they accumulate cellular debris.

The study supports the geroscience hypothesis: a unified theory proposing that a single underlying aging mechanism drives multiple seemingly unrelated diseases. Understanding how senescent macrophages accumulate could ultimately unlock treatments for atherosclerosis, Alzheimer's, cancer, and other conditions rooted in chronic inflammation. As Salladay-Perez summarized the broader vision: "If you really understand the basic mechanisms driving inflammation with aging, you can target those same mechanisms to treat not just fatty liver disease, but atherosclerosis, Alzheimer's and cancer."