Which Fat You Eat May Steer Immune Cells That Drive Liver Scarring

Not all fats act alike inside the immune system, and a new study suggests that the specific species of fatty acid circulating in our blood can quietly reprogram the inflammatory cells that help drive one of the world’s fastest-growing liver diseases. Researchers reporting in Physiological Reports have shown that palmitic acid, oleic acid, and palmitoleic acid, three of the most abundant fatty acids in human plasma, exert strikingly different effects on macrophage polarization and oxidative stress, with downstream consequences for how these immune cells communicate with hepatic stellate cells, the principal architects of liver fibrosis. The findings offer a molecular window into why diets rich in certain fats may accelerate the progression from simple fatty liver to metabolic dysfunction-associated steatohepatitis, or MASH, a condition that affects millions of people worldwide and can progress to cirrhosis and liver cancer.

MASH develops when excessive lipid accumulation in the liver triggers chronic inflammation, hepatocellular injury, and ultimately fibrosis, the scarring process that replaces functional tissue with rigid extracellular matrix. In Japan alone, several million individuals are estimated to be affected, and disease progression is tightly linked to obesity, diabetes, and dyslipidemia. When adipose tissue becomes inflamed, it floods the circulation with free fatty acids and pro-inflammatory cytokines, amplifying the hepatic inflammatory milieu. Yet while the broad role of lipid overload in liver disease is well established, the mechanisms by which individual fatty acid species contribute to fibrogenesis have remained frustratingly opaque.

Central to the new work is the concept of macrophage polarization, the remarkable plasticity of innate immune cells that allows them to shift between functionally distinct states. Resting M0 macrophages can polarize toward pro-inflammatory M1 phenotypes, driven by stimuli such as lipopolysaccharide and interferon-gamma and characterized by production of tumor necrosis factor-alpha and inducible nitric oxide synthase, or toward anti-inflammatory M2 phenotypes induced by interleukin-4 and interleukin-13, marked by expression of peroxisome proliferator-activated receptor gamma and other mediators of tissue repair and fibrosis. Because dysregulated macrophage polarization is a key determinant of chronic inflammatory diseases including MASH, the research team led by investigators at Osaka Metropolitan University set out to determine whether specific fatty acids could bias this process in cultured cells.

Using bone marrow-derived macrophages isolated from male Wistar rats, the researchers exposed cells to albumin-conjugated palmitic acid, oleic acid, or palmitoleic acid at non-cytotoxic concentrations of 0.1 and 0.2 millimolar while simultaneously polarizing them toward M1 or M2 states. The results were strikingly fatty acid-specific. Palmitic acid, the saturated species that accounts for roughly 31 percent of plasma free fatty acids, significantly increased Tnf mRNA and boosted both iNOS and TNF-alpha protein expression in M1 macrophages, effectively reinforcing their inflammatory profile. It also consistently suppressed PPARgamma, a master transcriptional regulator of alternative activation, in M2 macrophages. Because PPARgamma negatively constrains NF-kappaB-mediated inflammatory signaling, its suppression may tilt macrophages toward a more pro-inflammatory identity.

The unsaturated fatty acids told a different story. Oleic acid, which makes up approximately 27 percent of plasma free fatty acids, increased Nos2 and Tnf mRNA in resting and M2 macrophages, but these transcriptional changes never materialized into detectable protein, and their magnitude was trivial compared with genuine M1 induction. Nevertheless, oleic acid proved far from benign: it significantly enhanced reactive oxygen species production in M1 macrophages and reduced several M2-associated markers, including PPARgamma and Ym1. This context dependence is notable given previous reports of oleic acid’s anti-inflammatory effects mediated through the free fatty acid receptor FFAR4 and adiponectin-driven AMPK activation. Palmitoleic acid, the monounsaturated omega-7 species, displayed its own signature: it dampened iNOS protein expression and significantly suppressed ROS production in M1 macrophages, consistent with prior evidence that it can antagonize palmitate-induced inflammation through AMPK and TLR4-related mechanisms, yet it too reduced selected M2 markers.

Oxidative stress emerged as a central theme. Using the chemiluminescent probe L-012, the team measured superoxide generation in polarized macrophages and found that ROS production was markedly higher in M1 cells than in M0 or M2 counterparts, confirming that the inflammatory program is intrinsically linked to redox activity. Critically, fatty acid treatment had minimal effects on ROS in resting and M2 macrophages, but palmitic acid and oleic acid each significantly increased superoxide production in M1 macrophages, while palmitoleic acid significantly decreased it. Since the majority of detected ROS was superoxide, likely originating from mitochondria and NADPH oxidase complexes, the data suggest that palmitate-induced oxidative stress in inflammatory macrophages may involve NOX-dependent mechanisms, a hypothesis the authors note requires direct testing of TLR4 and NF-kappaB pathways in future work.

The researchers then turned to the other half of the fibrotic dialogue: hepatic stellate cells, which reside quiescently in the space of Disse storing vitamin A-laden lipid droplets until chronic injury drives them to activate, express alpha-smooth muscle actin, and deposit type I collagen. In a first series of experiments, direct treatment of primary rat stellate cells with any of the three fatty acids failed to alter alpha-SMA, cytoglobin, TGF-beta, or Col1a1 expression at either the mRNA or protein level, indicating that none of the tested lipids directly activates stellate cells under these conditions. This negative result is itself meaningful, redirecting attention from direct lipotoxicity toward immune-mediated indirect pathways.

When stellate cells were instead cultured in conditioned media harvested from fatty acid-treated polarized macrophages, the picture shifted. The dominant determinant of stellate cell marker expression was the macrophages’ polarization status rather than the fatty acid they had received. Conditioned media from M1 macrophages reduced both alpha-SMA and cytoglobin expression in stellate cells compared with media from resting M0 cells, and fatty acid-specific effects were modest and marker-dependent. Cytoglobin deserves particular attention: this antioxidant protein protects stellate cells by scavenging reactive oxygen species and maintaining redox homeostasis, and its reduction, as documented in previous studies of NASH, can weaken antioxidant defenses and heighten susceptibility to oxidative DNA damage. The authors therefore interpret their data as evidence that fatty acid-dependent macrophage reprogramming can secondarily modify stellate cell redox-related responses, without claiming that fatty acids directly drive stellate cell activation.

The study is not without limitations, which the authors candidly enumerate. TLR4-dependent signaling was not directly assessed, ROS measurements relied primarily on L-012 chemiluminescence rather than complementary probes, the entire system was in vitro and awaits validation in animal models of MASH, stellate cell activation was evaluated by marker expression rather than functional assays of proliferation, migration, and contractility, and some experiments employed relatively small numbers of biological replicates. The fatty acid concentrations chosen, 0.2 millimolar, were selected after preliminary cytotoxicity testing and sit below concentrations commonly used in the field, though local concentrations within the hepatic microenvironment may differ from systemic levels.

Even with these caveats, the work delivers a provocative message: lipid composition, not merely lipid quantity, is a meaningful determinant of immune-stromal interactions in the liver. By showing that a saturated fatty acid pushes macrophages toward inflammatory, ROS-generating phenotypes while a monounsaturated omega-7 species dampens them, and that these shifts ripple outward to alter antioxidant signaling in the cells that build scar tissue, the study sketches a plausible mechanism linking dietary fat quality to fibrogenic progression in metabolic liver disease. If confirmed in vivo, the findings could open new avenues for intervening in MASH not simply by reducing fat intake, but by reshaping the fatty acid milieu that instructs the immune system how to respond to it.

Subject of Research: Fatty acid species-dependent regulation of macrophage polarization, oxidative stress, and macrophage-hepatic stellate cell crosstalk in liver fibrosis.

Article Title: Fatty acid species differentially regulate macrophage polarization and oxidative stress with secondary effects on macrophage–HSC crosstalk

Article References: Nakanishi, K., Shinkawa, H., Takemura, S., Nakagawa, K., Minamiyama, Y., & Ishizawa, T. (2026). Fatty acid species differentially regulate macrophage polarization and oxidative stress with secondary effects on macrophage– HSC crosstalk. Physiological Reports, 14(17), Article e71082. https://doi.org/10.14814/phy2.71082

Image Credits: AI Generated

DOI: 10.14814/phy2.71082

Keywords: macrophage polarization, fatty acids, palmitic acid, oleic acid, palmitoleic acid, hepatic stellate cells, liver fibrosis, MASH, oxidative stress, PPARgamma, cytoglobin, TLR4