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Palmitic acid reduces cytosolic potassium and induces IL-1β production in lipopolysaccharide-primed human macrophages

Published online by Cambridge University Press:  27 November 2025

Jyue-Wei Chuang  and
Shao-Chun Lu Open the ORCID record for Shao-Chun Lu [Opens in a new window]
Jyue-Wei Chuang
Affiliation:
Department of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei, Taiwan
Shao-Chun Lu*
Affiliation:
Department of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei, Taiwan
*
Corresponding author: Shao-Chun Lu; Email: lsc@ntu.edu.tw

Abstract

Saturated fatty acids, particularly palmitic acid (PA), promote inflammation and contribute to chronic diseases such as type 2 diabetes and cardiovascular disease. PA induces interleukin-1 beta (IL-1β) production in lipopolysaccharide (LPS)-primed macrophages via NLRP3 inflammasome activation; but the underlying mechanism remains unclear. This study investigates whether PA-induced IL-1β production involves cytosolic potassium (K+) depletion. In LPS-primed macrophages, treatment with PA conjugated to bovine serum albumin (PA-BSA) significantly reduced cytosolic K+ levels and increased IL-1β production 2.4-fold. Stearic acid-BSA produced similar effects, whereas BSA-bound oleic, linoleic and docosahexaenoic acids had minimal impact. Voltage-gated potassium (Kv) channel blockers, 4-aminopyridine and tetraethylammonium chloride, attenuated PA-BSA-induced K+ efflux and IL-1β production in LPS-primed macrophages, implicating Kv channels as key mediators. These findings reveal a novel inflammatory pathway in which PA-BSA promotes IL-1β production via Kv channel-dependent K+ efflux, highlighting a mechanistic link between saturated fatty acid exposure and inflammatory signalling.

Keywords

interleukin-1 beta (IL-1β)macrophagespalmitic acidpotassium effluxsaturated fatty acids

Information

Type
Brief Report
Information
Journal of Nutritional Science , Volume 14 , 2025 , e84
Creative Commons
Creative Common License - CCCreative Common License - BYCreative Common License - NCCreative Common License - ND
This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives licence (https://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided that no alterations are made and the original article is properly cited. The written permission of Cambridge University Press or the rights holder(s) must be obtained prior to any commercial use and/or adaptation of the article.
Copyright
© The Author(s), 2025. Published by Cambridge University Press on behalf of The Nutrition Society

Introduction

Diets rich in saturated fatty acids (SFAs) are well-documented for their adverse health effects compared with those rich in unsaturated fatty acids (UFAs). Among SFAs, palmitic acid (PA) has emerged as a significant contributor to inflammation, a key factor implicated in the pathogenesis of chronic diseases such as insulin resistance, type 2 diabetes, and cardiovascular diseases.(Reference Ralston, Lyons and Kennedy1) One prominent inflammatory effect of PA is its activation of the NLRP3 inflammasome, which converts pro-interleukin-1 beta (pro-IL-1β) into active IL-1β in lipopolysaccharide (LPS)-primed macrophages, where nuclear factor kappa B (NF-κB) signalling is activated.(Reference Wen, Gris and Lei2) Recent studies have identified multiple mechanisms by which PA activates the NLRP3 inflammasome, including the formation of cytosolic PA crystals,(Reference Karasawa, Kawashima and Usui-Kawanishi3) the generation of reactive oxygen species, and the involvement of AMP-activated protein kinase and ULK1 autophagy signalling cascades.(Reference Wen, Gris and Lei2)

In a previous study, we demonstrated that PA synergizes with electronegative low-density lipoprotein (LDL(-)) to significantly enhance IL-1β production in human macrophages.(Reference Chang, Chang and Chang4) Our results suggested that LDL(-) primarily initiates NF-κB activation and mildly activates the NLRP3 inflammasome, while PA amplifies inflammasome activation and IL-1β production. Interestingly, the combined effect of PA and LDL(-) on IL-1β production was mitigated by two potassium (K+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium chloride (TEA). These findings led us to hypothesise that PA-induced activation of the NLRP3 inflammasome may be associated with a reduction in cytosolic K+, a well-established trigger of NLRP3 activation, via modulation of K+ channels.(Reference Muñoz-Planillo, Kuffa and Martínez-Colón5) However, in our prior study, macrophages were treated with PA and LDL(-) simultaneously, and the direct impact of PA on cytosolic K+ levels was not assessed. Therefore, whether PA alone can alter cytosolic K+ levels remains unclear.

In the present study, we investigate the effects of PA on cytosolic K+ levels and IL-1β production in LPS-primed human macrophages. In this model, NF-κB signalling is activated, leading to the production of pro-IL-1β. Subsequent activation of the NLRP3 inflammasome facilitates the processing of pro-IL-1β by activated caspase-1 to generate IL-1β.(Reference Lopez-Castejon and Brough6) Using LPS-primed Tohoku Hospital Pediatrics-1 (THP-1) macrophages, we examined the effect of bovine serum albumin-bound PA (PA-BSA) on cytosolic K+ and IL-1β production.

Materials and methods

Materials

THP-1, a human leukemic cell line, was obtained from the American Type Culture Collection (Manassas, VA, USA). Roswell Park Memorial Institute (RPMI) 1640 medium (containing 5.3 mM KCl.), fetal bovine serum and penicillin/streptomycin were purchased from GIBCO-BRL/Life-Technologies (Rockville, MD, USA). The palmitic acid (C16:0), stearic acid (SA, C18:0), oleic acid (OA, 18:1, ω9), linoleic acid (LA, C18:2, ω6), docosahexaenoic acid (DHA, C22:6, ω3), and fatty acid-free bovine serum albumin (BSA) were purchased from Sigma-Aldrich (St. Louis, MO). A human IL-1β enzyme-linked immunosorbent assay (ELISA) kit was obtained from R&D Systems (Minneapolis, MN, USA). Voltage-gated potassium (Kv) channel blockers, including 4-aminopyridine (4-AP) and TEA, were obtained from Abcam (Cambridge, UK).

Preparation of BSA-bound fatty acid

Free fatty acids (FFAs) and BSA were mixed at a molar ratio of 3:1. Briefly, 0.05 mmole of pure FFAs were incubated with 0.25 ml of 0.2 M NaOH (equimolar). Subsequently, 0.017 mmole of fatty acid-free BSA and 50 ml of 25 mM HEPES buffer (pH 7.0) were added, and the mixture was flushed with nitrogen gas. The mixture was shaken for 5 h at room temperature and then stored at −20°C until use.

Cell culture and treatment

THP-1 cells were cultured in RPMI 1640 medium and differentiated into macrophages as previously described.(Reference Chang, Chang and Chang4) After differentiation, the differentiation medium was removed and cells were washed with serum-free RPMI 1640. Macrophages were maintained in serum-free RPMI 1640 media and treated with LPS (100 ng/ml) for 6 h. Following LPS priming, cells were washed and treated with 100 μM PA-BSA or other BSA-bound fatty acids for an additional 6 h. Control cells were treated with BSA alone.

For Kv channel inhibition studies, cells were pre-treated for 30 min with Dimethyl sulfoxide (DMSO) (vehicle control), 5 μM 4-AP, or 50 mM TEA after LPS priming, followed by 100 μM PA-BSA treatment for 6 h. Culture supernatants were collected, and IL-1β levels were quantified using the Human IL-1β DuoSet ELISA kit (R&D system) according to the manufacturer’s instructions. Cell viability was assessed using the MTT assay, and viability remained above 95% relative to control cells in all experiments.

Measurement of cytosolic potassium (K+) level

Cytosolic K+ levels were measured using the PBFI-AM fluorescent probe (Invitrogen Molecular Probes, Thermo Fisher), which is cell-permeable and sensitive to intracellular K+ changes. Fluorescence was recorded at an excitation wavelength of 340 nm and emission wavelength of 500 nm using a SpectraMax i3x Multi-Mode Microplate Reader (Molecular Devices, San Jose, CA, USA). Macrophages were incubated in buffer (20 mM HEPES, 102 mM NaCl, 5.6 mM Na2HPO4, 1 mM CaCl2, 1 mM MgCl2, 2 mg/mL D-glucose, pH 7.4) containing 0 or 5 mM KCl as controls.

Statistical analysis

Data are presented as the mean ± standard error (SE). The effects of LPS-priming and PA-BSA treatment on IL-1β and cytosolic K+ levels (Figure 1) were evaluated using two-way analysis of variance (ANOVA), followed by Duncan’s multiple range test. Statistically significance was defined as p < 0.05. For LPS-primed cells, the effects of various fatty acids and Kv channel inhibitors on IL-1β and cytosolic K+ levels (Figures 2 and 3, respectively) were assessed using one-way ANOVA followed by Duncan’s multiple range test.

Figure 1. Effects of bovine serum albumin (BSA)-bound palmitic acid (PA-BSA) on interleukin-1β (IL-1β) production and cytosolic potassium (K+) levels in LPS-primed THP-1 macrophages.

THP-1 macrophages were either untreated or primed with LPS (100 ng/ml) for 6 h, followed by treatment with BSA or PA-BSA (100 μM) for an additional 6 h. (a) IL-1β concentrations in the culture medium were quantified using ELISA. (b) Cytosolic K+ levels were assessed using the fluorescent cytosolic indicator PBFI-AM. Control cells were incubated in buffer (20 mM HEPES, 102 mM NaCl, 5.6 mM Na2HPO4, 1 mM CaCl2, 1 mM MgCl2, 2 mg/mL D-glucose, pH 7.4) containing either 0 or 5 mM K+. Data are presented as mean ± SE (n = 5). Two-way ANOVA followed by Duncan’s multiple range test was used to assess the effects of PA-BSA and LPS-priming on IL-1β and cytosolic K+ levels. Group differences are indicated. *p < 0.01.

Figure 2. Effects of BSA-bound free fatty acids on IL-1β production and cytosolic K+ levels in LPS-primed THP-1 macrophages.

THP-1 macrophages were either unprimed or primed with LPS (100 ng/ml) for 6 h, followed by treatment with BSA, PA-BSA, SA-BSA, OA-BSA, LA-BSA, or DHA-BSA (100 μM) for an additional 6 h. BSA or PA-BSA-treated cells served as controls. (a) IL-1β levels in the culture medium were measured by ELISA. (b) Cytosolic K+ levels were evaluated using PBFI-AM indicator. Data are expressed as mean ± SE (n = 4). *p < 0.001, compared with the LPS-primed BSA, OA-BSA, LA-BSA or DHA-BSA groups; † p < 0.05, compared with the LPS-primed BSA group in (a). *p < 0.01, compared with the LPS-primed BSA, OA-BSA, LA-BSA or DHA-BSA groups; † p < 0.05 compared with other LPS-primed groups in (b).

Figure 3. Inhibition of PA-BSA-induced IL-1β production and cytosolic K+ levels depletion by voltage-gated potassium (Kv) channel blockers.

THP-1 macrophages were either unprimed or primed with LPS (100 ng/ml) for 6 h, followed by a 30-min pre-treatment with 5 μM 4-AP or 50 mM TEA. Control cells were treated with DMSO. PA-BSA was then added and incubated for an addition 6 h. (a) IL-1β concentrations in the culture medium were measured by ELISA. (b) Cytosolic K+ levels were measured using the PBFI-AM indicator. Data are shown as mean ± SE (n = 4). *p < 0.001; compared with the other LPS-primed groups; † p < 0.05, compared with the LPS prime BSA group in (a). *p < 0.001 compared with the other LPS-primed groups in (b).

Results

In macrophages not primed with LPS, treatment with PA-BSA (100 μM) did not significantly alter IL-1β levels in the culture medium. However, in LPS-primed macrophages, PA-BSA treatment led to a significant 2.4-fold increase in IL-1β levels compared with BSA-treated controls (Figure 1a). Cytosolic K+ levels were assessed using the PBFI-AM fluorescent probe. Cytosolic K+ levels remained comparable among control groups, including BSA-treated macrophages, PA-BSA-treated macrophages without LPS-priming, LPS-primed macrophages treated with BSA, and macrophages incubated in 5 mM K+ buffer. In contrast, PA-BSA treatment in LPS-primed macrophages significantly reduced cytosolic K+ levels, approaching those observed in macrophages incubated in 0 mM K+ buffer (Figure 1b). Two-way ANOVA revealed a significant interaction between LDL priming and PA-BSA treatment on both IL-1β production and cytosolic K+ levels (Figure 1a). These findings suggest that PA-BSA specifically enhances IL-1β production and reduces cytosolic K+ levels in LPS-primed macrophages.

To assess the impact of fatty acid saturation on these effects, we compared IL-1β production and cytosolic K+ levels in LPS-primed macrophages treated with SFAs (PA-BSA, SA-BSA) or UFAs (OA-BSA, LA-BSA, DHA-BSA). SFAs significantly increased IL-1β production (Figure 2a) and decreased cytosolic K+ levels (Figure 2b), whereas UFAs only modestly increased IL-1β production and did not reduce cytosolic K+ levels.

Building on our previous study, which showed that IL-1β production induced by co-treatment with LDL(-) and PA-BSA was significantly inhibited only by 4-AP and TEA, and not by blockers targeting large-conductance calcium-activated K+ channels, inwardly rectifying K+ channels, or two-pore domain K+ channels.(Reference Chang, Chang and Chang4) We next evaluated whether 4-AP and TEA could inhibit PA-BSA-induced effects in LPS-primed macrophages. Our results confirm that both 4-AP and TEA significantly suppressed PA-BSA-induced IL-1β production (Figure 3a) and reversed the associated reduction in cytosolic K+ levels (Figure 3b). Together, these findings demonstrate that, in primed macrophages, PA-BSA enhances IL-1β production and reduces cytosolic K+ levels in a Kv channel-dependent manner.

Discussion

This study demonstrates that PA-BSA reduces cytosolic K+ levels in LPS-primed macrophages via 4-AP and TEA-sensitive K+ channels, thereby enhancing IL-1β production. PA-BSA did not affect cytosolic K+ levels in unprimed macrophages, indicating that its action depends on LPS-priming, likely through modulation of Kv channel activity. Since 4-AP and TEA inhibit distinct Kv channel subsets, PA-BSA likely influences cytosolic K+ levels through these channels in LPS-primed cells. Previous studies have shown that LPS regulates Kv1.3 and Kv1.5 channels in macrophages, which localise to lipid rafts following LPS stimulation.(Reference Vicente, Villalonga and Calvo7,Reference Martens, O’Connell and Tamkun8) Lipid rafts cluster the LPS-TLR4-CD14 complex, essential for pro-inflammatory signalling and potentially mediating PA-BSA’s effects.(Reference Płóciennikowska, Hromada-Judycka and Borzęcka9) Kv1.5 has also been implicated in monosodium urate-induced NLRP3 inflammasome activation in LPS-primed macrophages.(Reference Li, Kurata and Taufiq10) Additionally, palmitic acid may enhance channel activity by increasing stability and surface expression through S-palmitoylation, a reversible post-translational modification. However, the specific Kv channel(s) mediating PA-BSA’s effects remain unidentified. Future research should aim to identify these channels and characterise their molecular interactions with PA-BSA.

SA-BSA exhibited effects similar to PA-BSA by significantly lowering cytosolic K+ levels and enhancing IL-1β production in LPS-primed macrophages. In contrast, UFAs such as OA-BSA, LA-BSA and DHA-BSA did not cause a notable reduction in cytosolic K+, and their increase in IL-1β production was much less pronounced. These results highlight that SFAs like PA and SA exert substantially stronger pro-inflammatory effects than UFAs. Moreover, UFAs have been shown to inhibit inflammasome activation. A previous study demonstrated that co-treatment with OA or LA suppresses PA- and SA-induced NLRP3 inflammasome activation in LPS-primed human macrophages.(Reference L’homme, Esser and Riva11) This inhibitory effect likely arises from competition between UFAs and SFAs in phosphatidylcholine synthesis, resulting in lower saturation and altered membrane fluidity.(Reference Gianfrancesco, Dehairs and L’homme12) Consequently, lipid raft formation is reduced, which may diminish Kv channel activity. This is consistent with the lack of cytosolic K+ depletion in OA- and LA-treated cells. Unlike OA and LA, DHA has been reported to inhibit both the priming and activation steps of inflammasome assembly.(Reference Yan, Jiang and Spinetti13)

Emerging evidence indicates that SFAs contribute to heightened inflammatory responses and are implicated in the development of metabolic disorders, including cardiovascular disease, insulin resistance and type 2 diabetes. PA, a major SFA, not only enhances IL-1β production but also induces a pro-inflammatory phenotypes in macrophages by upregulating cytokines and chemokines such as TNF-α, IL-6, CCL2, CCL4, cyclooxygenase-2 and matrix metallopeptidase-9. These effects are mediated through mechanisms involving de novo ceramide synthesis, activation of p38 and JNK mitogen-activated protein kinases, and the AP-1 transcription factor.(Reference Håversen, Danielsson and Fogelstrand14,Reference Korbecki and Bajdak-Rusinek15) Notably, these inflammatory effects are further amplified when macrophages are primed or co-stimulated with LPS. PA is endogenously synthesised and abundant in dietary sources such as palm oil, butter, meats and dairy products. Replacing dietary SFAs with diet rich in UFAs may mitigate these pro-inflammatory effects and reduce the risk of metabolic diseases.

In conclusion, these results establish a link between PA-BSA-induced cytosolic K+ reduction and IL-1β production, implicating 4-AP and TEA-sensitive Kv channels in LPS-primed macrophages. These findings highlight a potential mechanistic link between SFA exposure and inflammatory signalling, offering insight into how dietary lipids may influence immune response. Given the elevated plasma levels of FFAs in pathological conditions such as type 2 diabetes and post-myocardial infarction, Kv channels may represent promising therapeutic targets for modulating inflammation. This potential warrants further investigation in the context of inflammatory disease management.

Acknowledgements

We sincerely thank Dr. Shwu-Fen Chang for her invaluable contributions to the experimental design, data analysis, and interpretation, as well as her role in drafting the original manuscript. Dr. Chang was a Professor in the Graduate Institute of Medical Sciences, College of Medicine, Taipei Medical University, Taipei, Taiwan. We are deeply saddened to report that she passed away on July 15, 2023 after a courageous battle with colorectal cancer.

Financial support

This work was supported by the Ministry of Science and Technology of Taiwan [grant numbers MOST 108–2320-B-002-070 and MOST 109-2320-B-002-064].

Competing interests

The authors declare no conflicts of interest.

Author contributions

Mr. JW Chuang conducted the majority of the experiments and analysed the data. Dr. SC Lu played a key role in conceptualising the study, designing the experimental approach, securing grant support, and preparing the manuscript.

Use of artificial intelligence

AI tools (specifically Microsoft Copilot) were used during the preparation of this manuscript to assist with grammar, spelling, and clarity. All scientific content, data interpretation, and conclusions were solely developed by the authors.

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Figure 0

Figure 1. Effects of bovine serum albumin (BSA)-bound palmitic acid (PA-BSA) on interleukin-1β (IL-1β) production and cytosolic potassium (K+) levels in LPS-primed THP-1 macrophages.THP-1 macrophages were either untreated or primed with LPS (100 ng/ml) for 6 h, followed by treatment with BSA or PA-BSA (100 μM) for an additional 6 h. (a) IL-1β concentrations in the culture medium were quantified using ELISA. (b) Cytosolic K+ levels were assessed using the fluorescent cytosolic indicator PBFI-AM. Control cells were incubated in buffer (20 mM HEPES, 102 mM NaCl, 5.6 mM Na2HPO4, 1 mM CaCl2, 1 mM MgCl2, 2 mg/mL D-glucose, pH 7.4) containing either 0 or 5 mM K+. Data are presented as mean ± SE (n = 5). Two-way ANOVA followed by Duncan’s multiple range test was used to assess the effects of PA-BSA and LPS-priming on IL-1β and cytosolic K+ levels. Group differences are indicated. *p < 0.01.

Figure 1

Figure 2. Effects of BSA-bound free fatty acids on IL-1β production and cytosolic K+ levels in LPS-primed THP-1 macrophages.THP-1 macrophages were either unprimed or primed with LPS (100 ng/ml) for 6 h, followed by treatment with BSA, PA-BSA, SA-BSA, OA-BSA, LA-BSA, or DHA-BSA (100 μM) for an additional 6 h. BSA or PA-BSA-treated cells served as controls. (a) IL-1β levels in the culture medium were measured by ELISA. (b) Cytosolic K+ levels were evaluated using PBFI-AM indicator. Data are expressed as mean ± SE (n = 4). *p < 0.001, compared with the LPS-primed BSA, OA-BSA, LA-BSA or DHA-BSA groups; †p < 0.05, compared with the LPS-primed BSA group in (a). *p < 0.01, compared with the LPS-primed BSA, OA-BSA, LA-BSA or DHA-BSA groups; †p < 0.05 compared with other LPS-primed groups in (b).

Figure 2

Figure 3. Inhibition of PA-BSA-induced IL-1β production and cytosolic K+ levels depletion by voltage-gated potassium (Kv) channel blockers.THP-1 macrophages were either unprimed or primed with LPS (100 ng/ml) for 6 h, followed by a 30-min pre-treatment with 5 μM 4-AP or 50 mM TEA. Control cells were treated with DMSO. PA-BSA was then added and incubated for an addition 6 h. (a) IL-1β concentrations in the culture medium were measured by ELISA. (b) Cytosolic K+ levels were measured using the PBFI-AM indicator. Data are shown as mean ± SE (n = 4). *p < 0.001; compared with the other LPS-primed groups; †p < 0.05, compared with the LPS prime BSA group in (a). *p < 0.001 compared with the other LPS-primed groups in (b).