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Role of macrophage lipids in regulating tumoricidal activity
Abstract:
Peritoneal macrophages (M phi) from mice became cytotoxic after incubation with lymphokine (LK); tumoricidal activity was evident with M phi treated with LK for 4 hr, became maximal after 8-12 hr of incubation, and decreased to control levels by 24-36 hr. LK induced marked changes in M phi lipid composition: cellular content of cholesterol (CHOL) and polyunsaturated fatty acid (UFA) content of cellular lipids (especially 18:3) increased two- to threefold after 8 hr when the cells showed maximal tumoricidal activity. Cellular lipid and fatty-acid content returned to control levels by 24 hr when the M phi had lost tumoricidal activity. These changes were not observed with equal numbers of M phi cultured in control supernatants. To analyze the role of CHOL and UFA in M phi tumor cytotoxicity, casein-induced peritoneal M phi were enriched in CHOL or linolenic acid (18:3) and then tested for their ability to kill 1023 tumor cells. The 18:3-enriched cells were markedly tumoricidal, whereas controls cultured in delipidized medium alone or enriched with saturated fatty acid (18:0) were not cytotoxic. CHOL-enriched M phi were not tumoricidal; indeed, these cells were inhibited in their killing after treatment with LK compared to M phi cultured in delipidized medium with LK alone. The effects of 18:3 and CHOL enrichment of the M phi on their metabolic status, inflammatory function, and tumor cell-binding capacity were tested. The 18:3-enriched M phi were depressed in their ability to synthesize protein and in phagocytic activity compared to controls; these cells showed a transient increase in superoxide release. M phi cultured with 18:3 for 48 hr were also cytotoxic for P815 tumor cells, but did not show an enhanced capacity for P815 binding compared to controls. CHOL-enriched M phi were similar to control cells in their protein synthesizing and phagocytic activities; these cells also showed an early transient increase in superoxide release. CHOL-enriched M phi were not cytotoxic for P815 cells, but bound the tumor cells more readily than did the 18:3-enriched M phi. The data suggest that endogenous levels of 18:3 and CHOL can regulate M phi tumor cytotoxicity, but not through regulation of M phi protein synthesis, oxidative metabolism, or augmented capacity for tumor target binding.
Insights
Lymphokine treatment enhances macrophage tumoricidal activity by altering lipid composition, specifically increasing polyunsaturated fatty acids (UFAs). Enriched linolenic acid (18:3) boosts cytotoxicity, while cholesterol (CHOL) enrichment inhibits it, suggesting lipids regulate macrophage anti-tumor function.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Peritoneal macrophages (M phi) are key immune cells involved in tumor surveillance.
- Lymphokine (LK) treatment can activate macrophages, inducing tumoricidal activity.
- The precise molecular mechanisms underlying LK-induced macrophage activation remain under investigation.
Purpose of the Study:
- To investigate the role of cellular lipid composition, specifically cholesterol (CHOL) and polyunsaturated fatty acids (UFAs), in lymphokine-induced macrophage tumoricidal activity.
- To determine how enrichment with specific fatty acids (linolenic acid 18:3, saturated fatty acid 18:0) or cholesterol affects macrophage anti-tumor functions.
- To elucidate the relationship between lipid modulation, macrophage metabolic status, inflammatory function, and tumor cell binding.
Main Methods:
- Peritoneal macrophages were incubated with lymphokine (LK) and their tumoricidal activity was assessed over time.
- Cellular lipid composition, including cholesterol and fatty acid content (especially 18:3 and 18:0), was analyzed.
- Macrophages were enriched with CHOL or linolenic acid (18:3) and tested for cytotoxicity against tumor cells (1023 and P815).
- Metabolic status (protein synthesis, phagocytosis) and inflammatory function (superoxide release) were evaluated.
Main Results:
- LK treatment induced time-dependent increases in M phi tumoricidal activity, correlating with a two- to threefold increase in cellular CHOL and UFA (especially 18:3) content.
- Macrophages enriched with linolenic acid (18:3) exhibited marked tumoricidal activity, whereas CHOL-enriched macrophages were not cytotoxic and showed inhibited LK-induced killing.
- 18:3-enriched M phi showed depressed protein synthesis and phagocytosis but transient superoxide release, while CHOL-enriched M phi had normal metabolic functions but enhanced tumor cell binding.
Conclusions:
- Endogenous levels of linolenic acid (18:3) and cholesterol (CHOL) significantly regulate macrophage tumor cytotoxicity.
- 18:3 enrichment enhances M phi tumoricidal activity, independent of protein synthesis, oxidative metabolism, or tumor cell binding.
- CHOL enrichment impairs M phi tumoricidal activity, suggesting a complex regulatory role for lipids in macrophage-mediated anti-tumor immunity.