KPV Peptide (10MG)

This 1998 study in the Journal of Leukocyte Biology (Delgado et al.) tested three melanocortin peptides in a mouse microglial cell line: full-length α-MSH (1–13), the tripeptide KPV, written as α-MSH (11–13), and ACTH (1–24). Microglia are the resident immune cells of the brain.

Cells were stimulated with bacterial lipopolysaccharide (LPS) plus interferon-γ (IFN-γ) to trigger an inflammatory response. Researchers then measured tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and nitric oxide (NO). They also examined cAMP signaling, gene expression, and whether the microglia produce α-MSH themselves.

Scope: All experiments were performed in a cultured mouse cell line. The study did not include animals or people.

• Citation: Delgado R, Carlin A, Airaghi L, et al. J Leukoc Biol. 1998;63(6):740–745.
• Study type: In vitro cell culture
• Cells: N9 immortalized mouse microglial cell line
• Stimulus: LPS (10 ng/mL) plus IFN-γ (1 U/mL) for 24 hours
• Peptides compared: α-MSH (1–13), α-MSH (11–13) (KPV), ACTH (1–24)
• Measurements: TNF-α, IL-6 and nitrite (a marker of NO), intracellular cAMP, TNF-α and iNOS mRNA, and α-MSH released by the cells

Activated microglia release cytokines and nitric oxide that help defend the brain against infection. In excess, these mediators are linked to tissue injury in experimental models. Earlier work had shown that α-MSH reduces inflammatory mediators in peripheral immune cells such as monocytes and macrophages, and that those cells can make α-MSH themselves.

This study asked three questions about microglia:

• Do melanocortin peptides, including the short KPV fragment, reduce TNF-α, IL-6 and NO production?
• Do microglia release α-MSH when activated?
• Does blocking that endogenous α-MSH change the inflammatory response?

N9 cells were plated and treated with each peptide 10 minutes before LPS plus IFN-γ. In pilot work, the peptides were tested from femtomolar to millimolar concentrations. The authors chose micromolar concentrations for the main experiments because they gave the most consistent inhibition under these culture conditions. Cell viability stayed above 98% in all conditions.

The study used several assays:

• TNF-α by cell bioassay, IL-6 by ELISA, and nitrite by the Griess reaction
• Intracellular cAMP after peptide exposure, with forskolin as a comparison
• Northern blots for TNF-α and inducible nitric oxide synthase (iNOS) mRNA
• α-MSH released into the medium, measured by radioimmunoassay
• Immunoneutralization of endogenous α-MSH with an anti-α-MSH antibody, compared with control IgG

All three melanocortin peptides reduced TNF-α, IL-6 and nitrite production by activated microglia compared with stimulated cells without peptide:

• α-MSH (1–13): The greatest inhibition was at 10 µM, about 43% (TNF-α), 31% (IL-6) and 42% (nitrite). The concentration–response pattern was U-shaped, as earlier α-MSH studies had reported.
• KPV, α-MSH (11–13): About 45% (TNF-α), 50% (IL-6) and 40% (nitrite)
• ACTH (1–24): About 38% (TNF-α), 65% (IL-6) and 41% (nitrite)

The authors noted that the three-amino-acid KPV fragment was "likewise a very effective inhibitor" in this assay system.

Northern blot analysis showed that the lower TNF-α and nitrite output matched lower TNF-α mRNA and iNOS mRNA. This indicates the effect occurred at the level of gene expression.

Both α-MSH (1–13) and KPV increased intracellular cAMP in resting cells and in cells stimulated with LPS plus IFN-γ. The size of the increase was comparable to that caused by forskolin, an adenylyl cyclase activator. Citing earlier studies in which other cAMP-raising agents reduced TNF-α and iNOS in microglia, the authors proposed that increased cAMP probably contributes to the effects they observed.

Microglia stimulated with LPS plus IFN-γ released α-MSH into the culture medium. When endogenous α-MSH was neutralized with an antibody before and during stimulation, release of IL-6, TNF-α and nitrite rose by about 26%, 28% and 27% compared with cells given control IgG. α-MSH was undetectable in the antibody-treated samples.

The authors interpreted this as evidence of an autocrine circuit: activated microglia release α-MSH, which in turn limits their own production of inflammatory mediators in culture.

Five melanocortin receptors (MC1R to MC5R) had been identified by 1998, and all signal through cAMP. At that time, none had been shown to bind KPV. Earlier work found that KPV did not compete with α-MSH for binding at MC1R on melanoma cells. The authors left open whether KPV acts through a known melanocortin receptor in an unusual way or through a different, uncharacterized receptor.

• All results come from one immortalized mouse microglial cell line. Primary microglia or cells from other species may respond differently.
• Culture conditions, including the strong LPS plus IFN-γ stimulus and micromolar peptide concentrations, are specific to this experiment.
• A cell culture cannot model interactions with neurons, astrocytes, blood vessels and other cells in living tissue.
• KPV's receptor and mechanism were not resolved in this study.
• In vitro results do not establish effects, safety or appropriate exposure in animals or people.

Delgado et al. (1998) found that α-MSH (1–13), KPV and ACTH (1–24) each reduced TNF-α, IL-6 and nitric oxide production by activated mouse microglia in culture. The effects were accompanied by lower TNF-α and iNOS mRNA and higher intracellular cAMP. The microglia also released α-MSH, and blocking it increased inflammatory output by roughly 26–28%, which suggests an autocrine regulatory loop in this cell model.

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