Ll 37: Mechanism of Action

Mechanism of Action

Primary Antimicrobial: "Carpet-Like" Membrane Disruption

LL-37 disrupts bacterial membranes via electrostatic attraction to anionic bacterial surfaces → hydrophobic insertion → toroidal pore formation or micellization ("carpet-like" mechanism). It acts preferentially on Gram-negative bacteria but is effective against both Gram-positive and drug-resistant strains. Eukaryotic membranes are protected by high cholesterol content.[3][7]

Receptor Targets

Receptor	Type	Functional Effect
FPR2/FPRL1	GPCR	Chemotaxis of neutrophils, monocytes, T cells (Yang et al., 2000)[8]
P2X7	Purinergic	IL-1β processing; neutrophil survival (Elssner et al., 2004)[8]
EGFR	Receptor Tyrosine Kinase	Transactivation → keratinocyte migration → wound healing (Tokumaru et al., 2005)[9]
IGF-1R	Receptor Tyrosine Kinase	Partial agonist → proliferation (Girnita et al., 2012)[10]
CXCR2	Chemokine	Functional ligand on neutrophils (Zhang et al., 2009)[8]
MrgX2	GPCR	Mast cell degranulation (Subramanian et al., 2011)[8]
TLR9	Toll-like Receptor	DNA-LL-37 complexes trigger endosomal TLR9 (Lande et al., 2007)[10]

Downstream Signaling Cascades

• ERK1/2 and p38 MAPK: Crucial for keratinocyte migration and wound healing; modulates cytokine production in monocytes.[9]
• PI3K/Akt → CREB: Cell survival signaling via P2X7–SFK–Akt pathway in keratinocytes.[9]
• mTOR: Activation suppresses autophagy in pancreatic cancer → ROS accumulation → DNA damage.[11]
• NF-κB: Inhibits p50/p65 translocation in inflammation (anti-inflammatory); may activate in some cancers (context-dependent).[8]

Anti-Biofilm Activity

Inhibits quorum-sensing (Las/Rhl systems) and promotes twitching motility; effective at sub-MIC concentrations (0.5 µg/mL) — far below bactericidal thresholds.[7]

LPS Neutralization

Binds and neutralizes lipopolysaccharide (LPS), preventing endotoxin-induced macrophage activation and cytokine storm.[12]

Biphasic Dose-Response

LL-37 exhibits a distinct bell-shaped dose response: ≤1 µM = anti-apoptotic, pro-healing; >10 µM = cytotoxic. In clinical trials, 0.5 mg/mL was 6-fold more effective than placebo, while the highest dose (3.2 mg/mL) showed no improvement.[5]

vs. Related Compounds

Compound	Key Difference
hCAP18	Inactive 18 kDa precursor; LL-37 is the active C-terminal domain released by proteolysis
KR-12	Truncated fragment (residues 18–29); retains antimicrobial activity with less cytotoxicity
D-LL-37	Protease-resistant enantiomer; retains antimicrobial and anti-biofilm activity
CRAMP (mouse)	Murine ortholog; functional homology but non-identical sequence