Novel endogenous peptide agonists of cannabinoid receptors

Hemopressin (Hp), a 9-residue α-hemoglobin-derived peptide, was previously reported to function as a CB₁ cannabinoid receptor antagonist (1). In this study, we report that mass spectrometry (MS) data from peptidomics analyses of mouse brain extracts identified N-terminally extended forms of Hp containing either three (RVD-Hpα) or two (VD-Hpα) additional amino acids, as well as a β-hemoglobin-derived peptide with sequence similarity to that of hemopressin (VD-Hpβ). Characterization of the α-hemoglobin-derived peptides using binding and functional assays shows that in contrast to Hp, which functions as a CB₁ cannabinoid receptor antagonist, both RVD-Hpα and VD-Hpα function as agonists. Studies examining the increase in the phosphorylation of ERK1/2 levels or release of intracellular Ca²⁺ indicate that these peptides activate a signal transduction pathway distinct from that activated by the endocannabinoid, 2-arachidonoylglycerol, or the classic CB₁ agonist, Hu-210. This finding suggests an additional mode of regulation of endogenous cannabinoid receptor activity. Taken together, these results suggest that the CB₁ receptor is involved in the integration of signals from both lipid- and peptide-derived signaling molecules.