Prostaglandin E₂ Glyceryl Ester Contributes to Hyperalgesia in a Humanized Model Pf Sickle Cell Disease through P2Y₆ Receptors

Sickle Cell Disease (SCD) is the most common monogenic red blood cell disorder characterized by a remarkable phenotypic complexity. Common complications of SCD are unpredictable episodic and chronic pain. Transgenic homozygous HbSS-BERK mice expressing >99% human sickle hemoglobin and exhibiting both pain and neurochemical changes similar to humans, and HbAA control mice expressing normal human hemoglobin, were used to determine the contribution of the prostaglandin E₂ glyceryl ester (PGE₂-G), to mechanical and heat hyperalgesia and the mechanism of its pro-nociceptive effect. Mechanical hyperalgesia was defined by a decrease in paw withdrawal threshold, and heat hyperalgesia was defined by a decrease in paw withdrawal latency to radiant heat applied to the plantar hind paw. Considering that prostaglandin E2-G (PGE₂-G) is the product of oxidation of 2-arachidonylglycerol (2-AG) by cycloxygenase-2 (COX-2), the expression of COX-2 protein and COX-2 activity was measured in dorsal root ganglia (DRG). The levels of 2-AG and PGE₂-G in DRG were determined by HPLC-MS. Mechanical and heat hyperalgesia in HbSS mice were associated with a decreased level of 2-AG and increased level of PGE₂-G in DRGs of HbSS mice due to elevated level of COX-2 protein and COX-2 activity. In HbAA control mice, intraplantar (i.pl.) injection of PGE₂-G (1 µg/10 µl) produced mechanical hyperalgesia in the injected paw independently from PGE₂ receptors. Also, PGE₂-G (1 µg/10 µl, i.pl.) produced mechanical and heat hyperalgesia in the injected paw in C3H mice. The hyperalgesia was blocked by co-injection of PGE₂-G with MRS 2578, the purinergic P2Y₆ receptor antagonist (1.4 mg/10 ml, i.pl.). Importantly, MRS 2578 (4.7 mg/50 ml, i.p.) also attenuated hyperalgesia in HbSS mice. Thus, increased expression of COX-2 in DRG neurons of HbSS mice results in the oxidation of 2-AG and production of PGE₂-G, a pronociceptive mediator. PGE₂-G produced mechanical and heat hyperalgesia in HbAA and C3H mice, and these effects were mediated by P2Y₆ receptors.

Acknowledgments: Supported by the National Heart, Lung and Blood Institute (HL135895-16).