EP₁‐ and FP‐mediated cross‐desensitization of the alpha (α) and beta (β) isoforms of the human thromboxane A₂ receptor

Abstract

1 Heterologous desensitization or intermolecular cross-talk plays a critical role in regulating intracellular signalling by diverse members of the G-protein-coupled receptor superfamily. We have previously established that the a and b isoforms of the human thromboxane A 2 receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E 2 , an agonist of the EP 1 subtype of the PGE 2 receptor (EP) family. 2 Herein, we investigated the molecular basis of TPa and TPb desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE 2 and in response to the PGF 2a receptor (FP) agonist PGF 2a , and sought to identify the target site(s) of those desensitizations.3 Our results demonstrated that TPa and TPb receptors are subject to desensitization in response to both EP 1 and FP receptor activation and that these effects are mediated by direct protein kinase (PK)C phosphorylation of the individual TP isoforms within their unique carboxyl-terminal (C)-tail domains. 4 Moreover, deletion/site-directed mutagenesis and metabolic labelling studies identified Thr 337 , within TPa, and Thr 399 , within TPb, as the specific target residues for PKC phosphorylation and EP 1 -and FP-mediated desensitization of TPa and TPb signalling, respectively. 5 Hence, in conclusion, while the TPa and TPb diverge within their C-tail domains, they have evolved to share a similar mechanism of PKC-induced phosphorylation and desensitization in response to EP 1 and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.

Identification of an Interaction between the TPα and TPβ Isoforms of the Human Thromboxane A2 Receptor with Protein Kinase C-related Kinase (PRK) 1

“…TP␣ and TP␤ also undergo entirely distinct mechanisms of both agonist-induced (homologous) desensitization (24,25) and heterologous desensitization or cross-talk/regulation by other signaling systems such as prostacyclin and nitric oxide (23, 26 -29). Such differences in desensitization of TP␣ and TP␤ signaling occur because of differences in their phosphorylation, occurring mainly within their unique C-tail domains, by the second messenger-regulated protein kinases, including by protein kinase (PK) C, PKA, PKG, and/or by GPCR-regulated kinases 1/2 (23)(24)(25)(26)(27)29).…”

α1‐Adrenoceptor activation of PKC‐ε causes heterologous desensitization of thromboxane receptors in the aorta of spontaneously hypertensive rats

“…The present results thus support the interpretation that variations in PKC isozymes underlie the desensitization of TP receptors observed in the SHR aorta. A similar conclusion was reached in TP receptor desensitization studies on cell cultures (Spurney and Coffman, 1997;Yukawa et al, 1997;Spurney, 1998;Walsh and Kinsella, 2000;Kelley-Hickie and Kinsella, 2004).…”

“…This expression increase may result in a protective response in the vasculature of hypertensive animals. Crosstalk between TP, IP, EP, DP and FP receptors in transfected cells has been studied by others (Walsh and Kinsella, 2000;Foley et al, 2001;Kelley-Hickie and Kinsella, 2004). The present study reports a crosstalk between non-prostanoid and prostanoid receptors that occurs ex vivo and is modulated by disease.…”

“…However, the present results with the selective PKC-ε inhibitor ε-V1-2 (Lee et al, 1999) make the latter interpretation unlikely but suggest that PKC activation contributes to the TP receptor desensitization in the SHR aorta. Transfection studies on cell cultures also suggest that PKC mediates modifications of TP receptors causing their desensitization (Spurney and Coffman, 1997;Yukawa et al, 1997;Walsh and Kinsella, 2000;Yan et al, 2002;Kelley-Hickie and Kinsella, 2004). The present findings indicate that the kinase indeed contributes to TP receptor desensitization in aortae isolated from live animals and that such heterologous desensitization results from crosstalk with α1-adrenoceptors.…”

International Union of Basic and Clinical Pharmacology. LXXXIII: Classification of Prostanoid Receptors, Updating 15 Years of Progress

“…These include, but are not limited to, transactivation and crossdesensitization. EP 1 receptor stimulation results in desensitization of TP receptors by PKC-mediated phosphorylation of C-terminal residues (Kelley-Hickie and Kinsella, 2004). An EP 1 receptor-mediated transactivation of epidermal growth factor receptors, with resultant Akt activation, has also been reported (Han and Wu, 2005).…”