Protein kinase C-dependent coupling of alpha(2A/D)-adrenergic receptors to phospholipase D

To clarify the role of protein kinase C (PKC) in regulating the coupling pathway of alpha(2)-adrenergic receptors, we examined receptor activation of phospholipase D (PLD) in PC12 cells overexpressing alpha(2A/D) receptors, using [(3)H]phosphatidylbutanol formation as an index of PLD activity. In intact PC12/alpha(2A/D) cells, the ability of either epinephrine or the alpha(2)-receptor-selective agonist UK14304 to stimulate PLD was completely dependent on concomitant PKC activation. Pretreatment with the PKC activator phorbol dibutyrate revealed an agonist-stimulated PLD activity which was blocked by the alpha(2)-receptor-selective antagonist rauwolscine and by pertussis toxin treatment. Removal of extracellular calcium or tyrosine kinase inhibition by genistein pretreatment also eliminated the ability of epinephrine to stimulate PLD. These results indicate that alpha(2A/D)-adrenergic receptors couple via pertussis toxin-sensitive G proteins to PLD in a PKC-requiring and tyrosine kinase regulated manner.

A hydrophobic cluster between transmembrane helices 5 and 6 constrains the thyrotropin-releasing hormone receptor in an inactive conformation

We have studied the role of a highly conserved tryptophan and other aromatic residues of the thyrotropin-releasing hormone (TRH) receptor (TRH-R) that are predicted by computer modeling to form a hydrophobic cluster between transmembrane helix (TM)5 and TM6. The affinity of a mutant TRH-R, in which Trp279 was substituted by alanine (W279A TRH-R), for most tested agonists was higher than that of wild-type (WT) TRH-R, whereas its affinity for inverse agonists was diminished, suggesting that W279A TRH-R is constitutively active. We found that W279A TRH-R exhibited 3.9-fold more signaling activity than WT TRH-R in the absence of agonist. This increased basal activity was inhibited by the inverse agonist midazolam, confirming that the mutant receptor is constitutively active. Computer-simulated models of the unoccupied WT TRH-R, the TRH-occupied WT TRH-R, and various TRH-R mutants predict that a hydrophobic cluster of residues, including Trp279 (TM6), Tyr282, and Phe199 (TM5), constrains the receptor in an inactive conformation. In support of this model, we found that substitution of Phe199 by alanine or of Tyr282 by alanine or phenylalanine, but not of Tyr200 (by alanine or phenylalanine), resulted in a constitutively active receptor. We propose that a hydrophobic cluster including residues in TM5 and TM6 constrains the TRH-R in an inactive conformation via interhelical interactions. Disruption of these constraints by TRH binding or by mutation leads to changes in the relative positions of TM5 and TM6 and to the formation of an active form of TRH-R.

Reconstitution of alpha2D-adrenergic receptor coupling to phospholipase D in a PC12 cell lysate

We have previously shown that alpha2-adrenergic receptor-mediated coupling to phospholipase D (PLD) in vascular tissues requires a tyrosine kinase activity (Jinsi, A., Paradise, J., and Deth, R. C. (1996) Eur. J. Pharmacol. 302, 183-190). To further clarify this mode of regulation we reconstituted alpha2A/D-adrenergic receptor-stimulated PLD activity in PC12 cells expressing the cloned receptor. [3H]Myristic acid-labeled cells were lysed by nitrogen cavitation, and aliquots of subnuclear fraction were utilized in the PLD assay. Agonist-stimulated PLD activity was measured in the presence of 0.4% butanol as [3H]phosphatidylbutanol formation. Both GTP and its non-hydrolyzable analog guanosine 5'-O-(thiotriphosphate) stimulated PLD activity in a concentration- and time-dependent manner that required co-activation of protein kinase C by phorbol dibutyrate. Addition of epinephrine produced a 3-fold stimulation of PLD activity in the presence of GTP and GDP. This agonist-stimulated PLD activity was completely blocked by the alpha2-adrenergic receptor antagonist rauwolscine and by Clostridium botulinum toxin as well as by antibodies directed against either pp60(src), RhoA, or Ras GTPase-activating protein. These results indicate that coupling of the alpha2A/D-adrenergic receptor to PLD is complexly regulated by both the tyrosine kinase pp60(src) and the low molecular weight G protein RhoA.

Constitutive activity of native thyrotropin-releasing hormone receptors revealed using a protein kinase C-responsive reporter gene

The native TRH receptor (TRH-R), which is a G protein-coupled receptor that signals via the phosphoinositide transduction pathway, has been assumed to be inactive in the absence of agonist. In contrast, a mutant mouse TRH-R (C335Stop TRH-R) was shown previously to exhibit constitutive (or agonist-independent) signaling activity. In this report, we measured signaling activity of TRH-Rs using a protein kinase C-responsive reporter gene instead of formation of inositol phosphate second messenger molecules. Using this more sensitive system, we show that native mouse TRH-Rs exhibit agonist-independent signaling activity that is directly proportional to the number of receptors expressed in COS-1 cells and is inhibited by negative antagonist benzodiazepine drugs. As expected, the basal signaling activity of native TRH-Rs is lower than C335Stop TRH-Rs. Constitutive activity of native TRH-Rs is not peculiar to COS-1 cells in which receptor density is markedly elevated, because it can also be demonstrated in Madin Darby canine kidney cells stably expressing mouse TRH-Rs and GH4C1 cells endogenously expressing rat TRH-Rs. These findings support the thesis that native TRH-Rs oscillate between active and inactive states. We suggest that demonstration of constitutive activity of native receptors may depend on the sensitivity of the signaling assay employed.