Expression of the mRNA encoding truncated PPAR alpha does not correlate with hepatic insensitivity to peroxisome proliferators.

Two alternatively spliced forms of human PPAR alpha mRNA, PPAR alpha1 and PPAR alpha2, have been identified. PPAR alpha1 mRNA gives rise to an active PPAR alpha protein while PPAR alpha2 mRNA gives rise to a form of PPAR which lacks the ligand-binding domain. PPAR alpha2 is unable to activate a peroxisome proliferator response element (PPRE) reporter gene construct in transient transfection assays. Both PPAR alpha1 and PPAR alpha2 mRNA are present in human liver, kidney, testes, heart, small intestine, and smooth muscle. In human liver, PPAR alpha2 mRNA abundance is approximately half that of PPAR alpha1 mRNA; a correlation analysis of PPAR alpha1 and PPAR alpha2 mRNA mass revealed an r-value of 0.75 (n = 18). Additional studies with intact liver from various species, showed that the PPAR alpha2/PPAR alpha1 mRNA ratios in rat, rabbit, and mouse liver were less than 0.10; significantly lower than the 0.3 and 0.5 ratios observed in monkey and human livers, respectively. To determine if a high PPAR alpha2/PPAR alpha1 mRNA ratio was associated with insensitivity to peroxisome proliferators, we treated human, rat, and rabbit hepatocytes with WY14643, a potent PPAR alpha activator, and measured acyl CoA oxidase (ACO) mRNA levels. Rat ACO mRNA levels increased markedly in response to WY14643 while human and rabbit hepatocytes were unresponsive. Thus, although the PPAR alpha2/PPAR alpha1 mRNA ratio is low in rabbits, this species is not responsive to peroxisome proliferators. Further studies with male and female rats, which vary significantly in their response to peroxisome proliferators, showed little difference in the ratio of PPAR alpha2/PPAR alpha1 mRNA. These data suggest that selective PPAR alpha2 mRNA expression is not the basis for differential species or gender responses to peroxisome proliferators.

Two classes of structurally different antagonists display similar species preference for the human tachykinin neurokinin3 receptor.

Two classes of structurally different tachykinin neurokinin3 (NK3) antagonists were used to evaluate species difference in antagonist binding between human and rat NK3 receptors. In competition binding experiments with [125I-MePhe7]NKB as radioligand, PD 154740, PD 157672, SR 48968, and SR 142801 displayed lower Ki values for the human NK3 receptor (40 +/- 4, 12 +/- 1,350 +/- 50, and 0.40 +/- 0.05 nM, respectively) than for the rat NK3 receptor (2450 +/- 130, 288 +/- 25, > 10,000, and 11.0 +/- 0.5 nM, respectively). Data from in vitro functional assay showed similar species preference as observed with the competition binding assay. It was shown previously that substitution of only two amino acid residues in the rat receptor to their human counterparts could change the species selectivity of SR 48968, a weak NK3 antagonist. In the double-substituted rat mutant, all three antagonists (PD 154740, PD 157672, and SR 142801) displayed Ki values (76 +/- 8, 16 +/- 2, and 0.50 +/- 0.05 nM, respectively) very similar to the Ki values for the wild-type human NK3 receptor. Thus, in addition to their previously reported effects on SR 48968, these two amino acid residues are responsible for the species selectivity of these three additional NK3 antagonists. Because PD 154740 and PD 157672 are very different structurally from SR 48968 and SR 142801, our results indicate that the two identified residues may be involved in adopting a receptor conformation that favors the binding of NK3 antagonists that display species preference for the human NK3 receptor.

The non-peptide tachykinin NK2 receptor antagonist SR 48968 interacts with human, but not rat, cloned tachykinin NK3 receptors.

SR 48968, a non-peptide tachykinin NK2 receptor antagonist, has been shown to possess sub-micromolar affinity for NK3 receptors present in the guinea pig. In the present study, we have compared the binding affinities of SR 48968 to the cloned human and rat NK3 receptors expressed in CHO cells. Using [125I]-[MePhe7]-neurokinin B as the radioligand, SR48968 displayed an IC50 value of 350 nM for the human NK3 receptor as compared with a value of greater than 10 microM for the rat NK3 receptor. Exposure of cells transfected with human NK3 receptor cDNA to [Pro7]-neurokinin B increased inositol phospholipid turnover in a concentration-dependent manner and this response was blocked competitively by SR 48968. Our results demonstrate that SR 48968 is an antagonist at the human NK3 receptor and may be a useful tool for elucidating the species-dependent variations in the non-peptide antagonist binding site(s) on the NK3 receptor.

Identification of methionine134 and alanine146 in the second transmembrane segment of the human tachykinin NK3 receptor as reduces involved in species-selective binding to SR 48968.

We have shown that SR 48968 possess sub-micromolar affinity for the human tachyknin NK3 receptor; however, its affinity for the rat NK3 receptor is greater than 10 microM. To determine the functional domain(s) responsible for the species variation in binding affinities, we have constructed several human/rat chimeric NK3 receptors. Based on studies of these chimeric receptors, the species-specific binding sites for SR 48968 were localized to five residues in the 1st and 2nd transmembrane segments of the human NK3 receptor. We have individually mutated all five residues in the rat receptor to their corresponding residues in human. Only two single-substituted mutants (V121M and G133A) show a small increase in their binding affinities for SR 48968. However, a mutant containing both substitutions was shown to have the same affinity for SR 48968 as the wild type human NK3 receptor. It is concluded that collectively these two amino acid changes are responsible for the species difference in binding affinities for SR 48968.