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Tuberoinfundibular Peptide 39 Binds to the Parathyroid Hormone (PTH)/PTH-Related Peptide Receptor, but Functions as an Antagonist¹

Endocrine Unit (K.B.J., M.R.J., R.C.G., T.J.G., H.J.), Department of Medicine and Pediatric Endocrine Unit (R.C.G.), MassGeneral Hospital for Children (R.C.G., H.J.), Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114

Address all correspondence and requests for reprints to: Dr. Harald Jüppner, Endocrine Unit, Wellman 5, Massachusetts General Hospital, Boston, Massachusetts 02114. E-mail: jueppner{at}helix.mgh.harvard.edu

The tuberoinfundibular peptide TIP39 [TIP-(1–39)], which exhibits only limited amino acid sequence homology with PTH and PTH-related peptide (PTHrP), stimulates cAMP accumulation in cells expressing the PTH2 receptor (PTH2R), but it is inactive at the PTH/PTHrP receptor (PTH1R). However, when using either ¹²⁵I-labeled rat [Nle^8,21,Tyr³⁴]PTH-(1–34)amide (rPTH) or ¹²⁵I-labeled human [Tyr³⁶]PTHrP-(1–36)amide [PTHrP-(1–36)] for radioreceptor studies, TIP-(1–39) bound to LLCPK₁ cells stably expressing the PTH1R (HKrk-B7 cells), albeit with weak apparent affinity (243 ± 52 and 210 ± 64 nM, respectively). In comparison to the parent peptide, the apparent binding affinity of TIP-(3–39) was about 3-fold higher, and that of TIP-(9–39) was about 5.5-fold higher. However, despite their improved IC₅₀ values at the PTH1R, both truncated peptides failed to stimulate cAMP accumulation in HKrk-B7 cells. In contrast, the chimeric peptide PTHrP-(1–20)/TIP-(23–39) bound to HKrk-B7 cells with affinities of 31 ± 8.2 and 11 ± 4.0 nM when using radiolabeled rPTH and PTHrP-(1–36), respectively, and it stimulated cAMP accumulation in HKrk-B7 and SaOS-2 cells with potencies (EC₅₀, 1.40 ± 0.3 and 0.38 ± 0.12 nM, respectively) and efficacies (maximum levels, 39 ± 8 and 31 ± 3 pmol/well, respectively) similar to those of PTH-(1–34) and PTHrP-(1–36). In both cell lines, TIP(9–39) and, to a lesser extent, TIP-(1–39) inhibited the actions of the three agonists with efficiencies similar to those of [Leu¹¹,D-Trp¹²,Trp²³,Tyr³⁶]PTHrP-(7–36)amide, an established PTH1R antagonist. Taken together, the currently available data suggest that the carboxyl-terminal portion of TIP-(1–39) interacts efficiently with the PTH1R, at sites identical to or closely overlapping those used by PTH-(1–34) and PTHrP-(1–36). The amino-terminal residues of TIP-(1–39), however, are unable to interact productively with the PTH1R, thus enabling TIP-(1–39) and some of its truncated analogs to function as an antagonist at this receptor.

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