GIP Receptor Antagonism Eliminates Paradoxical Growth Hormone Secretion in Some Patients With Acromegaly

Abstract

Context: About 30% of patients with active acromegaly experience paradoxically increased growth hormone (GH) secretion during the diagnostic oral glucose tolerance test (OGTT). Endogenous glucose-dependent insulinotropic polypeptide (GIP) is implicated in this paradoxical secretion.

Objective: We used the GIP receptor (GIPR) antagonist GIP(3-30)NH2 to test the hypothesis that GIP mediates this paradoxical response when GIPR is abundantly expressed in somatotropinomas.

Methods: A total of 25 treatment-naive patients with acromegaly were enrolled. Each patient underwent one OGTT during simultaneous placebo infusion and one OGTT during a GIP(3-30)NH2 infusion. Blood samples were drawn at baseline and regularly after infusions to measure GH. We assessed pituitary adenoma size by magnetic resonance imaging and GIPR expression by immunohistochemistry on resected somatotropinomas. For mechanistic confirmation, we applied in vitro and ex vivo approaches. The main outcome measure was the effect of GIP(3-30)NH2 on paradoxical GH secretion during OGTT as a measure of GIP involvement.

Results: In 4 of 7 patients with paradoxical GH secretion, GIP(3-30)NH2 infusion completely abolished the paradoxical response (P = .0003). Somatotrophs were available from 3 of 4 of these patients, all showing abundant GIPR expression. Adenoma size did not differ between patients with and without paradoxical GH secretion.

Conclusion: Of 25 patients with acromegaly, 7 had paradoxical GH secretion during OGTT, and pharmaceutical GIPR blockade abolished this secretion in 4. Corresponding somatotroph adenomas abundantly expressed GIPR, suggesting a therapeutic target in this subpopulation of patients. In vitro and ex vivo analyses confirmed the role of GIP and the effects of the antagonist.

Keywords: GIP receptor antagonism; acromegaly; glucose-dependent insulinotropic polypeptide (GIP); growth hormone (GH); paradoxical GH secretion.

Figure 1.

Study design and plasma levels of GIP(3-30)NH₂ in the paradoxical and nonparadoxical groups. A) Blood sample time points are indicated with red drops, and yellow stars indicate blood pressure and heart rate measurements. The duration of infusion is highlighted with a yellow bar. B) Plasma levels of GIP(3-30)NH₂. Black circles, paradoxical group (n = 7); open circles, nonparadoxical group (n = 18). C) Corresponding bsAUC_{0-180 minutes}. Data are presented as mean ± SEM, and bsAUC_{0-180 minutes} were assessed for statistical differences using an unpaired t test. No statistically significant differences were found.

Figure 2.

GH secretion patterns during OGTTs. A) and B) Paradoxical group (n = 7). Solid lines highlight responders (n = 4) and dashed lines nonresponders (n = 3). C) and D) Nonparadoxical group (n = 18). Data are presented as relative (%) changes in GH secretion from basal levels. Each curve represents a corresponding patient, and identical colors reflect data from the same patient during OGTT with placebo and GIP(3-30)NH₂ infusion, respectively.

Figure 3.

GH and prolactin responses during OGTT with corresponding bsAUC_{0-180 minutes}. A) Serum GH levels presented as a relative (%) change from basal concentrations (mean of time points −60, −45, and −30 minutes; n = 7 in paradoxical group and n = 18 in nonparadoxical group). B) Serum GH levels (%) in responders (n = 4) and nonresponders (n = 3) of the paradoxical group. C) Serum prolactin levels presented as a relative (%) change from basal concentrations and D) serum prolactin levels (%) in responders (n = 4) and nonresponders (n = 3) of the paradoxical group. Data are presented as mean ± SEM. To assess statistical significance between bsAUC_{0-180 minutes} differences in the paradoxical and nonparadoxical groups and between the paradoxical-placebo and nonparadoxical-placebo groups, a matched 2-way repeated-measures ANOVA with Šidák multiple comparisons test was applied. To assess statistical significance between bsAUC_{0-180 minutes} differences in paradoxical-placebo vs nonparadoxical-GIP(3-30)NH₂ infusion and paradoxical-GIP(3-30)NH₂ infusion vs nonparadoxical-placebo infusion, a 2-way repeated-measures ANOVA with Tukey multiple comparisons test was applied. *P less than .05; ***P less than .001; ****P less than .0001.

Figure 4.

Representative images of adenomas with normal and abundant GIPR expression. Normal GIPR (glucose-dependent insulinotropic polypeptide receptor) expression visualized by immunohistochemistry in a nonparadoxical patient by A) brightfield microscopy, and C) and D) fluorescence microscopy. Abundant GIPR expression visualized in a responder by E, brightfield microscopy, and G and H, fluorescence microscopy. B) and F) Growth hormone (GH) visualization in red; C) and G) GIPR visualization in green; and D) and H) merged red and green staining showing GIPR colocalization with GH in the somatotrophs. All images were acquired with 40× magnification. Scale bars (white), 20 μm.

Figure 5.

Effect of GIP on GH secretion ex vivo and in vitro and on cAMP accumulation in vitro. A) GH secretion from somatotropinoma-derived primary cultures expressed as relative secretion from vehicle. Light blue bars, GIP 10 nM stimulation; dark blue bars, GIP 100 nM stimulation; black arrows, confirmed paradoxical GH secretion to OGTT. B) GIP dose response of cAMP accumulation in hGIPR transiently transfected GH3 cells. Results are expressed as relative cAMP accumulation compared to the maximal GIP-induced cAMP response. Solid line, closed circles: hGIPR vector; dashed line, open squares: empty pCMV vector. C) GIP and GIP + GIPR antagonist-induced GH secretion from hGIPR transiently transfected GH3 cells expressed as relative secretion with vehicle. Data are presented as mean ± SEM. One-way analysis of variance with Holm-Šídák multiple comparison test was used to assess statistical significance in the GH3 cell line, and an unpaired t test to assess statistical significance in primary cultures. *P less than .05 increase from vehicle. ^ΨP less than .05 decrease from GIP-stimulated cells.