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Human renal cell carcinoma expresses distinct binding sites for growth hormone-releasing hormone
G Halmos1, A V Schally, J L Varga
1Endocrine, Polypeptide and Cancer Institute, Veterans Affairs Medical Center and Department of Medicine, Tulane University School of Medicine, New Orleans, LA 70112-2699, USA.
Abstract:
Antagonists of growth hormone-releasing hormone (GHRH) inhibit the proliferation of various human cancers in vitro and in vivo by mechanisms that include apparent direct effects through specific binding sites expressed on tumors and that differ from pituitary human GHRH (hGHRH) receptors. In this study, GHRH antagonist JV-1-38 (20 microgram/day per animal s.c.) inhibited the growth of orthotopic CAKI-1 human renal cell carcinoma (RCC) by 83% and inhibited the development of metastases to lung and lymph nodes. Using ligand competition assays with (125)I-labeled GHRH antagonist JV-1-42, we demonstrated the presence of specific high-affinity (K(d) = 0.25 +/- 0.03 nM) binding sites for GHRH with a maximal binding capacity (B(max)) of 70.2 +/- 4.1 fmol/mg of membrane protein in CAKI-1 tumors. These receptors bind GHRH antagonists preferentially and display a lower affinity for hGHRH. The binding of (125)I-JV-1-42 is not inhibited by vasoactive intestinal peptide (VIP)-related peptides sharing structural homology with hGHRH. The receptors for GHRH antagonists on CAKI-1 tumors are distinct from binding sites detected with (125)I-VIP (K(d) = 0.89 +/- 0.14 nM; B(max) = 183.5 +/- 2.6 fmol/mg of protein) and also have different characteristics from GHRH receptors on rat pituitary as documented by the insignificant binding of [His(1),(125)I-Tyr(10), Nle(27)]hGHRH(1-32)NH(2). Reverse transcription-PCR revealed the expression of splice variants of hGHRH receptor in CAKI-1 RCC. Biodistribution studies demonstrate an in vivo uptake of (125)I-JV-1-42 by the RCC tumor tissue. The presence of specific receptor proteins that bind GHRH antagonists in CAKI-1 RCC supports the view that distinct binding sites that mediate the inhibitory effect of GHRH antagonists are present on various human cancers.
Insights
Growth hormone-releasing hormone (GHRH) antagonists significantly inhibit renal cell carcinoma (RCC) growth and metastasis. Specific GHRH antagonist binding sites on RCC tumors differ from pituitary receptors, indicating a potential targeted therapy.
Area of Science:
- Oncology
- Endocrinology
- Molecular Biology
Background:
- Growth hormone-releasing hormone (GHRH) antagonists show promise in inhibiting various human cancers.
- These antagonists exert effects through specific binding sites on tumors, distinct from pituitary GHRH receptors.
- Understanding these tumor-specific binding sites is crucial for developing targeted cancer therapies.
Purpose of the Study:
- To investigate the efficacy of GHRH antagonist JV-1-38 in inhibiting renal cell carcinoma (RCC) growth and metastasis.
- To characterize the specific binding sites for GHRH antagonists on CAKI-1 RCC tumors.
- To determine if these tumor-specific binding sites differ from known GHRH receptors and VIP binding sites.
Main Methods:
- Treatment of orthotopic CAKI-1 human RCC xenografts with GHRH antagonist JV-1-38.
- Ligand competition assays using radiolabeled GHRH antagonist JV-1-42 to assess binding affinity and capacity.
- Reverse transcription-PCR to identify GHRH receptor splice variants in CAKI-1 RCC.
- Biodistribution studies to evaluate in vivo uptake of radiolabeled JV-1-42 by tumor tissue.
Main Results:
- GHRH antagonist JV-1-38 inhibited CAKI-1 RCC tumor growth by 83% and reduced lung and lymph node metastases.
- Specific high-affinity binding sites (K(d) = 0.25 nM) for GHRH antagonists were identified in CAKI-1 tumors.
- These tumor binding sites preferentially bound GHRH antagonists over hGHRH and were distinct from VIP binding sites and pituitary GHRH receptors.
- Expression of hGHRH receptor splice variants was detected in CAKI-1 RCC, and in vivo studies confirmed tumor uptake of the GHRH antagonist.
Conclusions:
- GHRH antagonists effectively inhibit renal cell carcinoma growth and metastasis.
- CAKI-1 RCC tumors express distinct GHRH antagonist binding sites with high affinity.
- These findings support the potential of targeting these specific binding sites for novel RCC therapies.