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.