Effective treatment of experimental human non-Hodgkin's lymphomas with antagonists of growth hormone-releasing hormone

Abstract

Antagonists of growth hormone-releasing hormone (GHRH) were shown to inhibit the growth of various cancers. We investigated the antitumor activity and the mechanism of action of GHRH antagonists in human non-Hodgkin's lymphomas (NHL). Nude mice bearing xenografts of RL and HT human NHL were treated with GHRH antagonists MZ-5-156 and MZ-J-7-138 at a dose of 40 microg twice daily. The concentrations of serum IGF-1 and GHRH, bFGF, and VEGF in tumor tissue were measured by radioimmunoassays. Expression of GHRH and splice variant 1 of the GHRH receptor in both cell lines was examined by RT-PCR. The effects of MZ-5-156, MZ-J-7-138 and GHRH on cell proliferation were evaluated in vitro. Treatment with MZ-5-156 and MZ-J-7-138 significantly (P < 0.05) inhibited the growth of RL and HT tumors by 59.9-73.9%. High-affinity binding sites for GHRH and mRNA for GHRH and splice variant-1 of the GHRH receptors were found on RL and HT tumors. RL and HT cells contained GHRH peptide, and their growth in vitro was significantly inhibited by both antagonists. IGF-I levels in serum of mice were significantly decreased by antagonist MZ-5-156. Therapy with GHRH antagonists also significantly reduced tumoral bFGF, whereas VEGF levels were not suppressed. Our findings suggest that GHRH antagonists inhibit the growth of RL and HT lymphomas by direct effects mediated by tumoral receptors for GHRH. GHRH antagonists could offer a new therapeutic modality for the management of advanced NHL.

Fig. 1.

Mean tumor volumes in nude mice bearing RL human NHL xenografts during treatment with GHRH-antagonists MZ-5-156 and MZ-J-7-138 at a daily dose of 2 × 40 μg. Vertical bars indicate SE (^**, P < 0.01).

Fig. 2.

Mean tumor volumes in nude mice bearing HT human NHL xenografts during treatment with antagonists MZ-5-156 and MZ-J-7-138 at a daily dose of 2 × 40 μg. Vertical bars indicate SE (^**, P < 0.01).

Fig. 3.

Effect of GHRH antagonists MZ-5-156 and MZ-J-7-138 on the proliferation of RL (A) and HT (B) human NHL cell lines. These experiments are representative of three independent experiments per cell line, each performed in octuplicate wells. The relative cell number in treated and control wells was determined by crystal violet staining and expressed as percent T/C values, where T is the absorbance of treated cultures and C is absorbance of control cultures. (^*, P < 0.05.)

Fig. 4.

RT-PCR analysis of mRNA expression for GHRH receptor SV1 (A Top), GHRH (A Middle), β-actin (A Bottom), and IGF-IR (B). (A) RT-PCR was performed in two samples of each human NHL tumor tissues HT (lanes 2 and 3) and RL (lanes 4 and 5). As positive controls, amplifications of SV1 mRNA from human prostate cancer cell line LNCaP (Top, lane 1) and amplifications of GHRH and beta-actin mRNA from human pituitary tissue (Middle and Bottom, lane 1) were used. DNA molecular weight markers are shown under M. All PCR products resulted in the expected sizes of 523 bp for GHRH receptor SV1, 150 bp for GHRH, and 140 bp for β-actin. No amplification was detected in the negative control (lane 6). (B) Expected PCR-products of 446 bp for IGF-IR were detected in the RL (lane 1) and HT (lane 2) NHL tumors. No amplification was detected in the negative control (lane 3).