Growth hormone-releasing hormone antagonist MZ-5-156 inhibits growth of DU-145 human androgen-independent prostate carcinoma in nude mice and suppresses the levels and mRNA expression of insulin-like growth factor II in tumors

Abstract

Insulin-like growth factors I and II (IGF-I and -II) are potent mitogens for various cancers, including carcinoma of the prostate. In several experimental cancers, treatment with antagonists of growth hormone-releasing hormone (GH-RH) produces a reduction in IGF-I and -II, concomitant to inhibition of tumor growth. To investigate the mechanisms involved, we treated male nude mice bearing xenografts of DU-145 human androgen-independent prostate cancer for 8 weeks with potent GH-RH antagonist MZ-5-156 at a dose of 20 microg/animal s.c. twice a day. Tumor growth, serum and tumor levels of IGF-I and -II, and the mRNA expression of IGF-I and -II in tumors were evaluated. After 8 weeks of therapy, final volume and weight of DU-145 tumors in mice treated with MZ-5-156 were significantly (P < 0.01) decreased compared with controls, and serum IGF-I showed a significant reduction. Treatment of nude mice bearing DU-145 xenografts with MZ-5-156 also significantly (P < 0.01) diminished by 77% the levels of IGF-II in tumor tissue compared with controls, but did not affect the concentration of IGF-I. Reverse transcription-PCR analyses revealed a high expression of IGF-II mRNA in DU-145 tumors. Treatment with GH-RH antagonist MZ-5-156 decreased the expression of IGF-II mRNA by 58% (P < 0.01) as compared with controls. Our work suggests that GH-RH antagonist MZ-5-156 may inhibit the growth of DU-145 human androgen-independent prostate cancers through a reduction in the production and mRNA expression of IGF-II by the tumor tissue. These findings extend our observations on the mechanism of action of GH-RH antagonists and may explain how GH-RH antagonists inhibit tumor growth.

Figure 1

Tumor volume in athymic mice bearing subcutaneously transplanted DU-145 human androgen-independent prostate cancers after treatment with the GH-RH antagonist MZ-5-156 administered s.c. at a dose of 20 μg per animal twice a day (b.i.d.). Treatment was started when tumors measured approximately 40–50 mm³ and lasted for 8 weeks. Vertical lines indicate the SEM. ∗, P < 0.05; ∗∗, P < 0.01 versus control.

Figure 2

RT-PCR analysis of human IGF-II (a) and β-actin (b) mRNA expression in DU-145 prostate tumors. PCR products were separated by 1.8% agarose gel electrophoresis and stained with ethidium bromide. The sizes of expected PCR products were 538 and 518 bp for hIGF-II and hβ-actin, respectively. Lanes: M (molecular weight marker), MX174 HaeIII digest; 1, negative control; 2–6, samples from untreated animals; 7–11, tumor samples from animals treated with GH-RH antagonist MZ-5-156.

Figure 3

Southern blot analysis of cDNA for human IGF-II (a) and β-actin (b) obtained after RT-PCR. The PCR products were separated by 1.8% agarose gel electrophoresis and transferred to Hybond N⁺ membranes. Hybridizations were performed with cDNA probes specific for hIGF-II or hβ-actin. The blots were washed under stringent conditions and exposed to autoradiography. Numbering is the same as in Fig. 2.

Figure 4

Effect of GH-RH antagonist MZ-5-156 on the expression of mRNA for IGF-II in DU-145 human androgen-independent prostate cancers. After densitometry, the levels of mRNA for hIGF-II were standardized according to the levels of hβ-actin mRNA and are expressed as a percentage of control value. ∗∗, P < 0.01 versus control.