Testes-specific transgene expression in insulin-like growth factor-I transgenic mice

Insulin-like growth factor-1 [IGF-1] is a low molecular weight peptide that mediates the cell proliferating actions of growth hormone. Evidence exists indicating that IGF-1 is produced by various cell types and this growth fa ctor has been implicated in a variety of reproductive processes. To investi gate the effect of IGF-1 over-expression on reproductive systems, we genera ted three independent lines of transgenic mice harbouring a human IGF-1 cDN A (hIGF-1) under the control of a Cytomegalovirus immediate early (CMV) pro moter. The CMV promoter was used in an attempt to direct expression of IGF- 1 into a variety of tissues both reproductive and non-reproductive. Yet exp ression of the foreign hIGF-1 gene, determined by Northern blot, was found to occur only in the testicular tissues of the male mice, apparently due to methylation of the transgene in all the tissues tested except the testes, which demonstrate transgene hypomethylation. Evaluation of the transgene ex pression during testicular development revealed that expression begins betw een 10 and 15 days of development, coinciding with the appearance of the zy gotene and pachytene primary spermatocytes during early spermatogenesis, th erefore indicating germ line expression of the transgene. Extensive study o f the CMV-hIGF-1 transgenic lines of mice has revealed that the effects of the transgene expression do not extend beyond the testicular tissues. No si gnificant differences (P > 0.05) in the IGF-1 serum levels, growth rates, o r testicular histology have been observed between transgenic and non-transg enic male siblings. The ability of transgenic males to produce offspring al so appears unaffected. Evaluation of the IGF binding protein (IGFBP) levels in the testicular tissues of CMV-hIGF-1 transgenic mice by Western ligand blot revealed an increase in the concentration of testicular proteins with molecular weights corresponding to IGFBP-2 and IGFBP-3. These results sugge st that the testicular over-expression of IGF-1 induces increased IGFBP loc alization in this tissue. Inhibition of IGF activity by the IGFBPs would ex plain the lack of a dramatic physiological effect in the CMV-hIGF-1 transge nic mice, despite the presence of elevated testicular IGF-1. The observatio n that testis specific IGF-1 overexpression induces localization of IGFBPs in this tissue confirms the existence of a well regulated testicular IGF sy stem and supports the convention that this growth factor plays an important role in testicular function. (C) 1999 Wiley-Liss, Inc.