Liver-specific expression of human Insulin-like Growth Factor Binding Protein-1 (IGFBP-1) in transgenic mice: repercussions on reproduction, ante- and peri-natal mortality and post-natal growth

Study of the in vivo functions of the insulin-like growth factor binding proteins (IGFBPs) is complicated by their variety (six molecular species) and the differences in their expression related to tissue of origin and stage of development. To investigate the physiological role of IGFBP-1 in the bloodstream, we induced hepatic overexpression of IGFBP-1 in transgenic mice, placing human IGFBP-1 (hIGFBP-1) cDNA under the control of the alpha1-antitrypsin promoter so as to obtain liver-specific expression. Five transgenic founder mice were raised, only two of which (lines 124 and 149) produced transgenic offspring. Northern blotting revealed transgene expression exclusively in the liver during fetal life and unchanged through to adulthood, whereas expression of the endogenous gene was undetectable beyond 10-15 days postnatally. hIGFBP-1 was detected by western immunoblotting in the plasma of transgenic mice and IRMAs yielded mean concentrations of 2.41 +/- 0.33 ng/ml and 13.69 +/- 1.42 ng/ml in homozygous animals of lines 124 and 149, respectively. In the latter, IGFBP-1 levels were distinctly higher than in heterozygotes (2.99 +/- 0.39 ng/ml), P < 0.0001. These levels remained stable in each given animal and did not change with age. Plasma concentrations of IGF-I measured in line 149 exhibited the well-known profile of an increase from birth up to puberty. Values for heterozygotes were similar to those for wild-type mice, with adult levels (544 +/- 98 ng/ml) slightly below those of controls (630 +/- 56 ng/ml), P < 0.05. In homozygotes they were distinctly lower, with adult levels of 370 +/- 75 ng/ml, P = 0.001. In heterozygous and homozygous adults, there was a negative correlation between IGF-I and IGFBP-1 concentrations (r = 0.8, P < 0.0001), suggesting a link between transgene expression and IGF-I levels. Study of body weight gain in line 149 revealed growth retardation within the first weeks after birth, which was marked in homozygous males and females (P < 0.001) but also present in heterozygous males (P = 0.002), indicating some relationship with transgene expression. In addition, body weight in adult mice was negatively correlated to plasma concentrations of IGFBP-1 (r = 0.7, P < 0.0001). Reproductive function also appeared to be severely affected, especially in homozygous females: mating that failed to result in pregnancy in half of the homozygous females crossed with nontransgenic males, suggestive of impaired fertilization or implantation; interrupted or prolonged pregnancies with fetal and neonatal death. Litter size was reduced in transgenic females (by about half in homozygotes) and in nontransgenic females mated with homozygous males, resulting from pre- or neonatal mortality. Moreover, deaths occurred within the first 5 days of life, with an incidence of approximately 50% in the litters of homozygous females, 12-18% among heterozygotes mated with nontransgenic or heterozygous males, respectively, and 30% among those mated with homozygous males. These results, suggesting that fetal transgene expression largely accounted for ante- and perinatal mortality, were confirmed by the predominance of homozygotes among those that could be analyzed genetically. Similarly impaired reproductive function was seen in line 124, but to a lesser degree. Although the mechanisms responsible for these disorders remain to be determined, our results indicate that permanent and uncontrolled hepatic expression of IGFBP-1, even at low levels, affects fertility in females and both ante- and postnatal development.