LIVER-SPECIFIC EXPRESSION OF HUMAN INSULIN-LIKE-GROWTH-FACTOR BINDINGPROTEIN-1 IN TRANSGENIC MICE - REPERCUSSIONS ON REPRODUCTION, ANTENATAL-MORTALITY AND PERINATAL-MORTALITY AND POSTNATAL-GROWTH

Study of the in vivo functions of the insulin-like growth factor bindi ng proteins (IGFBPs) is complicated by their variety (six molecular sp ecies) and the differences in their expression related to tissue of or igin and stage of development. To investigate the physiological role o f IGFBP-1 in the bloodstream, we induced hepatic overexpression of IGF BP-1 in transgenic mice, placing human IGFBP-1 (hIGFBP-1) cDNA under t he control of the alpha 1-antitrypsin promoter so as to obtain liver-s pecific 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. hI GFBP-1 was detected by western immunoblotting in the plasma of transge nic 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, r espectively. In the latter, IGFBP-1 levels were distinctly higher than in heterozygotes (2.99 +/- 0.39 ng/ml), P < 0.0001. These levels rema ined stable in each given animal and did not change with age. Plasma c oncentrations of IGF-I measured in line 149 exhibited the well-known p rofile of an increase from birth up to puberty. Values for heterozygot es 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 3 70 +/- 75 ng/ml, P = 0.001. In heterozygous and homozygous adults, the re 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 grow th retardation within the first weeks after birth, which was marked in homozygous males and females (P < 0.001) but also present in heterozy gous males (P = 0.002), indicating some relationship with transgene ex pression. In addition, body weight in adult mice was negatively correl ated to plasma concentrations of IGFBP-1 (r = 0.7,P < 0.0001). Reprodu ctive function also appeared to be severely affected, especially in ho mozygous females: mating that failed to result in pregnancy in half of the homozygous females crossed with nontransgenic males, suggestive o f impaired fertilization or implantation; interrupted or prolonged pre gnancies with fetal and neonatal death. Litter size was reduced in tra nsgenic females (by about half in homozygotes) and in nontransgenic fe males mated with homozygous males, resulting from pre- or neonatal mor tality. Moreover, deaths occurred within the first 5 days of life, wit h an incidence of approximately 50% in the litters of homozygous femal es, 12-18% among heterozygotes mated with nontransgenic or heterozygou s males, respectively, and 30% among those mated with homozygous males . These results, suggesting that fetal transgene expression largely ac counted for ante- and perinatal mortality, were confirmed by the predo minance 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 disord ers remain to be determined, our results indicate that permanent and u ncontrolled hepatic expression of IGFBP-1, even at low levels, affects fertility in females and both ante- and postnatal development.