Transgenic hypersecretion of des(1-3) human insulin-like growth factor I in mouse milk has limited effects on the gastrointestinal tract in suckling pups

We tested the hypothesis that chronic ingestion of increased concentrations of milk-borne des(1-3) human insulin-like growth factor-1 (hIGF-I) stimula tes gastrointestinal growth and development in suckling mice. We used a tra nsgenic mouse with targeted, lactation-dependent, overexpression of des(1-3 ) hIGF-I in the mammary gland (IGF). Pups were suckled (7 pups per litter) from birth by either IGF (n = 3-6 litters) or control (n = 3-5 litters) dam s. In IGF and control pups, we measured the growth (protein and DNA content ) and protein synthesis rate (H-3-phenylalanine incorporation) of gastroint estinal and visceral organs in 4-, 8-, 12-, 16- and 29-d-old pups. Des(1-3) hIGF-I in milk from IGF dams was 40-200-fold higher than mouse IGF in eith er IGF or control dams, but was not detected in the plasma of pups suckling IGF dams. Small intestinal weight, protein and DNA content at 8 and 16 d w ere greater in pups suckling IGF dams than control dams; protein synthesis was also greater in IGF pups at 8 d. Total intestinal lactase activity at 8 and 12 d of age tended to be higher (P < 0.10) in IGF than in control pups . Hypersecretion of des(1-3) hIGF-I in milk ingested by suckling mice pups had limited effects on the growth and maturation of the gastrointestinal tr act. Moreover, there was little evidence that milk-borne IGF-I is absorbed into the circulation and stimulates visceral organ growth. This study also demonstrates the feasibility of using mammary-specific transgenes to increa se the concentration of milk-borne growth factors to examine whether they a ffect the growth and development of the suckling neonate.