Colostral and milk insulin-like growth factors and related substances: Mammary gland and neonatal (intestinal and systemic) targets

The identification of hormones and regulatory factors in colostrum and milk has led to intensive investigations on their roles in the development and maintenance of the mammary and neonatal tissues. Insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) in transgenic mice influence mammary biology gland towards the end of lactation. In the bovine, IGFBP-3 is the major IGFBP in mammary secretions. In addition to binding IGFs, IGFBP-3 also binds to lactoferrin (Lf). Secreted IGFBP-3 re-enters mammary epithelial cells and with the presence of a nuclear localization sequence, IGFBP-3 and Lf enter the nucleus. Nuclear IGFBP-3 affects apoptotic signaling through the retinoic-x-receptors, while Lf affects apoptotic events through unknown mechanisms. Such interactions likely influence mammary development and involution. Furthermore, ingested colostral bioactive factors can exert regulatory functions in neonates. Intestinal receptors for IGFs and insulin are modified by age and/or diet. Feeding IGF-I had no effect, but colostrum extracts had small intestinal effects (stimulation of proliferation and villus size), suggesting that several factors, rather than one single bioactive factor were responsible. Systemic changes of metabolic and endocrine profiles in neonates depend on composition, amounts, time and duration of feeding colostrum. Early postnatal colostrum intake is not only important for the provision and absorption of immunoglobulins. Thus, in neonatal calves the lack of colostrum intake during the first 24 h after birth results in a low immunoglobulin G, β-carotene and Vitamin A status that persists for weeks and plasma patterns of fatty acids, essential amino acids and the glutamine/glutamate ratios are affected. In calves oral administration of IGF-I had no and feeding of colostrum whey extracts had only minor effects on metabolic and endocrine traits. Thus, mammary secretions influence regulatory functions of mammary and neonatal tissues.