The insulin-like growth factor binding proteins (IGFBPs) constitute a family of proteins which modulate the activity of the IGFs. IGF-I is an important survival factor for mammary epithelial cells and overexpression of IGF-I in the mammary glands of lactating mice can delay involution. We have shown that the mouse, rat and pig mammary gland produces IGFBP-5 during the early stage of mammary involution and we proposed that the function of this binding protein is to inhibit IGF-mediated cell survival and thereby to induce apoptotic cell death. Prolactin is a potent inhibitor of IGFBP-5 expression and we believe this is an important aspect of its anti-apoptotic effect. Subsequently we have shown that over expression of IGFBP-5 in the mammary gland of lactating mice leads to impaired milk production, with a reduction in the number of mammary epithelial cells. The precise mechanism by which this occurs is currently under investigation. In ovine and bovine mammary gland IGFBP-5 is not yet implicated although IGFBP-3 concentrations increase in cows during mammary involution and it is possible that IGFBP-3 serves this function in ruminants. In addition, the IGFBPs have been shown to interact with several proteins present in milk including αs2-casein, lactoferrin and transferrin. These interactions implicate the IGFBPs in the regulation of plasminogen activation since plasminogen and t-PA also bind to αs2-casein. This is a key process in tissue remodelling although, at the moment, no unifying hypothesis exists with regard to the nature of these interactions. Some studies have suggested that these interactions of IGFBPs with milk proteins lead to increased growth factor availability whereas others suggest that this serves to function as a means of sequestering IGF-I from its receptor. Whatever their role, it is clear that the IGFBPs play an important role in mammary gland function.

We examined the effects of GH and prolactin deficiency upon milk production, apoptosis and IGFBP production by the mammary gland. GH deficiency produced a 15% reduction in milk yield, prolactin a 50% reduction and combined prolactin- and GH-deficiency an 85% reduction in milk production. Litter removal led to complete inhibition of milk synthesis within 24 h owing in large part to milk accumulation. GH- and prolactin-deficiency also led to significant loss of mammary cells within 48 h and this was owing at least in part to apoptosis as judged by the appearance of characteristic DNA ladders. Prolactin replacement therapy could prevent all of these changes whilst GH was partially effective. The effects of GH are believed to be mediated via IGF-I, however, we were unable to mimic the effects of GH with IGF-I, IGF-II or a combination of IGF-I, IGF-II and IGFBP-3. We hypothesized that the cell-survival effects of exogenous IGFs might be blocked by an inhibitory IGFBP produced by the gland. Indeed the involuting mammary gland produces large concentrations of an IGFBP which Northern blotting identified as IGFBP-5. There was also a small increase in IGFBP-4 mRNA expression. Both appear to be produced by the secretory epithelial cells as judged by in situ hybridization. Preliminary studies using mouse “mammosphere” cultures suggest that they will be useful for investigating a potential causal relationship between IGFBP-5 synthesis and apoptosis.