In vitro growth-promoting activity of porcine mammary secretions: initial characterization and relationship to known peptide growth factors.
We have examined the in vitro growth-promoting properties and growth factor content of porcine mammary secretions. Defatted, porcine colostrum stimulated the proliferation of fibroblast and epithelial-like cell lines of diverse species origins in serum-free medium and cellular DNA synthesis (4- to 119-fold) as monitored by uptake of 3H-thymidine into DNA of quiescent cells in culture. Porcine milk, although mitogenic, had reduced activity when compared with colostrum on an equivalent-volume basis. Furthermore, the relative mitogenic activity of milk, although still detectable at 3 wk, continued to decline with length of the lactation period. Fractionation of pig colostrum on gel-filtration columns revealed multiple peaks of (AKR-2B) fibroblast mitogenic activity (208, 66 and 4.6 kdaltons) and a heterogenous profile of epithelial cell mitogenicity. Polyclonal antibodies (IgG) specific for murine epidermal growth factor (EGF; the major mitogen in human and murine milk) or human platelet-derived growth factor (PDGF) did not inhibit the mitogenic activity of pig colostrum or milk, demonstrating lack of antigenic relatedness between the contributing porcine factors and mEGF or hPDGF. Also, we were unable to demonstrate similarity of the small Mr colostral factor with EGF by use of EGF radioreceptor assay. These results identify porcine colostrum and milk as sources of potentially important in vitro growth-promoting factors. The enhanced expression of these factors in early mammary secretions suggests their possible in vivo involvement in mammary and neonatal tissue growth processes.