Origin and fate of erythropoietin in human milk

Erythropoietin (Epo) is a normal constituent of human milk, but the origin and fate of this cytokine in milk are not known. Regarding its origin, we h ypothesized that cells of the mammary gland secrete Epo into milk actively and, therefore, that concentrations in milk do not correlate with concentra tions in serum. Regarding its fate, we hypothesized that Epo concentrations in milk change with time postpartum and that Epo in milk is protected from digestion in the neonatal gastrointestinal tract. To address these issues, we measured Epo concentrations in 103 milk samples (ELISA), 55 of which we re paired with serum. Mammary duct epithelial cells were evaluated as a sou rce of Epo by breast tissue immunohistochemistry and by cell culture. Circu lating and milk Epo were compared by Western analysis to detect size differ ences, possibly reflecting differences in processing. Epo stability in simu lated conditions of digestion was evaluated. We observed that milk Epo conc entrations increase as a function of duration of breast-feeding and have a negative correlation with serum Epo or milk protein concentration. Mammary duct epithelial cells from breast biopsies of lactating women had marked im munoreactivity to Epo, but such activity was minimal to absent in nonlactat ing breast tissue. Further evidence that mammary duct epithelia produce Epo was obtained by observing Epo mRNA and protein expression in cultured huma n mammary epithelial cells. The molecular size of Epo in milk and serum is identical. Recombinant Epo added to human milk or commercial infant formula s was relatively stable in conditions that simulate gastric and small intes tinal conditions of newborn infants; however, recombinant Epo added to D5W was not protected from digestion. We conclude that Epo concentrations in mi lk increase as a function of the duration of breast feeding, that Epo is ac tively secreted into human milk by mammary duct epithelia, and that the Epo within milk is largely protected from digestion.