Premature delivery impacts the concentration of plasminogen activators and a plasminogen activator inhibitor and the plasmin activity in human milk

Abstract

Background and aims: Plasmin in human milk partially hydrolyzes milk proteins within the mammary gland and may enhance the hydrolysis of milk proteins within the infant's stomach. This study examined the effects of extremely preterm (EP)-, very preterm (VP)-, and term-delivery on plasmin activity and the concentrations of plasminogen activators [urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA)], plasminogen activator inhibitor type 1 (PAI-1) and the complexes of PAI-1/uPA and PAI-1/tPA in human milk.

Materials and methods: Human milk samples were collected from mothers who delivered extremely preterm infants [24-27 weeks gestational age (GA), n = 20], very preterm infants (28-32 weeks GA, n = 12), and term infants (38-39 weeks GA, n = 8) during 2-72 days postnatally. Plasmin activity was determined using fluorometric substrate assay, whereas concentrations of uPA, tPA, PAI-1, the PAI-1/uPA complex and the PAI-1/tPA complex were quantified by ELISA.

Results: Plasmin activity, uPA and tPA were detected in all human milk samples, PAI-1 and the PAI-1/uPA complex were present in 42.5 and 32.5% of milk samples, respectively, and the PAI-1/tPA complex was not detected. Plasmin activity was correlated negatively with postnatal age and postmenstrual age (PMA) in the VP group and positively with postnatal age in the term group. uPA and tPA concentrations decreased with increasing postnatal age in both EP and VP groups but did not correlate in the term group. uPA concentration was correlated positively with GA in the VP group and tended to be elevated with increasing GA in the combined three groups. In contrast, tPA concentrations were correlated negatively with GA and PMA in the combined three groups (P < 0.008) and with PMA in the EP and VP groups. PAI-1 concentration tended to be correlated positively with postnatal age in the combined three groups. No correlation was detected with the PAI-1/uPA complex.

Conclusion: Premature delivery impacted the plasmin activity and the concentrations of uPA, tPA, and PAI-1 in human milk. Whether these changes in milk plasminogen activators and inhibitors have a role in balancing the proteolytic digestion of premature infants remains to be investigated.

Keywords: breastfeeding; digestive system; enzymes; immaturity; lactation; newborns; premature neonates; proteases.

FIGURE 1

Human milk’s plasmin system is composed of plasminogen (zymogen), plasmin (the active enzyme), urokinase-type plasminogen activator (uPA), and tissue-type plasminogen activator (tPA) (plasminogen activators), and human plasminogen activator inhibitor type 1 (PAI-1) (plasminogen activator inhibitor). Plasminogen (the inactive zymogen) must be cleaved by uPA or tPA to become the active plasmin. The PAI-1 in human milk can block the activators’ (both tPA and uPA) conversion of plasminogen to active plasmin.

FIGURE 2

The levels of plasmin system components in milk from extremely preterm-(EP), very preterm-(VP), and term-delivering mothers across postnatal age at sampling. (A) Plasmin activity across postnatal age; (B) human urokinase-type plasminogen activator (uPA) across postnatal age; (C) human tissue-type plasminogen activator (tPA) across postnatal age; (D) human plasminogen activator inhibitor type 1 (PAI-1) across postnatal age in human milk from EP (n = 20), VP (n = 12), term groups (n = 8), and combined groups (overall, n = 40). r, Spearman correlation.

FIGURE 3

The levels of plasmin system components in milk from extremely preterm-(EP), very preterm-(VP), and term-delivering mothers across gestational age (GA) at birth. (A) Plasmin activity across GA; (B) human urokinase-type plasminogen activator (uPA) across GA; (C) human tissue-type plasminogen activator (tPA) across GA; (D) human plasminogen activator inhibitor type 1 (PAI-1) across GA in human milk from EP (n = 20), VP (n = 12), term groups (n = 8), and combined groups (overall, n = 40). r, Spearman correlation.

FIGURE 4

The levels of plasmin system components in milk from extremely preterm-(EP), very preterm-(VP), and term-delivering mothers across postmenstrual age (PMA). (A) Plasmin activity across PMA; (B) human urokinase-type plasminogen activator (uPA) across PMA; (C) human tissue-type plasminogen activator (tPA) across PMA; (D) human plasminogen activator inhibitor type 1 (PAI-1) across PMA in human milk from EP (n = 20), VP (n = 12), term groups (n = 8), and combined groups (overall, n = 40). r, Spearman correlation.