In species expressing BSSL in the lactating mammary glands, the milk-borne enzyme is a prerequisite for efficient digestion of milk fat during the suckling period (21
). Therefore, we hypothesized that expression of endogenous pancreatic lipases during early infancy differ between species having BSSL supplied with the milk and species that do not and tested this hypothesis by comparing ontogeny of pancreatic lipases in one species that has BSSL supplied with the milk (the mouse) and one that does not (the rat).
BSSL mRNA and protein was detected in the pancreas of mice and rats from postnatal d 1. However, the temporal pattern of expression differed between the two species. In mice, expression increased significantly after the first 4 d of life. Both mRNA and protein levels reached a peak value at approximately d 7; thereafter, it declined to adult level around weaning. In contrast, BSSL in rats was constitutively expressed from birth into adulthood. It is tempting to speculate that the BSSL expression in the mouse (but not in rat) is insufficient at birth and during the first postnatal days, but that the milk-borne BSSL is able to compensate for the slower ontogeny of pancreatic BSSL expression in suckling mouse pups. This hypothesis is supported by a previous study suggesting a developmental window during the first week of life when the supply of milk BSSL is most critical (21
). BSSL heterozygous pups nursed by BSSL-deficient dams developed disruption of the epithelium due to massive accumulation of undigested lipids in the distal part of the small intestine. This problem was most prominent before postnatal d 5, which, according to our present data, coincides with ontogeny of pancreatic BSSL.
The gradual induction of BSSL mRNA in mouse pancreas during the first week of life may be a response to the onset of feeding. A similar increase of mRNA expression immediately after birth was previously reported for PLRP1, PLRP2, and colipase in rat pancreas (5
). Our present study confirms that PLRP2 mRNA and protein levels are higher at d 1–7 compared with later in life, although we did not observe any additional induction after birth. Exactly what triggers the response to onset of feeding is not known, but expression of pancreatic lipases, including BSSL, PLRP2, and PL, are all regulated by the amount and/or type of dietary fat (26
). However, the response to dietary fat differs between genes encoding the different pancreatic lipases. In suckling rats, PL and PLRP2 transcription was shown to be anticoordinately regulated by amount of dietary polyunsaturated fatty acids (PUFAs). High levels of PUFAs significantly down-regulated the PLRP2 mRNA expression during the preweaning phase (28
). The BSSL gene expression is not regulated by dietary triglycerides, but by high cholesterol levels in the diet (29
Leptin, a hormone involved in regulation of food intake and energy metabolism, was recently shown to decrease PL and PLRP1 transcription and to increase PLRP2 transcription in the exocrine pancreatic cell line AR42J (30
). Because leptin is present in human milk (31
), serum concentration is higher in breast-fed than in formula-fed infants (32
) and lower in colostrum compared with mature milk, in which it decreases with time of lactation (33
), it is tempting to speculate that a high concentration of milk leptin during the initial period of suckling (except the first days) increases serum leptin levels and induces PLRP2 and suppresses PL transcription. However, further studies are needed to explore the effect of leptin on pancreatic lipases in vivo
and whether leptin affects BSSL expression.
It is an intriguing question why BSSL and PLRP2, two nonspecific lipases with broad substrate specificities, seem to be the key players in fat digestion as long as milk is the main food. During infancy, the intraluminal level of bile salt is low compared with later in life (4
), and we know from earlier studies that milk fat globules are a poor substrate for PL even in the presence of colipase and bile salts (34
). Conversely, the triglyceride activity of human PLRP2 is inhibited by bile salts and addition of colipase does not completely restore the activity (10
). Due to the comparatively low bile salt concentration, this may be of less relevance in the newborn. It is tempting to speculate that the concerted action of gastric lipase, PLRP2, and BSSL are ideally suited to efficiently digest human milk fat globules under the intraluminal conditions of infancy. As the digestive system matures and expression of PL and PLA2 increases, the role of BSSL and PLRP2 may change. After weaning, BSSL and PLRP2 may function as complementary lipases to increase the efficiency of dietary fat hydrolysis and absorption. This hypothesis is supported by previous studies on genetically modified mice, in which BSSL and/or PLRP2 were suggested to compensate for the lack of PL and PLA2 activity in catalyzing triglyceride and phospholipid digestion, respectively (36
). BSSL and PLRP2 could also have more specific functions after weaning. For example, the main physiologic function of PLRP2 has been suggested to be to hydrolyze galactolipids (10
), a substrate also hydrolyzed by BSSL. BSSL has been shown to play a primary role in absorption of cholesterol esters (38
). Compared with infant formula, human milk is rich in cholesterol and the endogenous cholesterol synthesis rate is lower in breast-fed than in formula-fed infants (40
). When infants are fed an experimental formula supplemented with cholesterol to meet the concentration of human milk, the synthesis rate is reduced but not to the level of breast-fed infants (41
). This suggests that factors other than the cholesterol concentration in milk affect cholesterol metabolism in breast-fed infants. Duration of breast-feeding is negatively associated to serum cholesterol levels later in life, and a programming effect on later cholesterol metabolism has been proposed (42
). It is possible that milk-borne BSSL could affect the cholesterol metabolism and influence the mechanism underlying the programming effect (39
To our knowledge, this is the first report showing the temporal, developmental pattern of BSSL expression in the pancreas of mice and rats. We also confirmed and extended previous studies in rodents and humans showing a divergent temporal expression of PL and PLRP2 during the suckling period (5
). Our data clearly support and add to previous findings that it is BSSL together with PLRP2 that are the dominating pancreatic lipases during the first weeks of life. It is not until weaning (at approximately d 16–20) that significant expression of classic PL and PLA2 are induced. Further studies will focus on the concerted action of lipolytic enzymes under conditions resembling those in vivo
of preterm infants. The developmental pattern of expression described in this study may have important consequences for nutritional therapy of preterm infants and newborns.