Enzymes of the taurine biosynthetic pathway are expressed in rat mammary gland

Abstract

Taurine is the most abundant free amino acid in the body and is present at high concentrations during development and in the early milk. It is synthesized from cysteine via oxidation of cysteine to cysteinesulfinate by the enzyme cysteine dioxygenase (CDO), followed by the decarboxylation of cysteinesulfinate to hypotaurine, catalyzed by cysteine sulfinic acid decarboxylase (CSAD). To determine whether the taurine biosynthetic pathway is present in mammary gland and whether it is differentially expressed during pregnancy and lactation, and also to further explore the possible regulation of hepatic taurine synthesis during pregnancy and lactation, we measured mammary and hepatic CDO and CSAD mRNA and protein concentrations and tissue, plasma and milk taurine concentrations. CDO and CSAD mRNA and protein were expressed in mammary gland and liver regardless of physiological state. Immunohistochemistry demonstrated the expression of CDO in ductal cells of pregnant rats, but not in other mammary epithelial cells or in ductal cells of nonpregnant rats. CDO was also present in stromal adipocytes in mammary glands of both pregnant and nonpregnant rats. Our findings support an upregulation of taurine synthetic capacity in the mammary gland of pregnant rats, based on mammary taurine and hypotaurine concentrations and the intense immunohistochemical staining for CDO in ductal cells of pregnant rats. Hepatic taurine synthetic capacity, particularly CSAD, and taurine concentrations were highest in rats during the early stages of lactation, suggesting the liver may also play a role in the synthesis of taurine to support lactation or repletion of maternal reserves.

FIGURE 1

Western blot of CDO and CSAD in liver and mammary gland of rats during pregnancy and lactation. A volume of tissue lysate containing an equal amount of total soluble protein was loaded per lane. NP, nonpregnant/nulliparous.

FIGURE 2

Expression of CDO and CSAD mRNA and protein in rat mammary gland during pregnancy and lactation. Relative abundance of CDO mRNA (A) and CSAD mRNA (B) per unit of total RNA, as determined by Northern blot. Relative abundance of CDO (C) and of CSAD (D) per unit total soluble protein, as determined by Western blot. Each bar represents the mean ± SE (n = 4-5 rats). NP, nonpregnant/nulliparous. Bars not denoted by the same letter differ, P ≤ 0.05.

FIGURE 3

Concentration of taurine in rat milk (A) and of taurine (B) and hypotaurine (C) in rat mammary gland during pregnancy and lactation. Each bar represents the mean ± SE (n = 4-5 rats). Hypotaurine was not detected in milk samples. NP, nonpregnant/nulliparous. Bars not denoted by the same letter differ, P ≤ 0.05.

FIGURE 4

Expression of CDO and CSAD mRNA and protein in rat liver during pregnancy and lactation. Relative abundance of CDO mRNA (A) and CSAD mRNA (B) per unit of total RNA, as determined by Northern blot. Relative abundance of CDO (C) and of CSAD (D) per unit of total soluble protein, as determined by Western blot. Each bar represents the mean ± SE (n = 4-5 rats). NP, nonpregnant/nulliparous. Bars not denoted by the same letter differ, P ≤ 0.05.

FIGURE 5

Concentration of taurine in rat plasma (A) and liver (B) during pregnancy and lactation. Each bar represents the mean ± SE (n = 4-5 rats). NP, nonpregnant/nulliparous. Bars not denoted by the same letter differ, P ≤ 0.05.

FIGURE 6

Immunohistochemical localization of CDO. Mammary gland sections immunostained with anti-CDO (shown in red) and counterstained with Gill’s hematoxylin (shown in blue) are shown at 20× magnification. (A and B) Mammary gland of a nonpregnant rat. (A) Staining for CDO is detected in the cytoplasm and some nuclei of the adipocytes (indicated by arrows) in the stroma. No stain can be seen in the ducts. (B) The IgG control shows no specific staining in ductal epithelial cells (d) or adipocytes. (C and D) Mammary gland of a d 16.5 pregnant rat. (C) Staining for CDO is detected in the ductal epithelial cells with staining concentrated on the apical side of these cells. Little staining is observed in the alveolar structures (Av). Staining is found in the cytoplasm and some nuclei of the adipocytes. Note that the cytoplasm and nuclei of adipocytes are displaced toward the plasma membrane by the large central fat globule. (D) IgG control shows no specific staining in the ducts, alveolar structure or adipocytes. Representative sections of mammary adipoctyes from NP (E) or pregnant (F) rats stained for CDO or IgG control (G) from pregnant rats show staining in the cytoplasm and some nuclei of the adipocytes inside the plasma membrane, displaced by the fat droplet. The photos in panels E, F, and G were taken at 20× magnification, cropped and enlarged.