The objective of this study was to investigate the possible role of UDP-glucose dehydrogenase (UGDH) in osteoarthritis (OA) and uncover whether, furthermore how interleukin-1beta (IL-1β) affects UGDH gene expression.
We show that both UTP and GlcN 6-P are inhibitors of the yeast UDPG pyrophosphorylase and therefore their concentrations must be regulated to obtain maximum yields of UDP-GlcN.
ß1,4 Galactosyltransferase (GT) is the enzymatic subunit of lactose synthase. It is a glycoprotein with a molecular weight varying from 35-60 kDa, depending upon the amount of glycosylation and the degree of proteolytic degradation. ß1,4 Galactosyltransferase in milk is proteolytically clipped removing the cytoplasmic and transmembrane domains. The GT found in milk has a molecular weight of 35-45 kDa. Without the presence of -lactalbumin, the enzyme functions in the Golgi during glycoprotein biosynthesis to add galactose to oligosaccharides with terminal -acetylglucosamine residues in a ß1-->4 linkage. The GT transfers galactose from the donor, UDP-galactose, to the terminal -acetylglucosamine (GlcNAc) acceptor on the oligosaccharise complex of glycosylated proteins. Galactosyltransferase is found in most tissues of the body. It is only found on the inner surface of the Golgi apparatus.
Bovine, murine and porcine -lactalbumin genes have all been sequenced and their proteins have molecular weights of about 14 kD. -Lactalbumin is produced at a concentration of approximately 0.2 to 1.8 mg/ml in the milk of most mammals. It is synthesized in the rough endoplasmic reticulum and passes to the Golgi complex where it interacts with GT. In the mammary Golgi apparatus, -LA combines with G and alters the substrate specificity of galactosyltransferase from -acetylglucosamine (GlcNAc) to glucose. This modified complex transfers galactose to glucose rather than to -acetylglucosamine.
Lactose cannot diffuse out of the Golgi and secretory vesicles, so water is drawn into the vesicles to balance the osmotic pressure. Since lactose synthase is necessary for production of lactose and the subsequent movement of water into the mammary secretory vesicles, it is critical in the lactational control and secretion of milk. The substrates for lactose synthase do not appear to be limiting since results of many experiments indicate that infusion of glucose, increasing blood glucose, does not increase milk production. Addition of more than 0.5 mg/ml glucose (low levels) to dispersed mammary cells also does not increase milk production. There is a suggestion that glucose transport across the plasma membrane may be limiting, but this is inconclusive. The expression of the glucose transporter is not up-regulated in cattle when exogenous growth hormone is administered. This suggests that glucose transport across the plasma membrane is not normally a limiting factor in milk production. The glucose transporter from the cell cytoplasm to the Golgi has a very large capacity and it seems unlikely that glucose transport across the Golgi membrane is limiting.
Since these two reactions could proceed under the same conditions, a one-pot synthesis of UDP-D-glucose with ATP regeneration was designed from easily available starting materials, and conversion up to 40% by HPLC peak integration was achieved given a reaction time of 4 h.
In conclusion, UGDH plays a critical role in the PG synthesis of articular chondrocytes, of which the expression is suppressed in advanced OA. Meanwhile, IL-1β suppresses UGDH gene expression through activating SAP/JNK and p38 MAPK pathways and subsequently modulating the gene expression of UGDH’s trans-regulators including Sp1, Sp3 and c-Krox. Accordingly, we speculate that IL-1β might be involved in the suppression of UGDH gene expression in OA, which would probably contribute to the OA pathogenesis.
Additional figure that shows relative protein expression of UDP-glucose dehydrogenase (UGDH) and Mankin score of human and rat cartilage.