Hormonal Control of Glucose Transporter Expression in Rat (RFH) and Human Fetal Hepatocytes (HFH) • 1542

Abstract

During the fetal period, hepatocytes are exposed to relatively low constant levels of glucose. The fetal to neonatal transition is characterized by remarkable changes in hepatic energy metabolism because of adaptation to intermittent oral feedings. Glucose transport, although not rate-limiting, is critical to the continuing supply of glucose to hepatocytes for storage and energy needs. Freshly isolated and cultured HFH and RFH express large amounts of both the low Km GLUT1 and the high Km GLUT2. Because cultured HFH, in contrast to RFH, do not downregulate GLUT1 and upregulate GLUT2 mRNA abundance in response to glucose, the reason for the change in GLUT abundance after birth in HFH is unclear. To define the mechanism by which hepatocytes change to the adult pattern of GLUT expression (largely GLUT2), we examined hormonal and substrate control of these transporters. RFH were isolated at 20 d gestation (n=4) and HFH were isolated at 13-20 wk (n=8), cultured for 24 h in 1 mM glucose DMEM with 10% FCS and then incubated for 4 h in stripped serum medium and 1 or 8.3 mM glucose with insulin (1μM), glucagon (1μM), or TGFβ (10 ng/mL). GLUT1 and GLUT2 mRNA was quantitated by densitometry and expressed relative to controls following Northern blot analysis of total RNA. In RFH and HFH, both insulin and glucagon increased GLUT1 mRNA abundance by 30-50% (p < 0.05), whereas TGFβ had no effect. In contrast, insulin enhanced GLUT2 mRNA abundance in RFH only in 8.3 mM glucose (p < 0.05) and in HFH in both 1 and 8.3 mM glucose (p < 0.05). Glucagon had no effect on GLUT2 mRNA abundance. TGFβ suppressed GLUT2 mRNA in RFH by 50% (p < 0.05), but not in HFH. We conclude that exogenous glucose and the capacity of the hepatocyte for endogenous glucose production (HFH are capable of gluconeogenesis) alter GLUT2 response to insulin. Glucose availability, as for other carbohydrate-regulated genes, is permissive for the action of insulin. The differential response to TGFβ remains to be explored. Table