Regulation of Glucagon Receptor mRNA in Cultured Primary Rat Hepatocytes by Glucose and cAMP

Abstract

Glucagon, the pancreatic hormone secreted in response to hypoglycemia, is a key regulator of hepatic glucose production. Since the number of specific glucagon receptors expressed on the cell surface affects the sensitivity of the liver to glucagon, we have examined the regulation of glucagon receptor mRNA levels in cultured primary rat hepatocytes. By ribonuclease protection assay we have identified glucose and intracellular cAMP as regulators of glucagon receptor mRNA expression in cultured rat hepatocytes. We observed a concentration-dependent increase in glucagon receptor mRNA expression when hepatocytes were cultured in the presence of increasing glucose. A 2-fold induction in glucagon receptor mRNA levels was obtained in hepatocytes cultured for 24 h with 22.5 mM glucose as compared with 5.5 mM glucose. Factors such as 3-isobutyl-1-methylxanthine (IBMX), isoproterenol, and forskolin, which are known to raise intracellular cAMP levels, all caused a reduction in glucagon receptor mRNA expression. IBMX alone, IBMX together with isoproterenol, and forskolin reduced glucagon receptor mRNA expression to approximately 25, 10, and 50%, respectively. Glucagon was found to dose dependently decrease glucagon receptor mRNA expression in the hepatocytes with an approximately 70% reduction in response to 100 nM glucagon. Finally, we observed a marked reduction in the number of glucagon binding sites (35% of control) after hepatocytes were cultured with the combination of IBMX and isoproterenol. These results indicate that hepatic glucagon receptor mRNA levels can be regulated by glucose and intracellular cAMP and that this is also reflected at the protein level. Furthermore, the observed effects of cAMP and glucagon suggest that this may be a means by which glucagon can down-regulate its own receptor expression.

In Vivo and in Vitro Regulation of Hepatic Glucagon Receptor mRNA Concentration by Glucose Metabolism

“…This observation is reinforced by our in vitro results and data from others (45), which showed that glucose increased GR mRNA concentration in primary cultures of hepatocytes (Fig. 3).…”

Defective Glucose-Dependent Insulinotropic Polypeptide Receptor Expression in Diabetic Fatty Zucker Rats

“…Furthermore, we were unable to measure elevated ambient GIP levels in the VDF rat, indicating that GIP is not causing a loss of functional cell surface receptors, either by desensitization or downregulation, in these animals at this level of glucose intolerance. Expression of other G-proteincoupled receptors (such as the glucagon receptor) in the superfamily are regulated by glucose as well as the adenylyl cyclase activator forskolin (42). Thus, it is likely that GIP receptor downregulation and dysfunction are a result of inappropriate stimulation of the ␤-cell by abnormal levels of metabolites and hormones in this animal model.…”

Glucagon Receptor–Mediated Extracellular Signal–Regulated Kinase 1/2 Phosphorylation in Rat Mesangial Cells

“…Based on the concentration-dependent responses, we used 1 nM glucagon for subsequent experiments, because the K d of the glucagon receptor is reportedly close to this concentration. 24 Glucagon at 1 nM induced rapid ERK 1/2 phosphorylation beginning at 2 minutes (data not shown) and peaking at 5 minutes of stimulation (Figure 4 …”