Abstract
The β-adrenoceptor-cyclic adenosine monophosphate (AMP) dependent glycogenolytic cascade was examined in normal rat hepatocytes and rat ascites hepatoma AH130 cells. The cyclic AMP content in AH130 cells was half of that in normal hepatocytes, and the cyclic AMP levels in both kinds of cells were clearly increased by isoproterenol (IPN). Cyclic AMP-dependent protein kinase activity was higher in AH130 cells than in normal hepatocytes. Phosphorylase kinase activities in 10000 × g supernatant of normal hepatocytes and AH130 cells were also increased in the presence of cyclic AMP. Phosphorylase a activities in the supernatant of both kinds of cells gradually decreased during incubation with 40 mM glucose at 37 °C, and the enzyme activity of normal hepatocytes was completely restored by the addition of Mg2+-adenosine triphosphate (ATP), but in the case of the hepatoma cells the recovery was small. The decreased phosphorylase a activity in the hepatoma cells was increased by additional glycogen but did not exceed the level before the incubation. In the case of normal hepatocytes it was not affected by glycogen. This indicates that glycogen contained in the cells influences the activation of phosphorylase; the glycogen content in AH130 cells was far less than in normal hepatocytes. On the other hand, when intact cells were incubated with a high concentration of glucose, phosphorylase a activity in the homogenate of normal hepatocytes was decreased and could be restored by IPN and dibutyryl cyclic AMP, but the enzyme activity in the homogenate of AH130 cells was very low and hardly changed after the incubation and treatment with these agents. Phosphorylase phophatase activity was lower in AH130 cells than in normal hepatocytes. Only phosphorylase b converted from the a form by the phosphatase may be able to act as the substrate of phosphorylase kinase. The present study indicates that the key enzymes in the glycogenolytic cascade of AH130 cells functionally act as well as normal hepatocytes, but glycogen phosphorylase is unrsponsive to stimulation through cyclic AMP. A part of the lack of response of the phosphorylase in AH130 cells may be due to its low ability to interconvert between the active form and the inactive form in the tumor cells owing to their low glycogen content and low phosphatase activity.
