The primary appearance of phosphoenolpyruvate (P-pyruvate) carboxykinase RNA transcripts in fetal liver was induced by a number of different stimulii . This may occur as rapidly as an hour after ...injection in utero of N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) to fetuses, suggesting that all stimulii predominantly affect activation of the P-pyruvate carboxykinase gene. Bt2cAMP treatment induces the appearance of the enzyme RNA transcripts, predominantly of the mature type in the cytoplasm. However, insulin deficiency by streptozotocin treatment causes the appearance of large-size as well as mature mRNA in the nucleus, in addition to the appearance of P-pyruvate carboxykinase mRNA in the cytoplasm. Insulin treatment of such diabetic fetuses, prior to causing the disappearance of P-pyruvate carboxykinase mRNA, reduces nuclear transcripts but increases the abundance of mature cytoplasmic enzyme mRNA. Bt2cAMP treatment of insulin-deficient fetuses causes an additive effect, increasing the abundance of not only the mature but the large P-pyruvate carboxykinase RNA transcripts as well. The results are best interpreted as insulin acting both to inhibit transcription of and accelerate post-transcriptional processes affecting P-pyruvate carboxykinase RNA.
Streptozotocin treatment produces a typical experimental diabetes in neonates exhibiting hyperglycemia, glucosuria, ketonemia and increased level of free fatty acids in the blood. The liver is ...affected as well, with reduced activity of glycogen synthase and a corresponding decrease in the content of liver glycogen.
In contrast, the activity of liver cytosolic phosphoenolpyruvate carboxykinase and the level of its mRNA are not affected. Using a cDNA containing
P
‐pyruvate carboxykinase sequence, the relative abundance of the enzyme mRNA was estimated. The level of the mRNA was readily observed increasing by glucocorticoid treatment or decreasing in response to administered load of glucose. These parallel the changes observed in the activity of the enzyme under these treatments, indicating that the level of
P
‐pyruvate carboxykinase mRNA actually determines that of the cnzyme. The failure of diabetes to incrcase the level of enzyme mRNA and the limited response to glucose loading strongly suggest that the mechanisms controlling the level of
P
‐pyruvate carboxykinase mRNA in neonates are relatively resistant to insulin. This is unique to nconates, since in both the adult and the fetal liver,
P
‐pyruvate carboxykinase readily responds to insulin. The minimal levels of glucocorticoids characteristic of neonates may be associated with this phenomenon.
Streptozotocin treatment produces a typical experimental diabetes in neonates exhibiting hyperglycemia, glucosuria, ketonemia and increased level of free fatty acids in the blood. The liver is ...affected as well, with reduced activity of glycogen synthase and a corresponding decrease in the content of liver glycogen.
In contrast, the activity of liver cytosolic phosphoenolpyruvate carboxykinase and the level of its mRNA are not affected. Using a cDNA containing P‐pyruvate carboxykinase sequence, the relative abundance of the enzyme mRNA was estimated. The level of the mRNA was readily observed increasing by glucocorticoid treatment or decreasing in response to administered load of glucose. These parallel the changes observed in the activity of the enzyme under these treatments, indicating that the level of P‐pyruvate carboxykinase mRNA actually determines that of the cnzyme. The failure of diabetes to incrcase the level of enzyme mRNA and the limited response to glucose loading strongly suggest that the mechanisms controlling the level of P‐pyruvate carboxykinase mRNA in neonates are relatively resistant to insulin. This is unique to nconates, since in both the adult and the fetal liver, P‐pyruvate carboxykinase readily responds to insulin. The minimal levels of glucocorticoids characteristic of neonates may be associated with this phenomenon.
