Thyroid hormones and fat cell phosphorylation.
The phosphorylation of cytosolic and plasma membrane proteins was studied in isolated fat cells from euthyroid and thyroidectomized rats. The analysis, by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, of subcellular fractions of 32P-labelled fat cells revealed the presence of 10-12 phosphoprotein bands in the cytosol. The washed plasma membrane fraction contained 4 major phosphoproteins with estimated molecular weights of 70-67, 60, 42-40 and 26-22 kDa. Two-dimensional analysis of the 32P-labelled phosphoproteins showed that their isoelectric points were between 6.3 and 4.1. The profiles and the isoelectric points were similar in fat cells from euthyroid and thyroidectomized rats. The state of hypothyroidism did not affect the basal phosphorylation of fat cell proteins of the cytosolic or plasma membrane fractions. The incubation of fat cells from euthyroid rats in the presence of isoproterenol or dibutyryl adenosine cyclic monophosphate led to (a) an increase in the 32P labelling of cytosolic proteins which may be subunits of acetyl CoA carboxylase, ATP citrate lyase, hormone-sensitive lipase and other proteins, with apparent molecular weights between 50 and 42 kDa, and (b) an increase in the 32P labelling of plasma membrane proteins of 26-22 kDa. In the case of fat cells from hypothyroid rats, the dibutyryl adenosine cyclic monophosphate increased the 32P labelling of plasma membrane proteins, whereas in the presence of isoproterenol these reactions did not occur. These results show that thyroid hormones control the 32P labelling of proteins of the cytosol and plasma membrane fractions of rat fat cells and therefore, at least in some cases, the lipolytic and lipogenic pathways.