Angiotensinogen production by rat hepatoma cells in culture and analysis of its regulation by techniques of somatic cell genetics

Angiotensinogen was synthesized by cells derived from the Reuber H35 rat hepatoma. Independent clones produced similar amounts of angiotensinogen, which corresponded to about four times more than expected for normal hepatocytes. The protein was secreted rapidly but could be visualized within cells using immunofluorescence. For one clone, it is shown that maximal angiotensinogen synthesis occurred during mid-exponential growth. Somatic cell genetics techniques have been used to investigate the regulation of angiotensinogen expression. Eleven clones of dedifferentiated variant hepatoma cells that failed to produce most or all of the liver specific proteins analyzed including albumin fell into two groups: Seven clones produced only 1-3% as much angiotensinogen as the differentiated clones, and four showed a reduction to 10-30%. Clones of the latter class were the only ones among the eleven analyzed that retained the potential to give rise to revertants, showing restoration of the differentiated state. All revertants fully restored angiotensinogen production, but only some of them re-expressed albumin. Somatic hybrids between differentiated hepatoma cells and one of the variants showed a substantial reduction in angiotensinogen production, whereas for some clones, albumin synthesis was fully maintained. These results show that regulation of the expression of angiotensinogen and of a second serum protein, albumin, was independent and that angiotensinogen synthesis was a faithful indicator of the general differentiation profile of all classes of clones.