Disparate effects of monensin and colchicine on intracellular processing of secretory proteins in culture rat hepatocytes.

We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, α₁-protease inhibitor and α_2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, α₁-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with M_r 51000, and then processed to two endoglycosidase-H-resistant forms having M_r 51000 and 56000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for α_2u-globulin. In the cells lrcated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed α₁-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized α₁-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51000-M_r forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant α₁-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH₄Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins. [ABSTRACT FROM AUTHOR]