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

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Abstract
We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, α1-protease inhibitor and α2u-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 Mr 51000, and then processed to two endoglycosidase-H-resistant forms having Mr 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 α2u-globulin. In the cells treated 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-Mr 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.