Immunocytochemical localization of procollagen and fibronectin in human fibroblasts: effects of the monovalent ionophore, monensin.

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Abstract
The monovalent ionophore monensin inhibits the secretion of both procollagen and fibronectin from human fibroblasts in culture. Distribution of these proteins in control and inhibited (5 .times. 10-7 M monensin) cells was studied by immunofluorescence microscopy. In control cells, both antigens are present throughout the cytoplasm and in specific deposits in a region adjacent to the nucleus, identified as a Golgi zone by EM. Treatment of cells with monensin causes intracellular accumulation of procollagen and fibronectin, initially in the juxta-nuclear region and also subsequently in peripheral regions. EM studies reveal that in such cells the juxta-nuclear Golgi zone becomes filled with a new population of smooth-membraned vacuoles and that normal Golgi complexes are not found. Immunocytochemically detected procollagen and fibronectin are localized in the region of these vacuoles, whereas more peripheral deposits correspond to the dilated cisternae of rough endoplasmic reticulum, which are also caused by monensin. Procollagen and fibronectin are often co-distributed in these peripheral deposits. Accumulation of exportable proteins in Golgi-related vacuoles is consistent with previous analyses of the monensin effect. The subsequent development of dilated rough endoplasmic reticulum also containing accumulated proteins may indicate that there is an additional blockade at the exit from the endoplasmic reticulum, or that the synthesized proteins exceed the capacity of the Golgi compartment and that their accumulation extends into the endoplasmic reticulum.