Zinc transport and metallothionein secretion in the intestinal human cell line Caco-2

Caco-2, a human cell line, displays several biochemical and morphological c haracteristics of differentiated enterocytes. Among these is the ability to transport zinc from the apical to the basal compartment. This process was enhanced following exposure by the apical compartment to increasing concent rations of the metal. High pressure liquid chromatography fractionation of the media obtained from cells labeled with radioactive zinc showed that met allothioneins (MTs), small metal-binding, cysteine-rich proteins), were pre sent in the apical and basal media of controls as well as in cells grown in the presence of high concentrations of zinc. Following exposure to the met al, the levels of Zn-MTs in the apical medium increased, while in the basal compartment the greatest part of zinc appeared in a free form with minor c hanges in the levels of basal MTs. Metabolic labeling experiments with radi oactive cysteine confirmed the apical secretion of MTs. A stable transfecta nt clone of Caco-8 cells (CL11) was selected for its ability to express con stitutively high levels of the mouse metallothionein I protein. This cell l ine showed an enhanced transport of the metal following exposure to high co ncentrations of zinc and a constitutive secretion of the mouse metallothion ein I protein in the apical compartment. Together, these findings strongly support the hypothesis of a functional role between the biosynthesis and se cretion of MTs and the transport of zinc in intestinal cells.