ORGANIZATION OF THE ENDOPLASMIC RETICULUM-GOLGI SYSTEM IS RELATED TO THE STATE OF ENTEROCYTIC DIFFERENTIATION OF HUMAN HT-29 CELLS

The organization of the endoplasmic reticulum (ER)-intermediate compar tment (IC)-Golgi system was studied in tumoral HT-29 cells. Depending on the culture conditions, these cells are either undifferentiated or exhibit enterocytic differentiation after reaching confluence. In diff erentiated HT-29 cells, these organelles were organized as in most cel l types. They displayed a classical structure and appeared associated with microtubules, as nocodazole altered both their structure and intr acellular localization. Likewise, membrane dynamics of the Golgi appea red normal: as in many other cells, brefeldin A (BFA) induced retrogra de transport from the Golgi to the ER, demonstrated by tubulation of t he Golgi elements and shift of the galactosyltransferase activity from the Golgi- to the RER-enriched fraction, isolated by subcellular frac tionation. In contrast, atypical features were observed in undifferent iated HT-29 cells: the Golgi structure exhibited abnormal swellings; t he IC elements were very rare. Only cytochalasin D altered the structu re and intracellular localization of the three organelles, suggesting that they were associated with-microfilaments instead of microtubules. The membrane dynamics were unusual: brefeldin A led to a vesiculation of the Golgi elements with a slowed-down retrograde transport of gala ctosyltransferase. HT-29 cells engaged in the differentiation process, but which were still undifferentiated, showed mainly the features of undifferentiated cells, with a few characteristics of differentiated c ells. These results indicate that the structure of the Golgi apparatus , IC and ER, their relationships to cytoskeletal elements and membrane dynamics depend on the state of differentiation of HT-29 cells. Altho ugh they are tumoral, differentiated HT-29 cells exhibit features obse rved in non-tumoral polarized epithelial cells. On the contrary, undif ferentiated HT-29 cells display important abnormalities that may be re lated to their metastatic properties.