Organization of the endoplasmic reticulum-Golgi system is related to the state of enterocytic differentiation of human HT-29 cells

Morphological and biochemical analysis of the secretory pathway in melanoma cells with distinct metastatic potential

In this report, we have investigated whether alterations of the morphological and functional aspects of the biosecretory membrane system are associated with the metastatic potential of tumor cells. To this end, we have analyzed the morphology of the Golgi complex, the cytoskeleton organization and membrane trafficking steps of the secretory pathway in two human melanoma A375 cell line variants with low (A375-P) and high metastatic (A375-MM) potential. Immunofluorescence analysis showed that in A375-P cells, the Golgi complex showed a collapsed morphology. Conversely, in A375-MM cells, the Golgi complex presented a reticular and extended morphology. At the ultrastructural level, the Golgi complex of A375-P cells was fragmented and cisternae were swollen. When the cytoskeleton was analyzed, the microtubular network appeared normal in both cell variants, whereas actin stress fibers were largely absent in A375-P, but not in A375-MM cells. In addition, the F-actin content in A375-P cells was significantly lower than in A375-MM cells. These morphological differences in A375-P cells were accompanied by acceleration and an increase in the endoplasmic reticulum to Golgi and the trans-Golgi network to cell surface membrane transport, respectively. Our results indicate that in human A375 melanoma cells, metastatic potential correlates with a well-structured morphofunctional organization of the Golgi complex and actin cytoskeleton.

Atypical microtubule organization in undifferentiated human colon cancer cells

We previously reported that undifferentiated colonic cancer HT-29 cells, unlike the differentiated ones, exhibit unusual organelle distributions and atypical vesicle trafficking patterns, which are microtubule-independent and microfilament-dependent. In the present study, we have analyzed the microtubule network in both phenotypes, using confocal microscopy, and determined the expression levels of some microtubule-associated proteins by quantitative immunoblotting. Differentiated cells exhibited the microtubular organization of polarized epithelial cells. Non-polarized undifferentiated cells presented an atypical microtubule organization as microtubules were localized mainly at the cell 'top'. Immunoblot analysis indicated the absence or low content of several structural and motor microtubule-associated proteins in undifferentiated cells, compared to differentiated cells. This may explain in part their atypical microtubular organization. This study agrees with a crucial role for microfilaments in the intracellular organization of undifferentiated HT-29 cancer cells, while differentiated HT-29 cells exhibit intracellular organization similar to that of normal enterocytic cells, although they are also tumoral.

Evidence for a dual control of macroautophagic sequestration and intracellular trafficking of N-linked glycoproteins by the trimeric G(i3) protein in HT-29 cells

The trimeric G(i3) protein-dependent lysosomal-autophagic pathway is responsible for the degradation of a pool of N-linked glycoproteins in the human colon cancer HT-29 cell line. Here we have followed the fate of N-glycans using HT-29 cells either overexpressing the wild-type G alpha(i3) protein or transfected with different mutants of the G alpha(i3) protein. The stabilization of N-glycans was dependent upon the inhibition of autophagic sequestration by either 3-methyladenine (3-MA) or pertussis toxin (PTX). However, PTX allowed the processing of high-mannose glycans whereas 3-MA did not. The destabilization of the Golgi apparatus by brefeldin A, which interrupts the intracellular trafficking of N-linked glycoproteins along the secretory pathway, did not interfere with the macroautophagic pathway. These results suggest that the lysosomal-autophagic pathway is not dependent upon the integrity of the Golgi apparatus and points to differences between the molecular properties of two membrane flow processes (macroautophagy, exocytic pathway) controlled by the trimeric G(i3) protein.