We have previously reported that the expression in yeast of an integral membrane protein (p180) of the endoplasmic reticulum (ER), isolated for its ability to mediate ribosome binding, is capable of inducing new membrane biogenesis and an increase in secretory capacity. To demonstrate that p180 is necessary and sufficient for terminal differentiation and acquisition of a secretory phenotype in mammalian cells, we studied the differentiation of a secretory cell line where p180 levels had been significantly reduced using RNAi technology and by transiently expressing p180 in nonsecretory cells. A human monocytic (THP-1) cell line, that can acquire macrophage-like properties, failed to proliferate rough ER when p180 levels were lowered. The Golgi compartment and the secretion of apolipoprotein E (Apo E) were dramatically affected in cells expressing reduced p180 levels. On the other hand, expression of p180 in a human embryonic kidney nonsecretory cell line (HEK293) showed a significant increase in proliferation of rough ER membranes and Golgi complexes. The results obtained from knockdown and overexpression experiments demonstrate that p180 is both necessary and sufficient to induce a secretory phenotype in mammalian cells. These findings support a central role for p180 in the terminal differentiation of secretory cells and tissues. INTRODUCTIONAt the morphological level, the development of secretory cells and tissues has been well documented. From a molecular point of view, details of how cells acquire the ability to secrete at high levels as part of the process of their terminal differentiation are less well understood. Early in mammalian embryogenesis, the cytoplasm of cells of exocrine tissues, such as pancreas or liver, possesses a mere fraction of the intracellular organelles that they ultimately acquire upon terminal differentiation (Dallner et al., 1966a,b;Pictet et al., 1972;Slack, 1995;Debas, 1997). As differentiation of these tissues progresses, their cells take on a morphology that is dominated by membranous elements of the secretory pathway, most notably rough endoplasmic reticulum (ER) (Jamieson and Palade, 1967a,b). A similar situation exists in the immune system, in which B-lymphocytes differentiate into plasma cells as the result of being triggered by the appropriate antigen Shohat et al., 1973;Wiest et al., 1989Wiest et al., , 1990Chen-Kiang 1995;Gass et al., 2002;Iwakoshi et al., 2003;Federovitch et al., 2005;Fagone et al., 2007). A prerequisite for achieving such a "secretory phenotype" is intracellular membrane proliferation, which is in turn needed to expand the cell's capacity to synthesize, process, transport, and ultimately release peptides and proteins into the extracellular space.Limited success has been achieved, in a few cell types, in recapitulating membrane proliferation reminiscent of secretory cell differentiation. Through the ectopic expression of membrane proteins, ER-like membranes have been produced in yeast as well as in various mammalian cells (Wright et al., 1988;Vergeres et ...

No abstract