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Section of Physiology, Cornell University, Ithaca, New York 14853
Growth arrest
and cell differentiation are generally considered temporally and
functionally linked phenomena in small intestinal crypt cells and colon
tumor cell lines (Caco-2, HT-29). We have derived a Caco-2 subclone
(NGI3) that deviates from such a paradigm. In striking contrast with
the parental cells, proliferative and subconfluent NGI3 cells were
found to express sucrase-isomaltase (SI) mRNA and to synthesize
relatively high levels of SI, dipeptidyl peptidase IV, and
aminopeptidase N (APN). In postconfluent cells, little difference was
seen in SI mRNA levels between Caco-2 and NGI3 cells, but the latter
still expressed much higher levels of SI that could be attributed to
higher rates of translation. APN expression was also greatly enhanced
in NGI3 cells. To determine whether high levels of brush-border enzymes
correlated with expression of cell-cycle regulatory proteins, we
investigated their relative cellular levels in growing and
growth-arrested cells. The results showed that the cyclin-dependent
kinase inhibitors (p21 and p27) and D-type cyclins (D1 and D3) were all
induced in postconfluent cells, but NGI3 cells expressed much higher
levels of p21. This study demonstrated that cell growth and expression
of differentiated traits are not mutually exclusive in intestinal
epithelial cells and provided evidence indicating that
posttranscriptional events play an important role in regulation of SI expression.
cyclins; cyclin-dependent kinases; cyclin-dependent kinase inhibitors; brush-border enzymes
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