AJP - Gastrointestinal and Liver Physiology, Vol 265, Issue 1 143-G148, Copyright © 1993 by American Physiological Society
Effect of 1,25(OH)2D3 and its analogues on membrane phosphoinositide turnover and [Ca2+]i in Caco-2 cells
X. Y. Tien, T. A. Brasitus, B. M. Qasawa, A. W. Norman and M. D. Sitrin
Department of Medicine, University of Chicago, Illinois 60637.
Our laboratory has recently reported that 1,25-dihydroxyvitamin D3
[1,25(OH)2D3] rapidly stimulates membrane polyphosphoinositide turnover and
increases intracellular calcium concentration ([Ca2+]i) in Caco-2 cells.
The role of binding to the vitamin D receptor (VDR) in the regulation of
these rapid biochemical events, however, remains unclear. The present
studies were, therefore, conducted using analogues of 1,25(OH)2D3, which
differ markedly in their affinities for the VDR, to assess and compare
their effects on [Ca2+]i and on inositol 1,4,5-trisphosphate (IP3)
formation. Competitive binding studies performed with both intact cells and
high-salt cytosolic extracts from Caco-2 cells demonstrated that
1,25(OH)2D3 and 1,24-(OH)2-22-ene-24-cyclopropyl-D3 (BT) have high
affinities for the VDR; 25(OH)-16-ene,23-yne-D3 (AT), however, has a much
lower affinity (approximately 1,000-fold less) for the VDR. Despite these
large differences in binding affinities for the VDR, AT and BT produced
similar concentration-dependent increases in [Ca2+]i and in IP3 formation
while 1,25(OH)2D3 was approximately 10-fold less active. These results
indicate that the structural requirements for the rapid action of these
secosteroids on signal transduction in Caco-2 cells are different from
those for receptor binding and transcriptional regulation.
