Effect of retinoic acid on cell proliferation and differentiation as well as on lipid synthesis, lipoprotein secretion, and apolipoprotein biogenesis

Abstract

Dietary vitamin A and its active metabolites are essential nutrients for many functions as well as potent regulators of gene transcription and growth. Although the epithelium of the small intestine is characterized by rapid and constant renewal and enterocytes play a central role in the absorption and metabolism of alimentary retinol, very little is known about the function of retinoids in the human gastrointestinal epithelium and mechanisms by which programs engage the cell cycle are poorly understood. We have assessed the effects of 10 μM 9- and 13- cis-retinoic acid (RA) on proliferation and differentiation processes, lipid esterification, apolipoprotein (apo) biogenesis and lipoprotein secretion along with nuclear factor gene transcription. Treatment of Caco-2 cells with RA at different concentrations and incubation periods revealed the reduction of thymidine incorporation in 60% preconfluent or 100% confluent cells. Concomitantly, RA 1) modulated D-type cyclins by reducing the mitogen-sensitive cyclin D1 and upregulating cyclin D3 expressions and 2) caused a trend of increase in p38 MAPK, which triggers CDX2, a central protein in cell differentiation. RA remained without effect on lipoprotein output and apo synthesis, even for apo A-I that possesses RARE in its promoter. RA, in combination with 22-hydroxycholesterol, could induce apo A-I gene expression without any impact on apo A-I mass. Only the gene expression of peroxisome proliferator-activated receptor (PPAR)β, retinoic receptor (RAR)β, and RARγ was augmented and no alteration was noted in PPARα, PPARγ, liver X receptor (LXR)α, LXRβ, and retinoid X receptors. Taken together, these data highlight RA-induced cell differentiation via specific signaling without a significant impact on apo A-I synthesis.

Effect of Soman on JNK and P38 Mitogen Activated Protein Kinase (Mapk) Pathways

“…Although the physiological role of p38 is not fully understood, we found phospho-p38 values to be lower in cryptal enterocytes than in apical cells. This indicates that the p38 pathway may participate in the enterocyte differentiation process in vivo and is in accordance with in vitro experiments linking p38 to differentiation (26,27,28). Thus, soman poisoning might not alter p38 signalling in crypts in order not to impair the undifferentiated status of cryptal cells.…”

Transforming Growth Factor-β Stimulates Intestinal Epithelial Focal Adhesion Kinase Synthesis via Smad- and p38-Dependent Mechanisms

“…Although it would be interesting to repeat these studies in primary human intestinal epithelial cells, these are difficult to maintain in culture under sterile conditions, but both human Caco-2 cells and nontransformed rat IEC-6 cells are commonly used to model normal human intestinal epithelial biology. 30,[53][54][55][56][57] Cells modify focal adhesions in response to changes in composition, two-and three-dimensional structure, and physical forces of the extracellular matrix environment. 58 During dynamic tissue conditions such as wound healing, cells may remain spread with clustered integrins but without mature focal adhesions to adapt to changing environmental conditions.…”

Modulation of intestinal cholesterol absorption by high glucose levels: impact on cholesterol transporters, regulatory enzymes, and transcription factors

“…The inserts were placed into six-well culture plates, permitting separate access to the upper and lower compartments of the monolayers. Cells were cultured for various periods, including 21 days, at which the Caco-2 cells are highly differentiated and appropriate for lipid metabolism (34,54,55,61,73 ACAT activity assay. The activity of acyl-coenzyme A:cholesterol acyltransferase (ACAT) was determined at initial rates by adding 5 nmol of [ 14 C]oleoyl-CoA (specific activity ϳ167 Bq/nmol) to the mixture containing 190 g of cellular protein to initiate the reaction in a buffer solution (pH 7.5) consisting of cholesterol, 0.04 M KH 2PO4, 50 mM NaF, 0.25 M sucrose, and 1 mM EDTA (73).…”