AbstractRetinoic acid dramatically increases the size of domes in confluent MDCK monolayers in a hormonally defined medium (medium K‐1). After 4–5 days of retinoic acid treatment, enlarged domes began to appear in confluent MDCK monolayers. After 7days with 3 × 10−7M retinoic acid, the majority of the domes in the monolayers were between 27 and 80 × 10−3μ M2in area, whereas in control medium the majority of the domes were between 0 and 9 × 10−3μm2in area. The dependence of the retinoic acid effect on prostaglandin E1(PGE1) was examined. In normal MDCK cells, the effects of retinoic acid on dome size were observed only in medium K‐1 supplemented with PGE1. This observation indicated that retinoic acid did not elicit its effects simply by stimulating PGE production. In contrast, in monolayers of PGE1‐independent MDCK cells, retinoic acid treatment resulted in an increase in dome frequency even in medium K‐1 lacking PGE1This observation can be explained by the elevated cyclic adenosine monophosphate (cAMP) levels in these PGE1‐independent MDCK cells. Dibutyryl cAMP‐resistant MDCK cells, which normally do not form domes in medium K‐1, were also studied. Remarkably, the dibutyryl cAMP‐resistant MDCK cells were observed to form domes at a significant frequency when medium K‐1 was supplemented with retinoic acid. However in medium K‐1 lacking PGE1,an effect of retinoic acid on dome formation by dibutyryl cAMP‐resistant MDCK monolayers was not observed. The inability of dibutyryl cAMP‐resistant MDCK cells to form domes in medium K‐1 has previously been attributed to their decreased cAMP‐dependent protein kinase activity. The stimulatory effects of retinoic acid on dome formation may possibly be due to an increase in the activity of a particular cAMP‐dependent protein
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