Effect of okadaic acid on apo B and apo A-I secretion by CaCo-2 cells.
The effect of protein phosphorylation on the synthesis and secretion of apo B and apo A-I by CaCo-2 cells was investigated. Okadaic acid, a potent inhibitor of protein serine/threonine phosphatases 1 and 2A, caused a significant increase in total cellular protein phosphorylation. Apo B-48 was phosphorylated in control cells and this was increased significantly in the presence of okadaic acid. Under the experimental conditions, the phosphorylation of apo B-100 or apo A-I was not observed. No evidence of tyrosine phosphorylation of apo B-100, B-48, or apo A-I was found. Okadaic acid did not change the amount of apo B mass within cells but apo B mass secreted into the basolateral medium was decreased by 40%. Apo A-I mass within cells or in the basolateral medium was unaffected by okadaic acid. Despite causing an 18% decrease in total protein synthesis, okadaic acid did not alter the rate of synthesis of apo B-100, apo B-48, or apo A-I. Cellular turnover of labeled apo B-100 in cells incubated with okadaic acid was similar to controls, whereas apo B-48 and apo A-I turnover were slowed by okadaic acid. Compared to controls, however, 1 microM okadaic acid caused a 75% and 50% decrease in the secretion of newly synthesized apo B-100 and apo B-48, respectively, while decreasing labeled apo A-I secretion by 35%. In contrast to apo A-I mRNA levels, which were not altered by okadaic acid, apo B mRNA levels were significantly decreased by the polyether fatty acid. Despite differences observed in the phosphorylation state of apo B-100 and apo B-48, okadaic acid decreased the secretion of both forms of apo B without altering their synthesis. Okadaic acid, by increasing cellular protein phosphorylation, significantly disrupts the secretory processing of apo B by CaCo-2 cells.
Mathur
S N
SN
Department of Internal Medicine, University of Iowa, Iowa City 52242.
Born
E
E
Bishop
W P
WP
Field
F J
FJ
D016632:Apolipoprotein A-I / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D001055:Apolipoproteins B / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line / Q000187:drug effects / Q000378:metabolism; D004988:Ethers, Cyclic / Q000494:pharmacology*; D006801:Humans; D007422:Intestines; D019319:Okadaic Acid; D010766:Phosphorylation; D019000:Phosphotyrosine; D011233:Precipitin Tests; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000032:analysis; D014443:Tyrosine / Q000031:analogs & derivatives
D016632:Apolipoprotein A-I; D001055:Apolipoproteins B; D004988:Ethers, Cyclic; D012333:RNA, Messenger; D019319:Okadaic Acid; D019000:Phosphotyrosine; D014443:Tyrosine