title: Characterization of serine/threonine protein phosphatases in RINm5F insulinoma cells.
abstract: This study investigates the occurrence and regulation of serine/threonine protein phosphatases (PPases) in insulin-secreting RINm5F insulinoma cells. PPases types 1 and 2A were identified in crude RINm5F cell homogenates by both enzymatic assay and Western blot analysis. We then characterized and compared the inhibitory actions of several compounds isolated from cyanobacteria, marine dinoflagellates and marine sponges, (viz. okadaic acid, microcystin-LR, calyculin-A and nodularin) cation-independent PPase activities in RINm5F cell homogenates. It was found that okadaic acid was the least potent inhibitor (IC50 approximately 10(-9) M, IC100 approximately 10(-6) M), while the other compounds exhibited IC50 values of approximately 5 x 10(-10) M and IC100 approximately 5 x 10(-9) M. The findings indicate that the inhibitory substances employed in this study may be used pharmacologically to investigate the role of serine/threonine PPases in RINm5F cell insulin secretion, a process that is likely to be regulated to a major extent by protein phosphorylation.
authors.affiliation: University of Hawaii at Manoa, Cancer Research Center of Hawaii, Honolulu 96813.
authors.forename: A
authors.initials: A
authors.lastname: Sjöholm
authors.forename: R E
authors.initials: RE
authors.lastname: Honkanen
authors.forename: P O
authors.initials: PO
authors.lastname: Berggren
mesh_terms: D000818:Animals; D004988:Ethers, Cyclic / Q000494:pharmacology; D066298:In Vitro Techniques; D007340:Insulinoma / Q000201:enzymology*; D008387:Marine Toxins / Q000494:pharmacology; D052998:Microcystins; D019319:Okadaic Acid; D010080:Oxazoles / Q000494:pharmacology; D010190:Pancreatic Neoplasms / Q000201:enzymology*; D010456:Peptides, Cyclic / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000145:classification / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D014407:Tumor Cells, Cultured / Q000201:enzymology
chemical_list: D004988:Ethers, Cyclic; D008387:Marine Toxins; D052998:Microcystins; D010080:Oxazoles; D010456:Peptides, Cyclic; C063998:nodularin; D019319:Okadaic Acid; C059041:calyculin A; D010749:Phosphoprotein Phosphatases; C057862:cyanoginosin LR