Abbreviated cell cycle progression induced by the serine/threonine protein phosphatase inhibitor okadaic acid at concentrations that promote neoplastic transformation.

We examined cell cycle-related effects of the phosphatase inhibitor okadaic acid (OA) in T51B rat liver epithelial cells under conditions chosen to mimic early stages of tumor promotion by this compound. Optimal transformation (colony formation in soft agar) was seen after prolonged culture of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-initiated T51B cells in 7 nM OA. Paradoxically, T51B cells treated with 2-10 nM OA showed decreased, rather than increased, proliferation in response to epidermal growth factor (EGF), as measured by [3H]thymidine incorporation. Complete inhibition was observed within 24 h at 10 nM OA. This response paralleled a loss of EGF-stimulated cdk2 kinase activity and an increase in association of the inhibitors p21 (cip-1) and p27 (kip-1) with cdk2. An increase in p53 phosphorylated on serine 15 accompanied the rise in p21 (cip-1). Both phosphorylation of the retinoblastoma protein and induction of cyclin A by EGF were blocked in cells treated with OA, but there was an increase in cyclin E. Resting cells treated with OA alone also showed elevated cyclin E levels, together with reduced levels of the E2F regulator pRb2/p130. Taken together, these observations indicate transforming levels of okadaic acid elicit a G(1)-trapping effect by facilitating cell cycle progression to the G(1)/S checkpoint, where cells are trapped by mechanisms that include p21 (cip-1)-mediated inhibition of cdk2. They support the premise that disruption of cellular processes regulating the transitions from G(0) to G(1) to S-phase is an important early step in tumor promotion by low levels of okadaic acid.

The sodium channel from rat brain. Separation and characterization of subunits.

Procedures are described for separation of the alpha, beta 1, and beta 2 subunits of the voltage-sensitive sodium channel from rat brain by gel filtration in sodium dodecyl sulfate (SDS) before and after reduction of intersubunit disulfide bonds or by preparative SDS-gel electrophoresis. Partial proteolytic maps of the SDS-denatured subunits indicate that they are nonidentical polypeptides. They are all heavily glycosylated and contain complex carbohydrate chains that bind wheat germ agglutinin. The apparent molecular weights of the separated subunits were estimated by gradient SDS-gel electrophoresis, by Ferguson analysis of migration in SDS gels of fixed acrylamide concentration, or by gel filtration in SDS or guanidine hydrochloride. For the alpha subunit, SDS-gel electrophoresis under various conditions gives an average Mr of 260,000. Gel filtration methods give anomalously low values. Removal of carbohydrate by sequential treatment with neuraminidase and endoglycosidase F results in a sharp protein band with apparent Mr = 220,000, suggesting that 15% of the mass of the native alpha subunit is carbohydrate. Electrophoretic and gel filtration methods yield consistent molecular weight estimates for the beta subunits. The average values are: beta 1, Mr = 36,000, and beta 2, Mr = 33,000. Deglycosylation by treatment with endoglycosidase F, trifluoromethanesulfonic acid, or HF yields sharp protein bands with apparent Mr = 23,000 and 21,000 for the beta 1 and beta 2 subunits, respectively, suggesting that 36% of the mass of the native beta 1 and beta 2 subunits is carbohydrate.

The saxitoxin receptor of the sodium channel from rat brain. Evidence for two nonidentical beta subunits.

The saxitoxin receptor of the sodium channel purified from rat bran contains three types of subunits: alpha with Mr approximately 270,000, beta 1 with Mr approximately 39,000, and beta 2 with Mr approximately 37,000. These are the only polypeptides which quantitatively co-migrate with the purified saxitoxin receptor during velocity sedimentation through sucrose gradients. beta 1 and beta 2 are often poorly resolved by gel electrophoresis in sodium dodecyl sulfate (SDS), but analysis of the effect of beta-mercaptoethanol on the migration is covalently attached to the alpha subunit by disulfide bonds while the beta 1 subunit is not. The alpha and beta subunits of the sodium channel were covalently labeled in situ in synaptosomes using a photoreactive derivative of scorpion toxin. Treatment of SDS-solubilized synaptosomes with beta-mercaptoethanol decreases the apparent molecular weight of the alpha subunit band without change in the amount of 125I-labeled scorpion toxin associated with either the alpha or beta subunit bands. These results indicate that the alpha and beta 1 subunits are labeled by scorpion toxin whereas beta 1 is not and that the beta 2 subunit is covalently attached to alpha by disulfide bonds in situ as well as in purified preparations.