Reduced expression of the regulatory A subunit of serine/threonine protein phosphatase 2A in human breast cancer MCF-7 cells.
The beta1 subunit of integrin is serine/threonine phosphorylated in growth arrested human breast cancer MCF-7 cells, while it is not in quiescent normal human breast epithelial (HBE) cells. Using the affinity-purified antibodies PB788-9 against the synthetic oligopeptide that contained phosphothreonines corresponding to threonines 788 and 789 on beta1 integrin, beta1 integrin in MCF-7 cells, but not in HBE cells, was found to react with PB788-9. The beta1 integrin immunoprecipitates from HBE cells co-immunoprecipitated the core enzyme of serine/threonine protein phosphatase (PP) 2A, consisting of the regulatory A (PP2A-A) and the catalytic C (PP2A-C) subunits, with the protein phosphatase activity susceptible to okadaic acid (OA), an inhibitor of PP2A and PP1, but not to a PP1 inhibitor. In contrast, beta1 integrin from MCF-7 cells co-immunoprecipitated PP2A-C, but not PP2A-A, with no protein phosphatase activity. Immunoblotting of whole cell lysates revealed that a comparable amount of PP2A-C was present in either HBE or MCF-7 cells, but the amount of PP2A-A was significantly reduced in MCF-7 cells compared to that in HBE cells. The results suggest that the failure of beta1 integrin dephosphorylation at threonines 788 and 789 may be due to a significant reduction in the PP2A-A expression in MCF-7 cells.
Suzuki
Katsuo
K
Department of Biochemistry, Kanagawa Cancer Center Research Institute, Asahi-ku, Yokohama 241-0815, Japan.
Takahashi
Kazuhide
K
D000199:Actins / Q000378:metabolism; D020134:Catalytic Domain; D002455:Cell Division; D045744:Cell Line, Tumor; D003599:Cytoskeleton / Q000378:metabolism; D000939:Epitopes; D006801:Humans; D015151:Immunoblotting; D019012:Integrin beta1 / Q000737:chemistry / Q000378:metabolism; D008856:Microscopy, Fluorescence; D019319:Okadaic Acid / Q000737:chemistry / Q000378:metabolism; D009842:Oligopeptides / Q000737:chemistry; D010455:Peptides / Q000737:chemistry; D010749:Phosphoprotein Phosphatases / Q000737:chemistry*; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding; D054648:Protein Phosphatase 2; D017434:Protein Structure, Tertiary; D013912:Threonine / Q000737:chemistry / Q000378:metabolism
D000199:Actins; D000939:Epitopes; D019012:Integrin beta1; D009842:Oligopeptides; D010455:Peptides; D019319:Okadaic Acid; D013912:Threonine; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2