Bryostatin 1 affects P-glycoprotein phosphorylation but not function in multidrug-resistant human breast cancer cells.
The function of P-glycoprotein (Pgp), which confers multidrug resistance by active efflux of drug, is thought to be dependent on phosphorylation. Previous studies have suggested that protein kinase C (PKC) plays an important role in Pgp phosphorylation. We report here the effects of bryostatin 1, a unique PKC activator and inhibitor, on Pgp function in a multidrug-resistant MCF-7 human breast cancer subline which overexpresses PKC-alpha. Bryostatin 1 (100 nM) decreased Pgp phosphorylation after 24 h of treatment. In contrast, it did not affect Pgp function as demonstrated by the accumulation of [3H]vinblastine and rhodamine 123. We compared the effect of bryostatin 1 treatment on PKC-alpha with that of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (200 nM). 12-O-tetradecanoylphorbol-13-acetate caused translocation of PKC-alpha from the cytosol to the cell membrane after a 10-min treatment and its down-regulation after 24 h of treatment. Likewise, bryostatin 1 (100 nM) caused translocation, but only after longer treatment (1 h), and it caused down-regulation of PKC-alpha at 24 h of treatment. Thus, while the MCF-7TH cells overexpress the PKC-alpha isoform, and its down-regulation by bryostatin 1 is associated with decreased Pgp phosphorylation, these alterations do not modulate drug transport. We conclude that, while bryostatin 1 may be useful clinically because of its ability to inhibit PKC, it is not able to reverse Pgp-mediated multidrug resistance.
Scala
S
S
Medicine Branch, Clinical Oncology Program, Division of Cancer Treatment, National Cancer Institute, NIH, Bethesda, Maryland 20892, USA.
Dickstein
B
B
Regis
J
J
Szallasi
Z
Z
Blumberg
P M
PM
Bates
S E
SE
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000187:drug effects* / Q000502:physiology; D000970:Antineoplastic Agents / Q000378:metabolism / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000378:metabolism*; D054713:Bryostatins; D003630:Daunorubicin / Q000378:metabolism; D015536:Down-Regulation; D018432:Drug Resistance, Multiple; D019008:Drug Resistance, Neoplasm; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007783:Lactones / Q000494:pharmacology*; D018942:Macrolides; D009363:Neoplasm Proteins / Q000187:drug effects* / Q000502:physiology; D010766:Phosphorylation / Q000187:drug effects; D011493:Protein Kinase C / Q000378:metabolism*; D012235:Rhodamines / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000187:drug effects; D014747:Vinblastine / Q000378:metabolism
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D054713:Bryostatins; D007527:Isoenzymes; D007783:Lactones; D018942:Macrolides; D009363:Neoplasm Proteins; D012235:Rhodamines; C046785:bryostatin 1; D014747:Vinblastine; D011493:Protein Kinase C; D003630:Daunorubicin