ABSTRACT
The green tea polyphenol epigallocatechin-3-gallate (EGCG) regulates gene expression differentially in tumor and normal cells. In normal human primary epidermal keratinocytes (NHEK), one of the key mediators of EGCG action is p57/KIP2, a cyclin-dependent kinase (CDK) inhibitor. EGCG potently induces p57 in NHEK, but not in epithelial cancer cells. In humans, reduced expression of p57 often is associated with advanced tumors, and tumor cells with inactivated p57 undergo apoptosis when exposed to EGCG. The mechanism of p57 induction by EGCG is not well understood. Here, we show that in NHEK, EGCG-induces p57 via the p38 mitogen-activated protein kinase (MAPK) signaling pathway. In p57-negative tumor cells, JNK signaling mediates EGCG-induced apoptosis, and exogenous expression of p57 suppresses EGCG-induced apoptosis via inhibition of c-Jun N-terminal kinase (JNK). We also found that restoration of p57 expression in tumor cells significantly reduced tumorigenicity in athymic mice. These results suggest that p57 expression may be an useful indicator for the clinical course of cancers, and could be potentially useful as a target for cancer therapies.
Subject(s)
Catechin/analogs & derivatives , Cyclin-Dependent Kinase Inhibitor p57/metabolism , JNK Mitogen-Activated Protein Kinases/metabolism , Mouth Neoplasms/prevention & control , Animals , Anthracenes/pharmacology , Apoptosis/drug effects , Apoptosis/physiology , Blotting, Western , Catechin/pharmacology , Catechin/therapeutic use , Cell Line , Cell Line, Tumor , Cell Proliferation/drug effects , Cyclin-Dependent Kinase Inhibitor p57/genetics , Cytochromes c/metabolism , Disease-Free Survival , Enzyme Inhibitors/pharmacology , Female , Humans , Imidazoles/pharmacology , JNK Mitogen-Activated Protein Kinases/antagonists & inhibitors , Mice , Mice, Nude , Mouth Neoplasms/enzymology , Mouth Neoplasms/metabolism , Mouth Neoplasms/pathology , Phosphorylation/drug effects , Pyridines/pharmacology , Signal Transduction/drug effects , Signal Transduction/physiology , Transfection , Xenograft Model Antitumor Assays , bcl-2-Associated X Protein/metabolism , p38 Mitogen-Activated Protein Kinases/antagonists & inhibitors , p38 Mitogen-Activated Protein Kinases/metabolismABSTRACT
Prefrontal cortical functioning depends on dopaminergic neurotransmission, which in turn depends on a complex signal transduction pathway including protein phosphatase-1 (PP1). Targeted localization of PP1 by the scaffolding proteins, spinophilin and neurabin, is critical for dopaminergic modulation of glutamate neurotransmission. In this study, we report the preparation of an antiserum to neurabin, use it to study the subcellular localization of neurabin and compare that to our previous study of spinophilin, a closely related PP1 scaffold. Neurabin is found predominately in dendritic spines, but is also found in other compartments, including dendrites, axons, terminals and glia. This distribution contrasts with that of spinophilin in that neurabin is found in axon terminals where spinophilin is absent, and in parvalbumin-containing interneuron dendrites there is no significant neurabin though these dendrites contain substantial spinophilin. Within the dendritic spine compartment, however, the two proteins are similarly distributed. Both neurabin and spinophilin are concentrated in spines, and double-labeling reveals that they co-localize in most spines. Furthermore, post-embedding immunogold labeling demonstrates that within a spine, neurabin is distributed in the same pattern as spinophilin, concentrated in the postsynaptic density and the 100 nm just below. These results indicate that neurabin and spinophilin share important similarities and differences in their patterns of distribution. Varying patterns of scaffold localization may play an important role in determining the content and action of signal transduction pathways in different neuronal populations or compartments.
