Abstract
We describe here an important function of the novel calmodulin kinase I isoform, pregnancy-upregulated nonubiquitous calmodulin kinase (Pnck). Pnck (also known as CaM kinase Ibeta(2)) was previously shown to be differentially overexpressed in a subset of human primary breast cancers, compared with benign mammary epithelial tissue. In addition, during late pregnancy, Pnck mRNA was shown to be strongly upregulated in epithelial cells of the mouse mammary gland exhibiting decreased proliferation and terminal differentiation. Pnck mRNA is also significantly upregulated in confluent and serum-starved cells, compared with actively growing proliferating cells (Gardner HP, Seung HI, Reynolds C, Chodosh LA. Cancer Res 60: 5571-5577, 2000). Despite these suggestive data, the true physiological role(s) of, or the signaling mechanism(s) regulated by Pnck, remain unknown. We now report that epidermal growth factor receptor (EGFR) levels are significantly downregulated in a ligand-independent manner in human embryonic kidney-293 (HEK-293) cells overexpressing Pnck. MAP kinase activation was strongly inhibited by EGFR downregulation in the Pnck-overexpressing cells. The EGFR downregulation was not the result of reduced transcription of the EGFR gene but from protea-lysosomal degradation of EGFR protein. Knockdown of endogenous Pnck mRNA levels by small interfering RNA transfection in human breast cancer cells resulted in upregulation of unliganded EGFR, consistent with the effects observed in the overexpression model of Pnck-mediated ligand-independent EGFR downregulation. Pnck thus emerges as a new component of the poorly understood mechanism of ligand-independent EGFR degradation, and it may represent an attractive therapeutic target in EGFR-regulated oncogenesis.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
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Research Support, U.S. Gov't, Non-P.H.S.
MeSH terms
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Adaptor Proteins, Signal Transducing / metabolism
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Calcium-Calmodulin-Dependent Protein Kinase Type 1 / genetics
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Calcium-Calmodulin-Dependent Protein Kinase Type 1 / metabolism*
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Cell Line
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Down-Regulation
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Epidermal Growth Factor / pharmacology
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Epithelial Cells / drug effects
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Epithelial Cells / metabolism
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ErbB Receptors / metabolism*
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Extracellular Signal-Regulated MAP Kinases / metabolism
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Gene Expression / drug effects
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Humans
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Intracellular Signaling Peptides and Proteins / metabolism
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Leupeptins / pharmacology
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Ligands
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Lysosomes / metabolism
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Macrolides / pharmacology
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Phosphoproteins / metabolism
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Phosphorylation / drug effects
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Protease Inhibitors / pharmacology
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Proteasome Endopeptidase Complex / metabolism
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Proteasome Inhibitors
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Protein Kinase Inhibitors / pharmacology
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Quinazolines
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RNA, Small Interfering / genetics
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Recombinant Fusion Proteins / genetics
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Recombinant Fusion Proteins / metabolism
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Shc Signaling Adaptor Proteins
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Src Homology 2 Domain-Containing, Transforming Protein 1
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Transfection
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Tyrphostins / pharmacology
Substances
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Adaptor Proteins, Signal Transducing
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Intracellular Signaling Peptides and Proteins
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Leupeptins
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Ligands
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Macrolides
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Phosphoproteins
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Protease Inhibitors
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Proteasome Inhibitors
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Protein Kinase Inhibitors
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Quinazolines
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RNA, Small Interfering
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Recombinant Fusion Proteins
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SHC1 protein, human
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Shc Signaling Adaptor Proteins
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Shc1 protein, mouse
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Src Homology 2 Domain-Containing, Transforming Protein 1
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Tyrphostins
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RTKI cpd
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Epidermal Growth Factor
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bafilomycin A1
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ErbB Receptors
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Calcium-Calmodulin-Dependent Protein Kinase Type 1
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PNCK protein, human
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Extracellular Signal-Regulated MAP Kinases
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Proteasome Endopeptidase Complex
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benzyloxycarbonylleucyl-leucyl-leucine aldehyde