Nuclear entry of activated MAPK is restricted in primary ovarian and mammary epithelial cells.
BACKGROUND
The MAPK/ERK1/2 serine kinases are primary mediators of the Ras mitogenic signaling pathway. Phosphorylation by MEK activates MAPK/ERK in the cytoplasm, and phospho-ERK is thought to enter the nucleus readily to modulate transcription.


RESULTS
Here, however, we observe that in primary cultures of breast and ovarian epithelial cells, phosphorylation and activation of ERK1/2 are disassociated from nuclear translocalization and transcription of downstream targets, such as c-Fos, suggesting that nuclear translocation is limited in primary cells. Accordingly, in import assays in vitro, primary cells showed a lower import activity for ERK1/2 than cancer cells, in which activated MAPK readily translocated into the nucleus and activated c-Fos expression. Primary cells express lower levels of nuclear pore complex proteins and the nuclear transport factors, importin B1 and importin 7, which may explain the limiting ERK1/2 import found in primary cells. Additionally, reduction in expression of nucleoporin 153 by siRNA targeting reduced ERK1/2 nuclear activity in cancer cells.


CONCLUSIONS
ERK1/2 activation is dissociated from nuclear entry, which is a rate limiting step in primary cells and in vivo, and the restriction of nuclear entry is disrupted in transformed cells by the increased expression of nuclear pores and/or nuclear transport factors.
Smith
Elizabeth R
ER
Department of Medicine, University of Miami Miller School of Medicine, Miami, Florida, United States of America. esmith@med.miami.edu
Cai
Kathy Qi
KQ
Smedberg
Jennifer L
JL
Ribeiro
Melina M
MM
Rula
Malgorzata E
ME
Slater
Carolyn
C
Godwin
Andrew K
AK
Xu
Xiang-Xi
XX
D021581:Active Transport, Cell Nucleus; D001943:Breast Neoplasms / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism*; D002478:Cells, Cultured; D004789:Enzyme Activation; D004847:Epithelial Cells / Q000166:cytology / Q000201:enzymology* / Q000378:metabolism; D005260:Female; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D042361:Mammary Glands, Human / Q000166:cytology; D008856:Microscopy, Fluorescence; D019950:Mitogen-Activated Protein Kinase 1 / Q000235:genetics / Q000378:metabolism*; D048052:Mitogen-Activated Protein Kinase 3 / Q000235:genetics / Q000378:metabolism*; D028861:Nuclear Pore Complex Proteins / Q000235:genetics / Q000378:metabolism; D010051:Ovarian Neoplasms / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D010053:Ovary / Q000166:cytology; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D046888:Tissue Array Analysis
D028861:Nuclear Pore Complex Proteins; D016760:Proto-Oncogene Proteins c-fos; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3