Normal human mammary epithelial cells proliferate rapidly in the presence of elevated levels of the tumor suppressors p53 and p21(WAF1/CIP1).
In normal cells, p53 protein is maintained at low levels, but the levels increase after stress or inappropriate growth signals to coordinate growth arrest or apoptosis. Human mammary epithelial cells (HMECs) are unusual in that they exhibit two phases of growth. The second growth phase, referred to as post-selection, follows a period of temporary growth arrest and is characterized by the absence of p16(INK4a) (also known as CDK4I and p16-INK4a) expression. Previously, we observed that post-selection HMECs have elevated levels of p53. Exogenous p16(INK4a) expression decreased levels of both p53 transcript and protein, and this effect was inhibited by nutlin-3a, indicating that p16(INK4a) can regulate p53 expression by affecting both p53 transcription and Mdm2-dependent degradation of p53. The p53 in post-selection HMECs was wild type and, as expected, increased p53 expression was associated with elevated p21(WAF1/CIP1) and Mdm2 levels; the p53 response to DNA damage seemed normal. Despite elevated levels of wild-type p53 and p21(WAF1/CIP1), post-selection cells grew more rapidly than their pre-selection HMEC precursors. We found that the post-selection HMECs contain a truncated Mdm2 protein (p60), which presumably lacks the p53 ubiquitylation domain. We propose that the increased levels of p53 in post-selection HMECs are due to the presence of an Mdm2 fragment that binds p53 but does not result in its degradation.
Huschtscha
Lily I
LI
Children's Medical Research Institute, Westmead, New South Wales 2145, Australia.
Moore
Jonathan D
JD
Noble
Jane R
JR
Campbell
Hamish G
HG
Royds
Janice A
JA
Braithwaite
Antony W
AW
Reddel
Roger R
RR
D002455:Cell Division; D049109:Cell Proliferation; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000378:metabolism; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism*; D004249:DNA Damage; D004847:Epithelial Cells / Q000166:cytology* / Q000378:metabolism*; D006801:Humans; D042361:Mammary Glands, Human / Q000166:cytology*; D008954:Models, Biological; D011499:Protein Processing, Post-Translational; D055550:Protein Stability; D051736:Proto-Oncogene Proteins c-mdm2 / Q000378:metabolism; D014158:Transcription, Genetic; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism*
D019941:Cyclin-Dependent Kinase Inhibitor p16; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016159:Tumor Suppressor Protein p53; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2