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Differential expression of multiple MDM2 messenger RNAs and proteins in normal and tumorigenic breast epithelial
Abstract:
The MDM2 gene is a nuclear phosphoprotein that is regulated by p53 and functions, in one capacity, to inhibit the transcriptional activity of the wild-type p53 protein. Multiple MDM2 transcripts were detected in human breast epithelial cells. In estrogen receptor-negative normal, immortal, and tumorigenic breast epithelial cells, we found a good correlation between MDM2 mRNA levels and expression of wild-type p53. When wild-type p53 was overexpressed in estrogen receptor-negative tumor cells containing a mutant or no endogenous p53, MDM2 mRNA levels increased significantly, indicating that wild-type p53 positively influences MDM2 mRNA levels in these tumor cells. Because all estrogen receptor-positive breast tumor cells had high MDM2 mRNA levels regardless of the status of their endogenous p53 protein, other factors likely influence MDM2 expression in these cells. Distinct MDM2 proteins (range, Mr 54,000-68,000 and 90,000-100,000, respectively) were differentially expressed in human breast epithelial cells. The lower molecular weight MDM2 proteins were most abundant in the normal mammary cells but present at varying levels in many of the tumor cells examined. MDM2 was a nuclear protein; however, nuclear staining intensity did not always correlate with the amount of MDM2-immunoreactive protein as determined by Western blot analysis. This discrepancy suggests that MDM2 interacts with novel cellular proteins in different kinds of breast epithelial cells.
Insights
The MDM2 gene
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- The MDM2 gene encodes a nuclear phosphoprotein that regulates p53 activity.
- MDM2 inhibits the transcriptional activity of wild-type p53.
- Multiple MDM2 transcripts exist in human breast epithelial cells.
Purpose of the Study:
- To investigate the regulation of MDM2 mRNA and protein expression in different subtypes of human breast epithelial cells.
- To explore the relationship between MDM2 expression and p53 status in estrogen receptor-positive and -negative breast cancer cells.
Main Methods:
- Analysis of MDM2 mRNA levels via transcript detection.
- Overexpression of wild-type p53 in tumor cells.
- Western blot analysis to determine MDM2 protein levels.
- Immunohistochemical staining for nuclear MDM2.
Main Results:
- MDM2 mRNA levels correlated with wild-type p53 expression in estrogen receptor-negative cells.
- Overexpressing wild-type p53 increased MDM2 mRNA in these cells.
- Estrogen receptor-positive breast tumor cells consistently showed high MDM2 mRNA levels, irrespective of p53 status.
- Distinct MDM2 protein isoforms were differentially expressed, with lower molecular weight forms most abundant in normal cells.
Conclusions:
- Wild-type p53 positively influences MDM2 mRNA levels in estrogen receptor-negative breast tumor cells.
- Other factors likely regulate MDM2 expression in estrogen receptor-positive breast tumor cells.
- MDM2 protein expression varies across different breast epithelial cell types, suggesting interactions with novel cellular proteins.