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Role of Drosophila retinoblastoma protein instability element in cell growth and proliferation
Jared S Elenbaas1, Rima Mouawad, R William Henry
1a Department of Biochemistry and Molecular Biology ; Michigan State University ; East Lansing , MI USA.
Abstract:
The RB tumor suppressor, a regulator of the cell cycle, apoptosis, senescence, and differentiation, is frequently mutated in human cancers. We recently described an evolutionarily conserved C-terminal "instability element" (IE) of the Drosophila Rbf1 retinoblastoma protein that regulates its turnover. Misexpression of wild-type or non-phosphorylatable forms of the Rbf1 protein leads to repression of cell cycle genes. In contrast, overexpression of a defective form of Rbf1 lacking the IE (ΔIE), a stabilized but transcriptionally less active form of the protein, induced ectopic S phase in cell culture. To determine how mutations in the Rbf1 IE may induce dominant effects in a developmental context, we assessed the impact of in vivo expression of mutant Rbf1 proteins on wing development. ΔIE expression resulted in overgrowth of larval wing imaginal discs and larger adult wings containing larger cells. In contrast, a point mutation in a conserved lysine of the IE (K774A) generated severely disrupted, reduced wings. These contrasting effects appear to correlate with control of apoptosis; expression of the pro-apoptotic reaper gene and DNA fragmentation measured by acridine orange stain increased in flies expressing the K774A isoform and was suppressed by expression of Rbf1ΔIE. Intriguingly, cancer associated mutations affecting RB homologs p130 and p107 may similarly induce dominant phenotypes.
Insights
Mutations in the retinoblastoma protein's instability element (IE) impact cell cycle regulation and apoptosis. Specific mutations in Drosophila Rbf1 IE cause developmental defects, influencing cell growth and survival.
Area of Science:
- Developmental Biology
- Molecular Biology
- Cancer Research
Background:
- The RB tumor suppressor is crucial for cell cycle control and is often mutated in cancers.
- A conserved C-terminal "instability element" (IE) in Drosophila Rbf1 regulates protein turnover.
- Altered Rbf1 protein levels and activity affect cell cycle gene expression.
Purpose of the Study:
- To investigate the in vivo developmental effects of mutations in the Rbf1 instability element (IE).
- To understand how Rbf1 IE mutations might lead to dominant phenotypes in development.
- To correlate these effects with apoptosis control.
Main Methods:
- In vivo expression of wild-type, IE-deleted (ΔIE), and point-mutated (K774A) Rbf1 proteins in Drosophila.
- Assessment of larval wing imaginal disc growth and adult wing size and cell morphology.
- Analysis of apoptosis using expression of the reaper gene and acridine orange staining.
Main Results:
- Expression of Rbf1 ΔIE led to overgrowth of wing imaginal discs and larger adult wings with larger cells.
- A point mutation (K774A) in the IE caused severely disrupted and reduced adult wings.
- The K774A mutation increased apoptosis, while Rbf1 ΔIE expression suppressed it.
Conclusions:
- Mutations in the Rbf1 IE can induce dominant developmental phenotypes in vivo.
- The contrasting effects of Rbf1 mutants correlate with differential control of apoptosis.
- Cancer-associated mutations in RB homologs may exert similar dominant effects.
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