Evidence that Proteasome-Dependent Degradation of the Retinoblastoma Protein in Cells Lacking A-Type Lamins Occurs Independently of Gankyrin and MDM2

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Abstract
A-type lamins, predominantly lamins A and C, are nuclear intermediate filaments believed to act as scaffolds for assembly of transcription factors. Lamin A/C is necessary for the retinoblastoma protein (pRB) stabilization through unknown mechanism(s). Two oncoproteins, gankyrin and MDM2, are known to promote pRB degradation in other contexts. Consequently, we tested the hypothesis that gankyrin and/or MDM2 are required for enhanced pRB degradation in Lmna−/− fibroblasts. Principal Findings. To determine if gankyrin promotes pRB destabilization in the absence of lamin A/C, we first analyzed its protein levels in Lmna−/− fibroblasts. Both gankyrin mRNA levels and protein levels are increased in these cells, leading us to further investigate its role in pRB degradation. Consistent with prior reports, overexpression of gankyrin in Lmna+/+ cells destabilizes pRB. This decrease is functionally significant, since gankyrin overexpressing cells are resistant to p16ink4a-mediated cell cycle arrest. These findings suggest that lamin A-mediated degradation of pRB would be gankyrin-dependent. However, effective RNAi-enforced reduction of gankyrin expression in Lmna−/− cells was insufficient to restore pRB stability. To test the importance of MDM2, we disrupted the MDM2-pRB interaction by transfecting Lmna−/− cells with p14arf. p14arf expression was also insufficient to stabilize pRB or confer cell cycle arrest, suggesting that MDM2 also does not mediate pRB degradation in Lmna−/− cells. Our findings suggest that pRB degradation in Lmna−/− cells occurs by gankyrin and MDM2-independent mechanisms, leading us to propose the existence of a third proteasome-dependent pathway for pRB degradation. Two findings from this study also increase the likelihood that lamin A/C functions as a tumor suppressor. First, protein levels of the oncoprotein gankyrin are elevated in Lmna−/− fibroblasts. Second, Lmna−/− cells are refractory to p14arf-mediated cell cycle arrest, as was previously shown with p16ink4a. Potential roles of lamin A/C in the suppression of tumorigenesis are discussed.