Abstract 1052: 26S Proteasome: Intracellular Localization and Regulation by Nitric Oxide
Introduction: Nitric oxide (NO) is known to inhibit vascular smooth muscle cell (VSMC) proliferation by modulating cell cycle proteins. The 26S proteasome tightly regulates the degradation of most cell cycle proteins. Therefore, we previously studied the relationship between NO and the 26S proteasome, and found that NO directly inhibits 26S proteasome catalytic activity. The purpose of this study was two-fold:
to determine the effect of NO on 26S proteasome subunit expression, and
to determine the intracellular localization of these subunits.
Methods: Protein expression was examined using western blot analysis conducted on rat aortic VSMC exposed to varying concentrations of the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 125–1000 μM) for 24 hours. Intracellular localization was conducted with immunofluorescent staining of VSMC ± exposure to SNAP (500 μM). VSMC were also fractionated into their nuclear and cytoplasmic components and western blot analysis was conducted to confirm localization.
Results: Of the 26S proteasome subunits examined, the α5, α6, β1, and inducible β1 subunits demonstrated increased protein expression with increasing concentrations of NO. However, β2, inducible β2, β5, and inducible β5 protein expression did not change with NO exposure. Immunofluorescent staining of VSMC for these subunits confirmed these results. Additionally, each of these subunits showed a distinct pattern of staining: the α5 subunit was mostly perinuclear, the α6 subunit appeared mostly nuclear, and the β1 and inducible β1 subunits were mostly cytoplasmic. Western blot analysis following nuclear and cytoplasmic separation confirmed that α5, β1, and inducible β1 subunits were mostly in the cytoplasmic fraction. Interestingly, while the α6 subunit localizes to the nucleus by immunofluorescent staining, western blotting showed it to be mostly in the cytoplasmic fraction, indicating that its cellular distribution is perinuclear.
Conclusion: Our data indicate that the 26S proteasome subunits have distinct patterns of intracellular localization, and are differentially regulated by NO. Understanding the relationship between NO and the 26S proteasome provides insight into the mechanism by which NO inhibits VSMC proliferation.