Research Communication Modulation of Proteasome Activity by Vitamin E in THP-1 Monocytes Adelina Munteanu 1,3 , Roberta Ricciarelli 2 , Sara Massone 2 and Jean-Marc Zingg 1 1 Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland 2 Dipartimento Medicina Sperimentale, University of Genoa, Genoa, Italy 3 Physiology Department, Medical Faculty, ‘Carol Davila’ University of Medicine and Pharmacy, Bucharest, Romania Summary In THP-1 monocytes, cellular proteasome inhibition by ritonavir or ALLN is associated with increased production of oxidative stress. Both compounds produced comparable amounts of oxidative stress; however, normalization by a-tocopherol occurred solely after inhibition by ritonavir, and not by ALLN. Similar to that, a- tocopherol could normalize the reduced formation of 3-nitro- tyrosine-modified proteins only after ritonavir treatment. In the absence of any proteasome inhibitor, intrinsic cellular proteasome activity was not modulated by a-, b-, and g-tocopherols; however, d- tocopherol, a-tocotrienol, and a-tocopheryl phosphate could sig- nificantly inhibit cellular proteasome activity and increased the level of p27 Kip1 and p53. Since oxidative stress was reduced by a- tocopherol only after proteasome inhibition by ritonavir and not by ALLN, it is concluded that, in this experimental system, a- tocopherol does not act as an antioxidant but interferes with the inhibitory effect of ritonavir. IUBMB Life, 59: 771–780, 2007 Keywords Ritonavir; ALLN; oxidative stress; 3-nitrotyrosine; proteasome; proliferation; scavenger receptors; CD36; atherosclerosis. Abbreviations LDL, low density lipoproteins; ARPI, anti-retroviral protease inhibitor. INTRODUCTION Increased plasma cholesterol levels and oxidative stress are known to enhance the risk for cardiovascular diseases. Based on this, current strategies to prevent atherosclerosis are aimed either at lowering the cholesterol load of lipoproteins or at reducing oxidative stress, e.g., by supplementation with mole- cules able to scavenge free radicals, such as vitamin E and C. In fact, several studies describe preventive effects of vitamin E against atherosclerosis and a number of other diseases, but a secure correlation is not universally accepted (reviewed in 1, 2). a-Tocopherol, the most abundant isoform of vitamin E in human plasma, has been hypothesized to act at the molecular level by reducing the formation of oxidized small molecules, proteins and lipids, which are possible causes for cellular de- regulation and consequent atherosclerotic lesion development (3). In addition, it may affect many cellular events occurring during atherosclerosis by directly modulating signal transduc- tion and gene expression (reviewed in 2, 4). Increased oxidative stress as assessed by the amount of lipid peroxidation is also measured in patients with HIV infection, and can be prevented by treatment with vitamin E and C (5). Moreover, the prolonged treatment with inhibitors of human immunodeficiency (HIV)-encoded protease (ARPI) increases the risk of drug-induced premature atherosclerosis, and up- regulation of CD36 scavenger receptor by ARPI has been shown to be centrally involved in this process (6 – 8). In human THP-1 monocytes, the mechanism of CD36 up-regulation by one of the ARPI, ritonavir, involves inhibition of the ubiquitin-proteasome system, and a-tocopherol is able to normalize cellular 20S proteasome activity after ritonavir treatment (9, 10). Interestingly, the proteasome is known to be directly inhibited by oxidative modification occurring after coronary occlusion/reperfusion as well as in atherosclerotic plaques, suggesting that compounds with antioxidant action may normalize proteasome activity in diseases associated with increased oxidative stress (11, 12). Both, the 20S and the 26S proteasomes are known to be inhibited by oxidative stress, albeit with different potency (13). On the other hand, inhibition of the proteasome by several proteasome inhibitors (bortezomib, MG-132, PSI (Z-IE(OtBu)AL-CHO), lactacys- tin, epoxomicin) increases the production of oxidative stress, ultimately leading to apoptosis (14, 15). Bortezomib-induced Received 15 August 2007; accepted 21 September 2007 Address correspondence to: Jean-Marc Zingg, PhD, Institute of Biochemistry and Molecular Medicine, University of Bern, Bu¨hlstrasse 28, 3012 Bern, Switzerland. Tel: þ41 31 631 41 18. Fax: þ41 31 631 37 37. E-mail: zin34@swissonline.ch Present Address: Jean-Marc Zingg, Vascular Biology Laboratory, Office 621, JM USDA-Human Nutrition Research Centre On Aging, Tufts University, 711 Washington St., Boston, MA 02111, USA. E-mail: Jean-Marc.Zingg@tufts.edu IUBMB Life, 59(12): 771 – 780, December 2007 ISSN 1521-6543 print/ISSN 1521-6551 online Ó 2007 IUBMB DOI: 10.1080/15216540701697420