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Definition

Protease inhibitors are a novel class of drugs used for the treatment of viral diseases including human immunodeficiency virus (HIV) infection. Protease inhibitors selectively block HIV protease, an enzyme involved in the later stages of HIV replication.

Structural Characteristics

The four approved HIV-protease inhibitors are based on amino acid sequences recognized and cleaved in HIV proteins. Indinavir (Crixivan), nelfinavir (Viracept), ritonavir (Norvir), and saquinavir (Invirase and Fortovase). Most contain a synthetic analogue of the phenylalanine–proline sequence at positions 167 and 168 of the gag–pol polyprotein that is cleaved by the protease1.

Mode of Action

HIV, the retrovirus believed to be the cause of acquired immunodeficiency syndrome (AIDS), infects a variety of CD4+ cells, including lymphocytes and cells of the monocyte/macrophage lineage. Encoded in the HIV genome are several precursor proteins that must undergo proteolytic cleavage to yield functional proteins. The gag precursor protein of HIV (p55) is cleaved by a virally encoded aspartate protease (HIV protease). Since the cleavage of p55 is required for viral maturation and infectivity, HIV-protease inhibitors prevent cleavage of gag and gag–pol protein precursors in acutely and chronically infected cells, arresting maturation and thereby blocking the infectivity of nascent virions2. The main antiviral action of HIV-protease inhibitors is thus to prevent subsequent waves of infection; they have no effect on cells already harboring integrated proviral DNA. These agents are active against clinical isolates of HIV types 1 and 2, with the in vitro concentration of drug required to reduce viral production by 50 percent (IC50). Antiviral activity is correlated with the inhibition of enzyme activity, although the drug concentration required to reduce enzyme activity by 50 percent (Ki) is lower than the IC50.

Functions

Suppression of Preadipocyte Differentiation and Promotion of Adipocyte Death by HIV Protease Inhibitors: Many HIV-infected patients taking combination antiretroviral therapy that includes HIV protease inhibitors experience atrophy of peripheral subcutaneous adipose tissue. In a study the effects of HIV protease inhibitors on adipogenesis and adipocyte survival was investigated using the 3T3-L1 preadipocyte cell line. Several HIV protease inhibitors were found either to inhibit preadipocyte differentiation or to promote adipocyte cell death. It was found that, one protease inhibitor, nelfinavir, elicited both of these effects strongly. When induced to differentiate in the presence of nelfinavir, 3T3-L1 preadipocytes failed to accumulate cytoplasmic triacylglycerol and failed to express normal levels of the adipogenic transcription factors CCAAT/enhancer-binding protein a and peroxisome proliferator-activated receptor ?. The level of the proteolytically processed, active 68-kDa form of sterol regulatory element-binding protein-1, a transcription factor known to promote lipogenic gene expression, also was reduced markedly in nelfinavir-treated cells, whereas the level of the 125-kDa precursor form of this protein was unaffected. The inhibitory effect of nelfinavir occurred subsequent to critical early events in preadipocyte differentiation, expression of CCAAT/enhancer-binding protein ß and completion of the mitotic clonal expansion phase, because these events were unaffected by nelfinavir treatment. In addition, nelfinavir treatment of fully differentiated 3T3-L1 adipocytes resulted in DNA strand cleavage and severe loss of cell viability. In contrast, cell proliferation and viability of preadipocytes were unaffected by nelfinavir treatment. Thus, molecular or cellular changes that occur during acquisition of the adipocyte phenotype promote susceptibility to nelfinavir-induced cell death. Taken together, these results suggest that nelfinavir may promote adipose tissue atrophy by compromising adipocyte viability and preventing replacement of lost adipocytes by inhibiting preadipocyte differentiation3.

HIV protease inhibitors are potent anti-angiogenic molecules and promote regression of Kaposi sarcoma: Treatment with HIV-1 protease inhibitors is associated with a reduced incidence or regression of Kaposi sarcoma (KS). It as been shown that systemic administration of the PIs indinavir or saquinavir to nude mice blocks the development and induces regression of angioproliferative KS-like lesions promoted by primary human KS cells, basic fibroblast growth factor (bFGF), or bFGF and vascular endothelial growth factor (VEGF) combined. These protease inhibitors also block bFGF or VEGF-induced angiogenesis in the chorioallantoic membrane assay with potency similar to paclitaxel (Taxol). These effects are mediated by the inhibition of endothelial- and KS-cell invasion and of matrix metalloproteinase-2 proteolytic activation by protease inhibitors at concentrations present in plasma of treated individuals. As protease inhibitors also inhibit the in vivo growth and invasion of an angiogenic tumor-cell line, this data indicates that protease inhibitors are potent anti-angiogenic and anti-tumor molecules that might be used in treating non-HIV KS and in other HIV-associated tumors4.

References

  1. Debouck C (1992). The HIV-1 protease as a therapeutic target for AIDS. AIDS. Res. Hum. Retroviruses., 8:153-164.
  2. Bugelski PJ, Kirsh R, Hart TK (1994). HIV protease inhibitors: effects on viral maturation and physiologic function in macrophages. J. Leukoc. Biol., 56:374-380.
  3. Dowell P, Flexner C, Kwiterovich PO, Lane MD (2000). Suppression of Preadipocyte Differentiation and Promotion of Adipocyte Death by HIV Protease Inhibitors. Journal of Biological Chemistry, 275:41325-41332.
  4. Sgadari C, Barillari G, Toschi E, Carlei D, Bacigalupo I, Baccarini S, Palladino C, Leone P, Bugarini R, Malavasi L, Cafaro A, Falchi M, Valdembri D, Rezza G, Bussolino F, Monini P, Ensoli B (2002). HIV protease inhibitors are potent anti-angiogenic molecules and promote regression of Kaposi sarcoma. Nature Medicine, 8:225-232.

     

 

 

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