Effects of valproate on Tax and HBZ expression in HTLV-1 and HAM/TSP T lymphocytes.

A determinant of human T-lymphotropic virus-1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) development is the HTLV-1-infected cell burden. Viral proteins Tax and HBZ, encoded by the sense and antisense strands of the pX region, respectively, play key roles in HTLV-1 persistence. Tax drives CD4(+)-T cell clonal expansion and is the immunodominant viral antigen recognized by the immune response. Valproate (2-n-propylpentanoic acid, VPA), a histone deacetylase inhibitor, was thought to trigger Tax expression, thereby exposing the latent HTLV-1 reservoir to immune destruction. We evaluated the impact of VPA on Tax, Gag, and HBZ expressions in cultured lymphocytes from HTLV-1 asymptomatic carriers and HAM/TSP patients. Approximately one-fifth of provirus-positive CD4(+) T cells spontaneously became Tax-positive, but this fraction rose to two-thirds of Tax-positive-infected cells when cultured with VPA. Valproate enhanced Gag-p19 release. Tax- and Gag-mRNA levels peaked spontaneously, before declining concomitantly to HBZ-mRNA increase. VPA enhanced and prolonged Tax-mRNA expression, whereas it blocked HBZ expression. Our findings suggest that, in addition to modulating Tax expression, another mechanism involving HBZ repression might determine the outcome of VPA treatment on HTLV-1-infected-cell proliferation and survival.

A tryptophan-rich motif in the carboxyl terminus of the small envelope protein of hepatitis B virus is central to the assembly of hepatitis delta virus particles.

The small hepatitis B virus surface antigen (S-HBsAg) is capable of driving the assembly and secretion of hepatitis delta virus (HDV) particles by interacting with the HDV ribonucleoprotein (RNP). Previously, a specific domain of the S-HBsAg protein carboxyl terminus, including a tryptophan residue at position 196 (W196), was proven essential for HDV maturation (S. Jenna and C. Sureau, J. Virol. 73: 3351-3358, 1999). Mutation of W196 to phenylalanine (W196F) was permissive for HBV subviral particle (SVP) secretion but deleterious to HDV virion assembly. Here, the W196F S-HBsAg deficiency was assigned to a loss of its ability for interaction with the large HDV antigen (L-HDAg), a major component of the RNP. Because the overall S-HBsAg carboxyl terminus is particularly rich in tryptophan, an amino acid frequently involved in protein-protein interactions, site-directed mutagenesis was conducted to investigate the function of the S-HBsAg Trp-rich domain in HDV assembly. Single substitutions of tryptophan between positions 163 and 201 with alanine or phenylalanine were tolerated for SVP secretion, but those affecting W196, W199, and W201 were detrimental for HDV assembly. This was proven to result from a reduced capacity of the mutants for interaction with L-HDAg. In addition, a W196S S-HBsAg mutant, which has been described in HBV strains that arose in a few cases of lamivudine-treated HBV-infected patients, was deficient for HDV assembly as a consequence of its impaired capacity for interacting with L-HDAg. Interestingly, the fact that even the most conservative substitution of phenylalanine for tryptophan at positions 196, 199, or 201 was sufficient to ablate interaction of S-HBsAg with L-HDAg suggests that W196, W199, and W201 are located at a binding interface that is central to HDV maturation.