CELLULAR PATHWAYS INVOLVED IN THE EX-VIVO EXPRESSION OF BOVINE LEUKEMIA-VIRUS

Bovine leukemia virus (BLV) is the etiologic agent of enzootic bovine leukosis. The virus adopts a strategy based on the lack of viral expre ssion in vivo: only very rare BLV-infected B lymphocytes express viral information. When the cells are isolated from animals in persistent l ymphocytosis and cultivated ex vivo, a tremendous increase in viral ex pression occurs. To gain insight into this mechanism, we employed a ge neral approach using chemicals that interfere specifically with cellul ar pathways involved in signal transduction from the cell membrane to the nucleus. Our data demonstrate that BLV expression is not correlate d with the activity of protein kinase A (PKA) and is even inhibited by cyclic AMP (cAMP). The cAMP/PKA pathway is thus apparently not involv ed in ex vivo viral expression. In contrast, PKC appears to play a key role in this process. Phorbol myristate acetate can directly activate viral expression in B cells (in the absence of T cells). Furthermore, calphostin C, a highly specific inhibitor of PKC, partly decreases ex vivo BLV expression. Our data further demonstrate that calmodulin and calcineurin, a calmodulin-dependent phosphatase, play a key role in t he induction of viral expression. The involvement of this calmodulin-d ependent pathway could explain the induction of expression that cannot be assigned to PKC. Furthermore, it appears that the activation of vi ral expression requires a calmodulin but not a PKA-dependent pathway. These data highlight major differences between transient transfection and ex vivo experiments. Finally, despite their homologies, BLV and hu man T-cell leukemia virus appear to use different signal transduction pathways to induce viral expression.