The combined efforts of viruses to suppress apoptosis in general and caspase activity in particular suggest that this cellular response is to be avoided at all costs. However, caspases also have roles in cell proliferation, differentiation, and NF-κB activation (69
). These roles for caspase activation raise the question of whether viruses can positively modulate or utilize caspase activity to facilitate replication. The role of the HPV-16 E6 long isoform in caspase-8 degradation has already been highlighted. The E6 short isoform, which is only produced by so-called high-risk HPV types, can also bind to caspase-8. However, this molecular interaction results in caspase-8 stabilization (40
). The implications from these findings are that, like inhibition of caspase activity, stabilization of limited caspase-8 activity may be important for cell survival or maintaining a cell at a particular stage of differentiation. Thus, caspases may be targeted by certain viruses to prolong cell viability or aid in cellular transformation.
Examples of viruses that directly utilize caspase activity to facilitate replication also exist. Permissive replication of Aleutian mink disease parvovirus (ADV) in cell culture is associated with an apoptotic response that can be blocked by treatment with caspase inhibitors. There are numerous examples of virus infection in which blockage of apoptosis increases the virus titer recovered from treated cells, consistent with the antiviral role of apoptosis (23
). However, in the case of ADV, inhibition of specific caspases results in decreased virus yield, suggesting that caspase activity facilitates virus replication (13
). The major nonstructural protein of ADV, NS1, is required for many replication functions including control of viral and cellular gene transcription, viral DNA replication, and capsid assembly. Caspases mediate cleavage of NS1 early in virus replication, particularly by effector caspase-3, caspase-7, or both (12; S.M. Best, unpublished data). In the absence of cleavage, translocation of NS1 to the site of virus replication in the nucleus is impaired. It appears that the role of caspases is to generate a C-terminal cleavage product that contains the NS1 nuclear localization sequence. This product forms oligomers with full-length NS1 and facilitates transport of the latter to the nucleus (M.E. Bloom & S.M. Best, unpublished data).
Although a clear requirement exists for caspases in ADV replication, it is currently unknown how this contributes to virus pathogenesis. In infected mink kits, permissive replication of ADV occurs in pulmonary type II pneumocytes, resulting in high levels of cytopathology and mortality. In contrast, infection of adult mink results in restricted replication in lymph node macrophages and persistent infection. It is possible that cell-type-specific regulation of caspase activity modulates the degree of NS1 nuclear translocation and hence its function in DNA replication and viral gene expression. In adult mink, for example, tight regulation of caspase activity in macrophages may restrict nuclear translocation of NS1 and virus replication, contributing to persistent infection. However, apoptosis may be triggered during virus infection of pneumocytes, resulting in elevated caspase activation and permissive replication in mink kits.
Adenoviral proteins are also cleaved during virus replication. The early transcription units E1A (which encodes two proteins, 12S and 13S) and E1B encode proteins involved in transactivation of both cellular and viral transcription as well as cellular transformation. Following transient expression of the E1A proteins from Ad2 or Ad12, caspases cleave both 12S and 13S at multiple sites, resulting in progressive truncation from the N termini (48
). This cleavage disrupts interactions of E1A with cellular transcription-regulating proteins that bind to the N terminus but not to the C terminus (48
). Thus, caspases have a potential regulatory role in E1A-mediated gene expression. E1A is also proteolytically cleaved by caspases during Ad5 replication. However, cleavage of E1A proteins, as well as cellular proteins normally cleaved during apoptosis, is limited and dependent on cell type (48
). The degree of cleavage is likely influenced by temporal expression of adenovirus-encoded inhibitors of apoptosis such as E3, the protein responsible for the downregulation of cell surface death receptors (116
). Thus, an intricate interplay between caspase activation and inhibition in a particular cell type may be required for optimal virus replication.
Additional examples of viruses that utilize caspase activity include human astroviruses that cause viral gasteroenteritis. In this case, caspase-mediated cleavage of the capsid precursor protein facilitates the release of viral particles from the cell (80
). Other examples of virus-encoded proteins cleaved by caspases include NS5A of hepatitis C virus (46
), the nucleocapsid protein of influenza A virus (138
), and ICP22 of herpes simplex virus (85
), although the role of protein cleavage in replication of these viruses is unclear (11
). Cleavage may facilitate protein degradation and negatively impact virus replication as described for the granzyme H-mediated cleavage of the Ad5−100K protein. However, because mutation of the crucial Asp residue would eliminate the enzyme recognition site, conservation of these sequences suggests a selection for caspase-mediated cleavage in virus replication (11
). Hence, in addition to facilitating virus release, caspases may have roles in regulation of virus protein maturation and function as suggested by studies using ADV and adenoviruses.