Caspase-3-mediated cleavage of p65/RelA results in a carboxy-terminal fragment that inhibits IkappaBalpha and enhances HIV-1 replication in human T lymphocytes.
BACKGROUND
Degradation of p65/RelA has been involved in both the inhibition of NF-kappaB-dependent activity and the onset of apoptosis. However, the mechanisms of NF-kappaB degradation are unclear and can vary depending on the cell type. Cleavage of p65/RelA can produce an amino-terminal fragment that was shown to act as a dominant-negative inhibitor of NF-kappaB, thereby promoting apoptosis. However, the opposite situation has also been described and the production of a carboxy-terminal fragment that contains two potent transactivation domains has also been related to the onset of apoptosis. In this context, a carboxy-terminal fragment of p65/RelA (DeltaNH2p65), detected in non-apoptotic human T lymphocytes upon activation, has been studied. T cells constitute one of the long-lived cellular reservoirs of the human immunodeficiency virus type 1 (HIV-1). Because NF-kappaB is the most important inducible element involved in initiation of HIV-1 transcription, an adequate control of NF-kappaB response is of paramount importance for both T cell survival and viral spread. Its major inhibitor IkappaBalpha constitutes a master terminator of NF-kappaB response that is complemented by degradation of p65/RelA.


CONCLUSIONS
In this study, the function of a caspase-3-mediated carboxy-terminal fragment of p65/RelA, which was detected in activated human peripheral blood lymphocytes (PBLs), was analyzed. Cells producing this truncated p65/RelA did not undergo apoptosis but showed a high viability, in spite of caspase-3 activation. DeltaNH2p65 lacked most of DNA-binding domain but retained the dimerization domain, NLS and transactivation domains. Consequently, it could translocate to the nucleus, associate with NF-kappaB1/p50 and IkappaBalpha, but could not bind -kappaB consensus sites. However, although DeltaNH2p65 lacked transcriptional activity by itself, it could increase NF-kappaB activity in a dose-dependent manner by hijacking IkappaBalpha. Thus, its expression resulted in a persistent transactivation activity of wild-type p65/RelA, as well as an improvement of HIV-1 replication in PBLs. Moreover, DeltaNH2p65 was increased in the nuclei of PMA-, PHA-, and TNFalpha-activated T cells, proving this phenomenon was related to cell activation. These data suggest the existence of a novel mechanism for maintaining NF-kappaB activity in human T cells through the binding of the carboxy-terminal fragment of p65/RelA to IkappaBalpha in order to protect wild-type p65/RelA from IkappaBalpha inhibition.
Coiras
Mayte
M
AIDS Immunopathology Unit, National Center of Microbiology, Instituto de Salud Carlos III, 28220 Majadahonda, Madrid, Spain. mcoiras@isciii.es
López-Huertas
María Rosa
MR
Mateos
Elena
E
Alcamí
José
J
D053148:Caspase 3 / Q000378:metabolism*; D002478:Cells, Cultured; D019281:Dimerization; D015497:HIV-1 / Q000254:growth & development*; D006801:Humans; D020672:I-kappa B Proteins / Q000037:antagonists & inhibitors*; D000072000:NF-KappaB Inhibitor alpha; D011485:Protein Binding; D013601:T-Lymphocytes / Q000821:virology*; D051996:Transcription Factor RelA / Q000378:metabolism*
D020672:I-kappa B Proteins; C000606204:NFKBIA protein, human; D051996:Transcription Factor RelA; D000072000:NF-KappaB Inhibitor alpha; D053148:Caspase 3