Expression of K13/v-FLIP gene of human herpesvirus 8 and apoptosis in Kaposi's sarcoma spindle cells

Background: Human herpesvirus 8 (HHV8) infection is associated with all for ms of Kaposi's sarcoma (KS), The HHV8 genome locus ORFK13-72-73 (ORF = open reading frame) encodes proteins that may be important in HHV8-mediated pat hogenesis, i.e., the latency-associated nuclear antigen (encoded by ORF73), viral-cyc-D (v-cyc-D), a viral homologue of cellular cyclin D (encoded by ORF72), and viral-FLIP (v-FLIP), a homologue of the cellular FLICE (Fas-ass ociated death domain-like interleukin 1 beta-converting enzyme) inhibitory protein (encoded by ORFK13; is an inhibitor of apoptosis [programmed cell d eath]), Through differential splicing events, this locus expresses individu al RNA transcripts that encode all three proteins (tricistronic transcripts ) or just two of them (v-FLIP and v-cyc-D; bicistronic transcripts). We exa mined expression of these transcripts in KS tissues. Methods: We collected tissues from patients with KS of different stages. By use of an optimized i n situ hybridization procedure, we examined different ORFK13-72-73 locus tr anscripts in HHV8-infected cells in skin lesions and in one adjacent lymph node. Apoptosis in KS lesions was determined by use of an in situ assay. Re sults and Conclusions: Our results indicate the following: 1) Transcripts f rom the ORFK13-72-73 locus appear to be spliced differentially in latently infected KS cells in skin lesions and in HHV8-infected cells in lymph nodes ; specifically, ORFK13-ORF72 bicistronic transcripts were expressed abundan tly in KS cells, whereas ORFK13-ORF72-ORF73 tricistronic transcripts were d etected only in lymph node cells. 2) Sequences encoding the antiapoptotic p rotein v-FLIP are expressed at very low levels in early KS lesions, but exp ression increases dramatically in late-stage lesions. 3) The increase in ex pression of v-FLIP-encoding transcripts is associated with a reduction in a poptosis in KS lesions. Implications: These data suggest that functional v- FLIP is produced in vivo and that antiapoptotic mechanisms may be involved in the rapid growth of KS lesions, where only a few cells undergoing mitosi s are generally observed.