Immortalization of Primary Endothelial Cells by the K1 Protein of Kaposi's Sarcoma–Associated Herpesvirus

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked to three different human cancers: Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The Kaposi's sarcoma lesion expresses high levels of angiogenic factors and is comprised of a mixed cell population, including endothelial cells that are infected with KSHV. We find that the KSHV K1 protein is expressed in Kaposi's sarcoma lesions and can immortalize and extend the life span of primary human umbilical vein endothelial cells in culture. Vascular endothelial growth factor (VEGF) is critical for the survival of endothelial cells, and we show that expression of K1 in endothelial cells resulted in increased levels of secreted VEGF and the activation of key signaling pathways, including the VEGF/VEGF receptor and the phosphatidylinositol-3V-OH-kinase (PI3K) pathway. The SH2 binding motifs present in the cytoplasmic tail of K1 were critical for K1's ability to activate these pathways. Activation of PI3K by K1 results in activation of Akt kinase and mammalian target of rapamycin and inactivation of the proapoptotic proteins FKHR, glycogen synthase kinase-3, and Bad, which are events indicative of cell survival. Because activation of the PI3K pathway is critical for transformation of many human cells, we suggest that PI3K activation by K1 is involved in endothelial cell immortalization and contributes to KSHV-associated tumorigenesis. We also report that K1 enhances angiogenesis in vivo and increases tumor vasculature and tumor size. (Cancer Res 2006; 66(7): 3658-66)

Dual inhibition of PI3K and mTOR inhibits autocrine and paracrine proliferative loops in PI3K/Akt/mTOR-addicted lymphomas

“…[12][13][14][15][16] KSHV-infected PELs also display constitutive high-level activation of the PI3K/Akt/ mTOR pathway. 10 Our findings indicate that inhibiting this pathway is an effective modality for treating PEL.…”

“…We, and others, have previously shown that individual viral proteins, such as KSHV K1, activate the PI3K/Akt pathway in B cells and endothelial cells. [12][13][14][15][16] Another viral protein, KSHV vGPCR, can also activate this pathway in endothelial and epithelial cells. [17][18][19][20][21] KSHV-infected PELs display constitutive activation of the PI3K/ Akt/mTOR pathway.…”

KSHV and the pathogenesis of Kaposi sarcoma: listening to human biology and medicine

“…Chief among these is ORF-K1, a transmembrane signaling molecule whose activity mimics signaling via the B cell antigen receptor (159). These findings may be important in KS pathogenesis, since it has been shown that K1 overexpression in primary endothelial cells can substantially extend their lifespan (160).…”

“…KS lesions express all 3 factors, and lytic infection is associated with strong upregulation of angiopoietin 2 (174,175). And v-GPCR is not the only gene expressed in lytic infection that can upregulate VEGF; ORF-K1 expression, which is strongly upregulated during lytic infection (66,176), also has this activity (177,178). Other lytic viral proteins can upregulate inflammatory signaling by the host.…”

K1 protein of human herpesvirus 8 suppresses lymphoma cell Fas-mediated apoptosis

“…[14][15][16] Others have shown that K1 immortalizes cells. 11 To determine whether K1 regulates apoptosis, we focused on the Fas (Apo-1/ CD95) pathway, the most important apoptosis pathway that determines the fate of lymphocytes. 17,18 To delineate the role of K1 in cell survival, in our present study we tested the ability of this protein to interfere with apoptosis in K1-transfected mice and in BJAB lymphoma cells and hematopoietic and endothelium-related cell lines (THP-1 macrophages/monocytes, U937 monocytes, and KS SLK cells) expressing K1 or a K1 mutant with an ITAM (K1m) with the Fas ligand, an anti-Fas antibody, and other agents that induce apoptosis.…”