Kaposi's sarcoma-associated herpesvirus downregulates transforming growth factor β2 to promote enhanced stability of capillary-like tube formation.

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of Kaposi's sarcoma (KS), the most common tumor of AIDS patients worldwide. A key characteristic of KS tumors is extremely high levels of vascular slits and extravasated red blood cells, making neoangiogenesis a key component of the tumor. The main KS tumor cell is the spindle cell, a cell of endothelial origin that maintains KSHV predominantly in the latent state. In cultured endothelial cells, latent KSHV infection induces angiogenic phenotypes, including longer-term stabilization of capillary-like tube formation in Matrigel, a basement membrane matrix. The present studies show that KSHV infection of endothelial cells strongly downregulates transforming growth factor β2 (TGF-β2). This downregulation allows the stabilization of capillary-like tube formation during latent infection, as the addition of exogenous TGF-β2 inhibits the KSHV-induced stability of these structures. While two KSHV microRNAs are sufficient to downregulate TGF-β2 in endothelial cells, they are not required during KSHV infection. However, activation of the gp130 cell surface receptor is both necessary and sufficient for downregulation of TGF-β2 in KSHV-infected cells.

Objective: Kaposi's sarcoma is a highly vascularized, endothelial cell-based tumor supporting large amounts of angiogenesis. There is evidence that KSHV, the etiologic agent of KS, induces aberrant angiogenesis. For example, KSHV induces stabilization of capillary-like tube formation in cultured endothelial cells. A clearer understanding of how KSHV regulates angiogenesis could provide potential therapeutic targets for KS. We found that KSHV downregulates TGF-β2, a cytokine related to TGF-β1 that is known to inhibit angiogenesis. The downregulation of this inhibitor promotes the stability of capillary-like tube formation insofar as adding back TGF-β2 to infected cells blocks KSHV-induced long-term tubule stability. Therefore, KSHV downregulation of TGF-β2 may increase aberrant vascularization in KS tumors through increased capillary formation and thereby aid in KS tumor promotion.