Dangerous liaisons: molecular basis for a syndemic relationship between Kaposi's sarcoma and P. falciparum malaria

Abstract

The most severe manifestations of malaria (caused by Plasmodium falciparum) occur as a direct result of parasitemia following invasion of erythrocytes by post-liver blood-stage merozoites, and during subsequent cyto-adherence of infected erythrocytes to the vascular endothelium. However, the disproportionate epidemiologic clustering of severe malaria with aggressive forms of endemic diseases such as Kaposi's sarcoma (KS), a neoplasm that is etiologically linked to infection with KS-associated herpesvirus (KSHV), underscores the significance of previously unexplored co-pathogenetic interactions that have the potential to modify the overall disease burden in co-infected individuals. Based on recent studies of the mechanisms that P. falciparum and KSHV have evolved to interact with their mutual human host, several new perspectives are emerging that highlight a surprising convergence of biological themes potentially underlying their associated co-morbidities. Against this background, ongoing studies are rapidly constructing a fascinating new paradigm in which the major host receptors that control parasite invasion (Basigin/CD147) and cyto-adherence (CD36) are, surprisingly, also important targets for exploitation by KSHV. In this article, we consider the major pathobiological implications of the co-option of Basigin/CD147 and CD36 signaling pathways by both P. falciparum and KSHV, not only as essential host factors for parasite persistence but also as important mediators of the pro-angiogenic phenotype within the virus-infected endothelial microenvironment. Consequently, the triangulation of interactions between P. falciparum, KSHV, and their mutual human host articulates a syndemic relationship that points to a conceptual framework for prevalence of aggressive forms of KS in malaria-endemic areas, with implications for the possibility of dual-use therapies against these debilitating infections in resource-limited parts of the world.

Keywords: Basigin/CD147; CD36; HHV-8; Kaposi’s sarcoma; PfEMP-1; PfRh5; malaria.

FIGURE 1

Basic interactions of P. falciparum with human erythrocytes during invasion (A) and with blood capillary endothelium during cyto-adherence (B). Upon exit from the liver, invasion of red blood cells by the blood-stage merozoites leads to replication and subsequent surface expression of the multi-domain PfEMP-1 molecule that mediates cyto-adherence via the binding activities of DBL1, CIDR1, and DBL2 domains of PfEMP-1 with various host adhesion receptors. Shown here is a region of PfEMP-1 encoded by FBR3S1.2-var1 (Chen et al., 2000). The CIDR1 domain primarily binds to CD36 and to members of the immunoglobulin superfamily, including IgM and CD31/PECAM (platelet endothelial cell adhesion molecule), whereas the DBL2 domain binds mainly to CD31/PECAM-1.

FIGURE 2

(A) Upregulation of CD36 by both the PfEMP-1-CIDR1α-derived MC179 peptide (lanes 5 and 6) and by KSHV gB-derived heptad peptide ligands HR2 (lanes 3 and 4), and the less potent HR1 (lanes 1 and 2) are both blocked by a monoclonal anti-CD36 antibody. Note that anti-CD36 did not block control upregulation of CD36 in response to treatment with sodium butyrate (NaB; lanes 7 and 8). (B) Activation of KSHV RTA by MC179 occurs via a CD36-dependent mechanism that can be blocked by anti-CD36 antibody FA6-152. Methodology: briefly, melanoma-derived Mel1700 cells were seeded in a six-well plate and either left untreated or pre-incubated with 10 μg/ml of anti-CD36 monoclonal antibody clone FA6-152 for 25 min. at 25°C. After washing to remove excess antibody, cells were incubated with either 100 μg/ml of KSHV gB-derived HR1, 200 μg/ml HR2 peptide, 100 μg/ml recombinant MC179, or 2 mM sodium butyrate (NaB). Forty-eight hours after treatment, total RNA was isolated and used as template in semi-quantitative RT-PCR with primers to an internal fragment of human CD36, viral RTA, or human glyceraldehyde 3-phosphate dehydrogenase (GAPDH) loading control. (C) Specific activation of the KSHV lytic switch protein, RTA, by the PfEMP-1-CIDR1α-derived peptide MC179 from Pf Malayan Camp strain, but not by peptides derived from CIDR1α domains of A4tres (which binds ICAM-1) or the Vietnam Oak Knoll strain (FVO).

FIGURE 3

Hypothetical model of CD36-dependent RTA activation. A motif displayed by PfEMP-1 on the surface of parasitized erythrocytes interacts with its cognate epitope within the ectodomain of CD36 on the surface of microvascular endothelial cells. This interaction activates one or more of the Src-like kinases, which in turn initiate a phosphorylation cascade that results in p38 and ERK/MAPK activation. This process culminates in activation of a cellular transcription factor, likely AP-1 (dimer of c-jun and c-fos), which translocates into the nucleus and stimulates KSHV RTA-dependent transcription of viral lytic cycle genes, starting with immediate early (IE), which then activate delayed early (DE), followed by late structural genes involved in assembly of an infectious virus particle.

FIGURE 4

Cell type-specific outcomes of KSHV infection in skin versus endothelial cells with respect to CD36 and CD147 expression. Semi-quantitative RT-PCR (A) and Western blot (B) analysis of CD147 and CD36 expression in uninfected (-) versus infected (+) endothelial (lymphatic) or melanoma-derived Mel1700 cells. KSHV upregulates both CD36 and CD147 in melanoma-derived cells, whereas in lymphatic and telomerase-immortalized mixed dermal microvascular and brain endothelial cells, KSHV upregulates CD147 (confirming a recent study; Qin et al., 2010) but downregulates expression of CD36 (and its angiostatic ligand, TSP-1; data not shown); in vivo, these dichotomous effects are consistent with promotion of angiogenesis, invasion, and tumor metastasis in disease-relevant cell types.

FIGURE 5

Cell type-specific outcomes of KSHV interactions with CD36 and CD147 signaling pathways. (A) In melanoma-derived cells (Mel1700), KSHV upregulates both CD36 (and its ligand, TSP-1) and CD147 (as shown in Figure 4). Binding of the PfEMP-1 CIDR1α-derived peptide, MC179, to virus-upregulated CD36 prevents Akt phosphorylation while inducing RTA-dependent KSHV reactivation via the MAPK/p38 pathway. Remarkably, structural mimics of MC179, such as the helical heptad repeat (HR) regions derived from KSHV glycoprotein B (gB), can, like MC179, also induce virus reactivation in a CD36-dependent manner (as shown in Figure 3). (B) In lymphatic (LEC) and other endothelial cells such as mixed tolemerase-immortalized dermal microvascular and brain endothelial cells (tDMB), KSHV upregulates CD147 but unlike in Melanoma cells, the virus downregulates both CD36 and its angiostatic ligand, TSP-1; in vivo, these effects are likely to promote angiogenesis, invasion, and tumor metastasis. (C) As a key receptor for the merozoite invasion antigen, PfRh5, KSHV-induced upregulation of CD147 on the surface of infected endothelial cells increases the frequency of contacts between merozoite-bound or soluble PfRh5 and blood or dermal microvascular endothelial surfaces, resulting in induction of CD147-mediated signals that could alter the microenvironment and cause pathologic outcomes in a variety of physiological sites in the co-infected host.