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Overview of Candida Research
Candidiasis is an increasingly common complication of cancer treatment with high morbidity and mortality. Candidemia now constitutes the third most common cause of positive blood cultures. Because clinical isolates are increasingly resistant to the available antifungal agents, new approaches are needed to prevent and treat these infections in cancer patients. Adhesion of C. albicans to host tissues is important for the transition from commensal colonization of the gastrointestinal tract to disseminated candidemia. The host extracellular matrix protein fibronectin is specifically recognized by this pathogen. We previously found that growth in complex medium induced expression of two fibronectin receptors on C. albicans (Figure 1). We subsequently found that hemoglobin specifically induces expression of the low affinity C. albicans receptor for fibronectin. This response to hemoglobin is conserved among pathogenic species in the Candida genus but not in other yeasts. Hemoglobin specifically induces adhesion to several host matrix proteins and to endothelial cell monolayers.
Hemoglobin acts by binding to a specific cell surface receptor. The plasma hemoglobin-binding protein haptoglobin inhibits this interaction. The iron in hemoglobin is not required for activity, as cobalt-hemoglobin is equally active. However, globin alone is inactive, as are myoglobin and all other ferroproteins that we examined. Therefore, the receptor is specific for the native conformation of hemoglobin. This is the first host factor identified that can modify behavior of this pathogen.

We identified a hemoglobin-induced cell wall protein that binds to fibronectin and cloned several novel genes that are rapidly induced in cells exposed to hemoglobin in vitro or by incubation in the vascular compartment of rabbits. One of these genes, HBR1, is essential for vegetative growth and haplo-insufficient for hyphal differentiation and some stress responses. These genes define a new differentiation pathway by which this pathogen adapts to the vascular compartment of its host. Understanding the signal transduction pathways that regulate this differentiation pathway could lead to new therapeutic targets to manage disseminated candidemia. To achieve this goal, we are investigating the signaling pathways that control expression of the newly identified genes and the hemoglobin-induced matrix receptor.

Overview of Thrombospondin Research
We are investigating the mechanisms by which the adhesive glycoproteins TSP1 and TSP2 regulate tumor growth, metastasis, and angiogenesis. Tumor xenograft and transgenic mouse models demonstrated that TSP1 and TSP2 are suppressors of tumor progression. Although the suppressive activity of TSP1 was initially ascribed to inhibition of angiogenesis, we have obtained evidence for significant direct effects of TSP1 on both tumor cells and the host immune response. Using synthetic peptides, recombinant TSP1 fragments, and mutagenesis of TSP1, we are defining the domains and specific amino acid sequences in TSP1 that mediate activities of TSP1 toward each of these cell types.
We identified peptide sequences from the type 1 repeats of TSP1 that mimic the anti-angiogenic activities of the whole molecule (see schematic model of TSP1). These sequences bind to heparan sulfate proteoglycans and antagonize the angiogenic factor FGF-2. This inhibitory sequence is distinct from the CD36-binding sequence in the type 1 repeats, which also inhibits angiogenesis [Iruela-Arispe, 1999]. Stable analogs of the heparin-binding peptides inhibit angiogenesis in several animal models and are being developing for therapeutic applications [Hugo, 1999; Bogdanov, 1999; Shafiee, 2000]. A third functional sequence has been identified in the type 1 repeats that activates latent transforming growth factor-b.
We also identified a pro-angiogenic sequence in TSP1 and identified its endothelial cell receptor as a3b1 integrin. We first identified a3b1 integrin as a TSP1-binding integrin in breast carcinoma cells [Chandrasekaran, 1999]. Subsequently, we found that the same integrin plays important roles in the interactions of small cell lung carcinoma cells and endothelial cells with TSP1 [Guo, 2000; Chandrasekaran, 2000]. This integrin mediated adhesion of the three cells types to immobilized TSP1. Furthermore, a3b1 integrin mediated cell-specific effects of TSP1 on motility and proliferation: stimulating chemotaxis of breast carcinoma cells, inhibiting small cell lung carcinoma proliferation but stimulating their neurotypic differentiation, and stimulating or inhibiting endothelial cell proliferation in a context-dependent manner. Thus, interactions of TSP1 with a3b1 integrin mediate several behaviors of tumor and endothelial cells that are relevant to angiogenesis and tumor progression.
We localized the binding site in TSP1 for a3b1 integrin to the N-terminal globular domain of TSP1 and identified a specific peptide sequence that was recognized by this integrin [Krutzsch, 1999] (see model). A synthetic peptide based on this sequence supported a3b1 integrin-dependent adhesion and antagonized many of the biological responses to TSP1 and to other know ligands for this integrin. The peptide showed anti-proliferative activity for endothelial cells in vitro and inhibited angiogenesis in the chick chorioallantoic membrane assay.
The specific responses of tumor cells, T lymphocytes, and endothelial cells to TSP1 arise from the utilization of distinct combinations of cell surface TSP1 receptors and result in different intracellular signals in each cell type. Recognition of TSP1 by the integrin 3 1 on breast carcinoma cells is specifically modulated by IGF1 and CD98. The same integrin is stimulated by EGF but not by IGF1 in small cell lung carcinoma and by VE-cadherin in endothelial cells.
In T lymphocytes, TSP1 responses are mediated by CD47, heparan sulfate proteoglycan, and a4b1 integrin [Wilson, 1999; Li, 2002; Li, 2001]. The a4b1 integrin also recognizes TSP2, whereas a3b1 is specific for TSP1. We are defining the cell-specific signal transduction pathways for each TSP receptor and identifying genes that are regulated by these TSP1-initiated signals in specific cell types and in TSP1-null transgenic mice. Using DNA microarrays, we have identified TSP1-regulated genes in T cells and demonstrated that TSP1 acts globally to suppress T cell receptor signaling [Li, 2001]. Two TSP1 receptors, CD47 and heparan sulfate proteoglycan, mediate this activity.

Useful Links:

Thrombospondin-1

Thrombospondin-2

TSP sequences and maps from the Bornstein Lab

Integrins and their subunits

Confused by all those CD numbers?

American Society for Matrix Biology

Role of thrombospondin-1 in Cancer

Scientific abstracts published in current journals can be found on http://www4.ncbi.nlm.nih.gov/PubMed.