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Biochemical Pathology

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Dr. Roberts' Homepage

Biochemical Pathology Staff

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Cell-cell and cell-matrix interactions are important regulators of normal cell growth and differentiation and play essential roles in pathological conditions such as tumor metastasis and infection by pathogens. We are defining functions of adhesion molecules, their cell surface and matrix receptors, and the signal transduction pathways that regulate their activities in specific diseases. These studies will identify new molecular targets and could provide a basis for designing novel therapeutic agents. The primary research projects in our laboratory are: 1) investigating the role of thrombospondin-1 (TSP1) in regulating tumor progression and 2) defining the host-pathogen interactions that are required for establishing disseminated infections by the pathogenic yeast Candida albicans. These projects are in divergent fields but share a common goal to define the mechanisms by which cells regulate their interactions with extracellular matrix. The cross fertilization between these projects has helped us to make significant contributions to both fields.

Regulation of angiogenesis and innate immunity by thrombospondins.

We are investigating the mechanisms by which TSP1 and TSP2 modulate and tumor angiogenesis and host immune responses to the tumor. We have defined specific domains and peptide sequences that mediate these activities. Stable analogs of inhibitory peptides prevent angiogenesis in several animal models and are being developing for potential therapeutic use. We also identified a pro-angiogenic region in TSP1 and three receptors that mediate this activity. Recombinant regions of TSP1 containing these sequences stimulate rather than inhibit angiogenesis in vivo. Thus, we propose that TSP1 should be viewed as a conditional modulator of angiogenesis, wherein endothelial cell responses to TSP1 are qualitatively and quantitatively regulated by their context (see http://ccr.cancer.gov/news/inthejournals/Roberts_02.pdf). We have identified several signals that control how endothelial cells respond to TSP1. Recognition of TSP1 by the integrin a3b1 in endothelial cells is specifically modulated by VE-cadherin. The TSP1/2-binding integrins a4b1 and a6b1 are also differentially regulated in their expression and function between large vessel and microvascular endothelial cells.

We found that interaction with a6b1 integrin also plays an important role in the effects of TSP1 on anti-tumor innate immunity. TSP1 engaging this integrin enhances superoxide production by tumor-infiltrating macrophages. Tumors expressing TSP1 have higher percentages of M1-differentiated infiltrating macrophages that can release cytotoxic levels of this reactive oxygen species. This provides a second mechanism in addition to inhibiting angiogenesis by which TSP1 expression can limit tumor growth (see CCR in the journals).

We have identified an important signaling node that mediates the anti-angiogenic activity of TSP1 (see NIH Researchers Identify Molecular Regulator for Blood Flow). Physiological levels of nitric oxide (NO) stimulate angiogenesis via cGMP signaling. We found that TSP1 potently inhibits this pathway both upstream and downstream of cGMP and that this plays an important role in inhibition of angiogenesis by TSP1. However, angiogenesis is not the only physiological process controlled by this pathway. Using functional magnetic resonance imaging, we found endogenous TSP1 to be a potent regulator of vascular homeostasis in vivo through its antagonism of NO signaling. Through this pathway, endogenous TSP1 acutely regulates vascular tone and blood pressure and enhances platelet aggregation during hemostasis by blocking the anti-thrombotic activity of NO. In addition to uncovering these physiological functions, TSP1 we found that TSP1 plays an important role in tissue survival of ischemic injuries. Tissue levels of TSP1 become elevated with aging and in several chronic diseases. Under these conditions, the beneficial effects of endogenous NO on tissue survival following fixed ischemia or ischemia/reperfusion injuries are blocked by TSP1. We are developing antagonists of this activity of TSP1 and have shown that they enhance survival of fixed ischemic injuries in both young and aged animals. The same antagonists also minimize damage to the liver following ischemia/reperfusion injury, and so may be useful to improve the outcome of organ transplantation. Furthermore, these antagonists selectively protect normal tissues from damage caused by ionizing radiation and so may be useful as adjuvants for radiotherapy of cancers.

Two TSP1 receptors, CD36 and CD47, mediate its inhibition of cGMP signaling, but only CD47 is necessary for this response. Although a predicted CD47-binding sequence in TSP1 is conserved in other members of the thrombospondin family, we recently found that TSP2 and TSP4 are much less active than TSP1 for signaling through CD47. TSP2 null mice do not have the enhanced ability to recover from ischemic injuries exhibited by TSP1 null mice. We are defining the signaling pathways downstream of CD47 through which TSP1 inhibits cGMP signaling and developing novel therapeutic approaches to modulate this activity of TSP1.

Regulation of host colonization and vascular dissemination of Candida albicans

Candidiasis is an increasingly common complication of cancer treatment with high morbidity and mortality. Candidemia has increased tenfold in the past decade and now constitutes the third most common cause of positive blood cultures. Candidemia in neutropenic cancer patients causes septic shock and multi-organ failure. Because clinical isolates are increasingly resistant to available antifungal agents, new approaches are needed to prevent and treat these infections in cancer patients. Our current studies of the pathogenic yeast C. albicans are based on our discovery that hemoglobin specifically induces expression of a receptor for the extracellular matrix protein fibronectin. This response to hemoglobin is conserved among pathogenic species in the Candida genus but not other yeasts. Hemoglobin induces adhesion to several host matrix proteins and to endothelial cell monolayers. We have cloned several novel genes that are specifically induced by hemoglobin. One of these, HBR1, is an essential gene for growth of the organism and controls its ability to undergo mating. A second hemoglobin-induced gene was identified as a heme oxygenase These genes define a new differentiation pathway by which the pathogen adapts to the vascular compartment of its host. Understanding the molecular mechanisms for regulation of this differentiation pathway could lead to new therapeutic targets for managing infections caused by this opportunistic pathogen.

Recent Publications:

Recent Thrombospondin Publications

Candida Publications

Redox Signaling in Angiogenesis

Autotaxin

 
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