We are studying the function of host extracellular matrix components in the pathogenesis of Candida albicans. Fibronectin mediates adhesion of C. albicans through two receptors expressed by the pathogen. A high affinity receptor recognizes the collagen-binding domain of fibronectin, and a low affinity receptor recognizes the cell-binding domain of fibronectin. Expression of both fibronectin receptors is tightly regulated by growth conditions. We previously identified hemoglobin as a highly specific inducer of the low affinity receptor. Hemoglobin-inducible binding was observed in all clinical isolates of C. albicans and in other pathogenic members of the Candida genus. The ability to respond to hemoglobin may play an important role in pathogenesis, since only pathogenic strains of C. albicans express hemolytic activity that can release hemoglobin from erythrocytes. Hemoglobin may therefore be a host environmental cue that triggers extracellular matrix receptor expression at a septic site. In addition to regulating expression of adhesion receptors on the yeast, we have identified several genes whose expression is induced by hemoglobin. Inhibitors of signaling pathways stimulated by hemoglobin may decrease the pathogenicity of C. albicans in immunocompromised cancer patients and thereby prevent disseminated candidemia. One of the genes regulated by hemoglobin is a C. albicans heme oxygenase (CaHMX1). Hemoglobin transcriptionally induces CaHMX1 independent of the presence of inorganic iron in the medium. A Renilla luciferase reporter driven by the CaHMX1 promoter demonstrated rapid activation of transcription by hemoglobin. In contrast, iron deficiency or exogenous hemin did not activate the reporter until after 3 h, suggesting that hemoglobin induction of the promoter is mediated by hemoglobin receptor signaling rather than heme or iron flux into the cell. As observed following disruption of the Saccharomyces cerevisiae ortholog, HMX1, a CaHMX1 null mutant was unable to grow under iron restriction. This suggests a role for CaHMX1 in inorganic iron acquisition, as has been postulated for HMX1. CaHMX1 encodes a functional heme oxygenase based on isolation and purification of the reaction product a-biliverdin from cells grown in the presence of hemin or hemoglobin. CaHMX1 is required for utilization of these exogenous substrates, indicating that C. albicans heme oxygenase confers a nutritional advantage for growth in mammalian hosts. Phenotypic switching from white to the opaque phase is a necessary step for mating in C. albicans. Another gene specifically induced following growth in the presence of exogenous hemoglobin, HBR1, is a repressor of white-opaque switching. Deletion of a single HBR1 allele allowed opaque phase switching and mating competence, accompanied by a lack of detectable MTL a1 and a2 gene expression and enhanced MTLa1 gene expression. Conversely, over-expression of Hbr1p or exposure to hemoglobin increased MTLa gene expression. The a1/a2 repressed target gene CAG1 was derepressed in the same mutant in a hemoglobin-sensitive manner. Regulation of CAG1 by hemoglobin required an intact MTLa1 gene. Several additional Mtlp targets were perturbed in HBR1 mutants in a manner consistent with commitment to an a mating phenotype, including YEL007w, MFa, HST6, and RAM2. Therefore, Hbr1 is part of an host factor-regulated signaling pathway that controls white/opaque switching and mating in the absence of allelic deletion at the MTL locus. Suppressing switching during vascular dissemination of the organism may be advantageous because opaque cells are more susceptible to host defenses.
