Emerging roles for hyaluronidase in cancer metastasis and therapy

Abstract

Hyaluronidases are a family of five human enzymes that have been differentially implicated in the progression of many solid tumor types, both clinically and in functional studies. Advances in the past 5 years have clarified many apparent contradictions: (1) by demonstrating that specific hyaluronidases have alternative substrates to hyaluronan (HA) or do not exhibit any enzymatic activity, (2) that high-molecular weight HA polymers elicit signaling effects that are opposite those of the hyaluronidase-digested HA oligomers, and (3) that it is actually the combined overexpression of HA synthesizing enzymes with hyaluronidases that confers tumorigenic potential. This review examines the literature supporting these conclusions and discusses novel mechanisms by which hyaluronidases impact invasive tumor cell processes. In addition, a detailed structural and functional comparison of the hyaluronidases is presented with insights into substrate selectivity and potential for therapeutic targeting. Finally, technological advances in targeting hyaluronidase for tumor imaging and cancer therapy are summarized.

Keywords: Cancer therapy; Hyaluronan; Hyaluronidase; Metastasis; Tumor biology; Tumor imaging.

Figure 1. Model for HA impact on tumor progression

HA polymers are synthesized at the cell surface by membrane embedded HAS. Reuptake and/or degradation of HA may require secreted hyaluronidases to generate low MW oligomers of HA. HA is retained by ligation to specific cell surface receptors or residual association with HAS, and can act on both tumor cells and associated stromal cells. Tumor cells may signal in HA and/or Hyal1-dependent fashion to endothelial cells of lymphatic vessels, lymph node or bone marrow sinusoids via other HA receptors. These signals may be released at the primary site to prepare metastatic target tissues and render them hospitable for tumor invasion, or tumor cells bearing HA may generate signals locally upon arrest in metastatic target tissues. The context of the HA signal is assumed to be free extracellular HA polymer or HA oligomer, but HA and/or Hyal1 delivery via exosomes or microvesicles is an emerging possibility. HA internalized by tumor epithelial cells may contribute to cellular transformation, proliferation, motility, and ultimately may be required for sustained tumor growth and metastasis.

Figure 2. The structure of human Hyal1

(A) Surface representation of human Hyal1. A HA tetrasaccharide (solid spheres with carbons colored in green, oxygen in red, and nitrogen in blue) was docked in the enzyme active site in an orientation comparable to that observed in the bee venom hyaluronidase structure (pdb codes: Hyal1 2PE4; bee venom 1FCV). The surface of the protein is colored based on the sequence conservation among the five human hyaluronidases, with perfectly conserved residues colored in dark blue and residues with no sequence conservation colored in red. Residues found in 4 out of 5 hyaluronidases are colored in light blue, 3 out of 5 in grey, and 2 out of 5 in pink. The most highly conserved residues are generally located within the enzyme active site or key structural elements. (B) Conserved active site features of human hyaluronidases. A ribbon representation of Hyal1 is color-coded as in Panel A, with the HA tetrasaccharide shown in ball and stick representation. Key active site residues are shown in stick representation. Several perfectly conserved residues are clustered at the site of the substrate-assisted cleavage.