Regulation of Hyaluronan Synthesis and Signaling via CD44 in Cancer
Sammanfattning: Hyaluronan is a ubiquitous glycosaminoglycan which is an important constituent of the extracellular matrix (ECM). In addition to organizing the extracellular matrix and regulating tissue homeostasis, hyaluronan, by binding to its main cell surface receptor CD44, is involved in intracellular signaling pathways regulating major cellular processes during development, wound healing, inflammation and cancer. Accumulation of hyaluronan in cancer promotes progression of the disease and correlates with poor prognosis. This thesis focuses on the regulation of hyaluronan synthesis and its signaling in normal and cancer cells.Cancer cells in solid tumors are surrounded by stroma, which has an essential role in the growth and metastasis of tumors. Prominent members of the tumor stroma are fibroblasts, which synthesize ECM components, such as hyaluronan, and secrete growth factors, and activate intracellular signaling pathways. We demonstrate a cross-talk between the receptors for platelet-derived growth factor BB (PDGF-BB), transforming growth factor β (TGFβ) and CD44 in dermal fibroblasts. We found that PDGF-BB can activate the Smad signaling pathway downstream of the TGFβ receptor I (TβRI), and that PDGF-BB-induced migration depends on TβRI. CD44 forms a ternary complex with the receptors for PDGF-BB and TGFβ, and negatively regulates their signaling. Furthermore, we demonstrate that TGFβ stimulation of mammary epithelial cells transcriptionally upregulates hyaluronan synthase 2 (HAS2), which is essential for TGFβ-induced epithelial-mesenchymal transition (EMT); in this process, polarized epithelial cells adapt a mesenchymal phenotype which facilitates migration and invasion.HAS2 protein activity and stability is regulated by posttranslational modifications, including ubiquitination. We investigated the ubiquitination of HAS2 in aggressive breast cancer cells, whose metastasizing capability depends on HAS2-synthesized hyaluronan. We identified two deubiquitinating enzymes, USP4 and USP17, which target HAS2 and affect its activity and stability.In summary, these studies increase the knowledge about the regulation of hyaluronan production and its role in cancer progression.
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