Modulation of TGF-β signaling by Hypoxia

A tumor is a fast-growing malignant tissue. This creates areas inside the tumor that are distant from local blood vessels to be able to get enough oxygen. This hypoxic condition activates a transcription factor called hypoxia inducible factor (HIF). HIF responses help a cell to adapt to decreased oxygen by activating glycolytic and angiogenesis pathways and by regulating apoptotic responses. Hypoxia drives the upregulation of a growth factor called transforming growth factor beta (TGF-beta). Similar to a hypoxia response, TGF is an important regulator of cell fate. TGF-β and HIF pathways regulate partially overlapping target genes. This regulation can also be cooperative. The TGF-beta signal is initiated by activation of plasma membrane receptors that then activate effector proteins called small mothers against decapentaplegic (Smad) homologs. In healthy tissue, TGF-β keeps cell proliferation and growth under control. During cancer progression, TGF-beta has shown a dual role, whereby it inhibits initial tumor formation but, conversely, in an existent tumor, TGF-beta drives malignant progression. Along with HIF and TGF-beta also protein dephosphorylation is an important regulatory mechanism of cell fate. Protein dephosphorylation is catalyzed by protein phosphatases such as Protein phosphatase 2A (PP2A). PP2A is a ubiquitous phosphatase that can exist in various active forms. PP2A can specifically regulate TGF-beta signaling either by enhancing or inhibiting the receptor activity. This work demonstrates that during hypoxia, PP2A is able to fine-tune TGF-beta signal by specifically targeting Smad3 effector in a Smad7-dependent manner. Inactivation of Smad3 in hypoxia leads to malignant conversion of TGF-beta signaling.

The Effects of Fibroblast Growth Factor 8b on Reproductive Organs and Prostate Tumorigenesis

Fibroblast growth factors (FGFs) are involved in the development and homeostasis of the prostate and other reproductive organs. FGF signaling is altered in prostate cancer. Fibroblast growth factor 8 (FGF8) is a mitogenic growth factor and its expression is elevated in prostate cancer and in premalignant prostatic intraepithelial neoplasia (PIN) lesions. FGF8b is the most transforming isoform of FGF8. Experimental models show that FGF8b promotes several phases of prostate tumorigenesis - including cancer initiation, tumor growth, angiogenesis, invasion and development of bone metastasis. The mechanisms activated by FGF8b in the prostate are unclear. In the present study, to examine the tumorigenic effects of FGF8b on the prostate and other FGF8b expressing organs, an FGF8b transgenic (TG) mouse model was generated. The effect of estrogen receptor beta (ERβ) deficiency on FGF8binduced prostate tumorigenesis was studied by breeding FGF8b-TG mice with ERβ knockout mice (BERKOFVB). Overexpression of FGF8b caused progressive histological and morphological changes in the prostate, epididymis and testis of FGF8b-TG-mice. In the prostate, hyperplastic, preneoplastic and neoplastic changes, including mouse PIN (mPIN) lesions, adenocarcinomas, sarcomas and carcinosarcomas were present in the epithelium and stroma. In the epididymis, a highly cancer-resistant tissue, the epithelium contained dysplasias and the stroma had neoplasias and hyperplasias with atypical cells. Besides similar histological changes in the prostate and epididymis, overexpression of FGF8b induced similar changes in the expression of genes such as osteopontin (Spp1), connective tissue growth factor (Ctgf) and FGF receptors (Fgfrs) in these two tissues. In the testes of the FGF8b-TG mice, the seminiferous epithelium was frequently degenerative and the number of spermatids was decreased. A portion of the FGF8b-TG male mice was infertile. Deficiency of ERβ did not accelerate prostate tumorigenesis in the FGF8b-TG mice, but increased significantly the frequency of mucinous metaplasia and slightly the frequency of inflammation in the prostate. This suggests putative differentiation promoting and anti-inflammatory roles for ERβ. In summary, these results underscore the importance of FGF signaling in male reproductive organs and provide novel evidence for a role of FGF8b in stromal activation and prostate tumorigenesis.

Fibroblast Growth Factor 8 and its Receptors in The Growth and Angiogenesis of Experimental Breast Cancer . With Special Reference to Thrombospondin-1 as a Novel Target Gene for FGF-8

The growth of breast cancer is regulated by hormones and growth factors. Recently, aberrant fibroblast growth factor (FGF) signalling has been strongly implicated in promoting the progression of breast cancer and is thought to have a role in the development of endocrine resistant disease. FGFs mediate their auto- and paracrine signals through binding to FGF receptors 1-4 (FGFR1-4) and their isoforms. Specific targets of FGFs in breast cancer cells and the differential role of FGFRs, however, are poorly described. FGF-8 is expressed at elevated levels in breast cancer, and it has been shown to act as an angiogenic, growth promoting factor in experimental models of breast cancer. Furthermore, it plays an important role in mediating androgen effects in prostate cancer and in some breast cancer cell lines. We aimed to study testosterone (Te) and FGF-8 regulated genes in Shionogi 115 (S115) breast cancer cells, characterise FGF-8 activated intracellular signalling pathways and clarify the role of FGFR1, -2 and -3 in these cells. Thrombospondin-1 (TSP-1), an endogenous inhibitor of angiogenesis, was recognised as a Te and FGF-8 regulated gene. Te repression of TSP-1 was androgen receptor (AR)-dependent. It required de novo protein synthesis, but it was independent of FGF-8 expression. FGF-8, in turn, downregulated TSP-1 transcription by activating the ERK and PI3K pathways, and the effect could be reversed by specific kinase inhibitors. Differential FGFR1-3 action was studied by silencing each receptor by shRNA expression in S115 cells. FGFR1 expression was a prerequisite for the growth of S115 tumours, whereas FGFR2 expression alone was not able to promote tumour growth. High FGFR1 expression led to a growth advantage that was associated with strong ERK activation, increased angiogenesis and reduced apoptosis, and all of these effects could be reversed by an FGFR inhibitor. Taken together, the results of this thesis show that FGF-8 and FGFRs contribute strongly to the regulation of the growth and angiogenesis of experimental breast cancer and support the evidence for FGF-FGFR signalling as one of the major players in breast cancers.