The transcription factor HIF-1α regulates hypoxia-mediated gene expression which promotes tumour growth, metastasis and desensitisation to chemo-and radiotherapy. HIF-1 is functional in a range of thyroid-carcinoma cells and is regulated by hypoxia and the PI3K and MAPK-signalling pathways.
We aimed to determine if HIF-1 and PI3K were important in the development of a metastatic phenotype and play a role in desensitisation to radiotherapy in thyroid carcinomas.
Thyroid cell migration and adhesion were monitored as metastatic characteristics in vitro. Migration increased with decreasing oxygen tension and was inhibited by i) the PI3K-inhibitors PI-103 and GDC-0941; ii) reintroduction of PTEN into PTEN-null FTC133 cells and iii) inhibiting HIF-1 function using a dominant negative protein (dnHIF) that reduced expression of HIF-1-target genes CA-9, LDH-A, GLUT-1, VEGF. PTEN-expressing FTC133s, dnHIF-FTC133s and cells treated with GDC-0941 also had reduced ability to adhere/spread on fibronectin. PI-103 and GDC-0941 inhibited expression of PI3K target proteins (pAKT, pGSK-3β) and HIF-1 target genes. Radiation induced HIF-1 expression. GDC-0941 combined with radiation blocked HIF-1 induction, prolonged DNA double-strand breaks and reduced clonogenic survival suggesting a radio-sensitising effect.
In vivo, PI-103 and GDC-0941 reduced PI3K and HIF-1α activity and downstream target genes in follicular and anaplastic tumours. Mice bearing dnHIF-FTC133 tumours and those treated with GDC-0941 had decreased metastatic colonies in the lungs. FTC133, dnHIF-FTC133 and 8505c cells were manufactured to express eGFP. Spontaneous metastatic colonization to the lungs was confirmed by viewing eGFP-positive colonies grown from digested lung tissue and by immuno-blotting for eGFP.
These data link PI3K, HIF-1-activation and aggressive disease in thyroid-carcinoma. With the known desensitising effects of HIF-1 and PI3K activity on radiation treatment, a combined approach involving both radiation and a PI3K inhibitor may improve both the therapeutic response within the tumour and reduce metastatic potential in thyroid carcinomas. This is a focus of ongoing studies.