Dysregulation of hypoxia pathways in fumarate hydratase-deficient cells is independent of defective mitochondrial metabolism.

Mutations in the gene encoding the Krebs cycle enzyme fumarate hydratase (FH) predispose to hereditary leiomyomatosis and renal cell cancer in affected individuals. FH-associated neoplasia is characterized by defective mitochondrial function and by upregulation of transcriptional pathways mediated by hypoxia-inducible factor (HIF), although whether and by what means these processes are linked has been disputed. We analysed the HIF pathway in Fh1-/- mouse embryonic fibroblasts (MEFs), in FH-defective neoplastic tissues and in Fh1-/- MEFs re-expressing either wild-type or an extra-mitochondrial restricted form of FH. These experiments demonstrated that upregulation of HIF-1alpha occurs as a direct consequence of FH inactivation. Fh1-/- cells accumulated intracellular fumarate and manifested severe impairment of HIF prolyl but not asparaginyl hydroxylation which was corrected by provision of exogenous 2-oxoglutarate (2-OG). Re-expression of the extra-mitochondrial form of FH in Fh1-/- cells was sufficient to reduce intracellular fumarate and to correct dysregulation of the HIF pathway completely, even in cells that remained profoundly defective in mitochondrial energy metabolism. The findings indicate that upregulation of HIF-1alpha arises from competitive inhibition of the 2-OG-dependent HIF hydroxylases by fumarate and not from disruption of mitochondrial energy metabolism.

Elimination of colon cancer in germ-free transforming growth factor beta 1-deficient mice.

Patients with ulcerative colitis are at risk for colon cancer and frequently have microsatellite instability,which, in turn, is usually associated with inactivation of transforming growth factor (TGF) beta signaling. TGF-beta1 deficiency in mice can lead to colon cancer that is preceded by precancerous lesions having submucosal inflammation and hyperplastic crypts. Germ-free TGF-beta1-deficient mice are free of inflammation, hyperplasia, and cancer, but when reintroduced into a Helicobacter hepaticus-containing specific pathogen-free room, these lesions reappear. Because adenoma/carcinoma but not inflammation/hyperplasia is dependent on the genetic backgrounds tested, colitis is required, but not sufficient, for carcinogenesis. This animal model should provide insight into the protective role of TGF-beta1 in early stages of ulcerative colitis-associated human colon cancer.