HIF-1 is involved in tumor progression/metastasis and activated in various cancers. Here we show that HIF-1α, which plays a major role in HIF-1 activation, is over-expressed in preneoplastic hepatocytic lesions from a very early stage during hepatocarcinogenesis in mice and man. Transcriptional targets of HIF-1, such as VEGF, glut-1, c-met and Igf-2, were also over-expressed in mouse lesions. Measurement of oxygen tension using the needle electrode demonstrated that the partial pressure of oxygen within the lesions was not different from that of the normal hepatic tissues. Furthermore, the immunohistochemical method using pimonidazole did not demonstrate positive staining in the preneoplastic lesion, indicating that HIF-1α expression was independent of hypoxia. On the other hand, Akt, the pathway of which can up-regulate HIF-1α expression, was activated in the mouse lesions, while HIF-1α was markedly downregulated in the mouse HCC cell lines after treatment with a PI3 kinase (PI3K) inhibitor, LY294002, indicating that HIF-1α expression is dependent on PI3K/Akt signaling. Conversely, HIF-1α knockdown by siRNA in the HCC cell line resulted in decreased expression of activated Akt together with the HIF-1 target genes, indicating that Akt activation is reversely dependent on HIF-1 activation. Moreover, although subcutaneous inoculation of the original HCC cells resulted in tumor formation in mice, the HIF-1α knockdown cells temporally formed small tumors at once, but these tumors disappeared due to extensive necrosis. Treating the HCC cells with Igf-2 or EGF upregulated both phospho-Akt and HIF-1α, while inhibition of Igf-2 or EGF signaling downregulated them both, suggesting that Igf-2 and EGF can, at least in part, mediate the activation of Akt and HIF-1α. However, Akt was not activated by Igf-2 or EGF in the HIF-1α knockdown cells, indicating that expression of the HIF-1 target genes is necessary for the Akt activation. These findings suggest that the reciprocal activation of PI3K/Akt signaling and HIF-1α may be important in progression of hepatocarcinogenesis.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA