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 mice lesions. HIF-1α expression was independent of hypoxia, because pimonidazole immunostaining was negative in the mouse lesions after treatment with pimonidazole. 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. Treating the HCC cells with Igf-2 upregulated both phospho-Akt and HIF-1α, while inhibition of Igf-2 by the neutralizing Igf-2 antibody downregulated them both, suggesting that Igf-2 may, 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 genesis 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.
