STAT3 is considered to be an oncogene since it cause the activation of cyclin D1, c-myc, and bcl-xL expression, and also is involved in promotion of cell-cycle progression, cellular transformation, and the prevention of apoptosis (Darnell, 1997
; Bowman et al., 2000
). In our previous report, we have demonstrated that STAT3 is a pivotal regulator of HIF-1-mediated VEGF expression in hypoxic solid tumor cells (Jung et al., 2005
). Thus, STAT3 is considered to be an oncogenic inducer in solid tumor as well as HIF-1. Also, we found that hypoxia-induced active STAT3 accelerated the accumulation of HIF-1α protein and prolonged its half-life in solid tumor cells (Jung et al., 2005
). However, its specific mechanisms are not fully understood.
In this study, we investigated the role of STAT3 in the mechanism of pVHL-mediated HIF-1α stability. We found that STAT3 interacts with the C-terminal domain of HIF-1α and stabilizes HIF-1α by competition with pVHL for binding to HIF-1α. The interaction between HIF-1α and negative regulator of HIF-1α stability, pVHL was interfered by dose-dependently overexpressed constitutive active STAT3. Moreover, we found that the enhanced HIF-1α protein levels by active STAT3 are due to decrease of poly-ubiquitination of HIF-1α protein via inhibition of interaction between pVHL and HIF-1α. Taken together, our results suggest that STAT3 decreases the pVHL-mediated ubiquitination of HIF-1α through competition with pVHL for binding to HIF-1α, and then stabilizes HIF-1α protein levels.
Under normoxic conditions, HIF-1α is rapidly degraded by the ubiquitin-proteasome pathway (Ohh et al., 2000
). Ubiquitination of HIF-1α is mediated by interaction with pVHL. HIF-1α is targeted for VHL E3 ligase complex by proline hydroxylation of oxygen-dependent degradation (ODD) domain (Semenza, 2001
; Safran and Kaelin, 2003
). Hydroxylated HIF-1α is associated with pVHL which has E3 ubiquitin ligase activity and is rapidly degraded by the ubiquitin-proteasome pathway (Ivan et al., 2001
). The ODD domain of HIF-1α is mainly critical domain for regulation of HIF-1α stabilization, because it is reported that ODD domain is interacted with pVHL in HIF-1α degradation by ubiquitin-proteasome pathway. Thus, it is conceivable that the binding of STAT3 to HIF-1α occurs in the ODD domain of HIF-1α and consequently, the inhibition of pVHL binding to HIF-1α by competent binding of STAT3 occurs in the ODD domain. However, interestingly, our result showed that STAT3 interacts with HIF-1α in the C-terminal region, the very next region of ODD domain of HIF-1α (). Since STAT3 protein has a high molecular weight and forms hetero or homo-dimer structure in its active state, there is the possibility that the heavy and complicated structured STAT3 protein can interfere with the binding of pVHL to HIF-1α. It still remains to be solved by in vitro
assays whether the interaction between STAT3 and HIF-1α is direct or indirect.
In conclusion, the results of this study demonstrate that active STAT3 enhances HIF-1α protein stability through inhibition of pVHL binding to the HIF-1α and pVHL-mediated ubiquitinantion of HIF-1α. In addition, it is suggested that STAT3 is a superior target for the development of anticancer drugs which has focused on targeting HIF-1α.