Solid tumors possess unique microenvironments that are exposed to chronic hypoxic conditions ("tumor hypoxia").
Although more than half a century has passed since it was suggested that tumor hypoxia correlated with poor treatment
outcomes and contributed to cancer recurrence, a fundamental solution to this problem has yet to be found. Hypoxia-inducible
factor (HIF-1) is the main transcription factor that regulates the cellular response to hypoxia. It induces various
genes whose functions are strongly associated with malignant alteration of the entire tumor. The cellular changes
induced by HIF-1 are extremely important targets of cancer therapy, particularly in therapy against refractory cancers.
Imaging of the HIF-1-active microenvironment is therefore important for cancer therapy. To image HIF-1activity in vivo,
we developed a PTD-ODD fusion protein, POHA, which was uniquely labeled with near-infrared fluorescent dye at the
C-terminal. POHA has two functional domains: protein transduction domain (PTD) and VHL-mediated protein
destruction motif in oxygen-dependent degradation (ODD) domain of the alpha subunit of HIF-1 (HIF-1α). It can
therefore be delivered to the entire body and remain stabilized in the HIF-1-active cells. When it was intravenously
injected into tumor-bearing mice, a tumor-specific fluorescence signal was detected in the tumor 6 h after the injection.
These results suggest that POHA can be used an imaging probe for tumor malignancy.
More malignant tumors contain more hypoxic regions. In hypoxic tumor cells, expression of a series of hypoxiaresponsive
genes related to malignant phenotype such as angiogenesis and metastasis are induced. Hypoxia-inducible
factor-1 (HIF-1) is a master transcriptional activator of such genes, and thus imaging of hypoxic tumor cells where HIF-1
is active, is important in cancer therapy. We have been developing PTD-ODD fusion proteins, which contain protein
transduction domain (PTD) and the VHL-mediated protein destruction motif in oxygen-dependent degradation (ODD)
domain of HIF-1 alpha subunit (HIF-1&agr;). Thus PTD-ODD fusion proteins can be delivered to any tissue in vivo through
PTD function and specifically stabilized in hypoxic cells through ODD function. To investigate if PTD-ODD fusion
protein can be applied to construct hypoxia-specific imaging probes, we first constructed a fluorescent probe because
optical imaging enable us to evaluate a probe easily, quickly and economically in a small animal. We first construct a
model fusion porein PTD-ODD-EGFP-Cy5.5 named POEC, which is PTD-ODD protein fused with EGFP for in vitro
imaging and stabilization of fusion protein, and conjugated with a near-infrared dye Cy5.5. This probe is designed to be
degraded in normoxic cells through the function of ODD domain and followed by quick clearance of free fluorescent
dye. On the other hand, this prove is stabilized in hypoxic tumor cells and thus the dye is stayed in the cells. Between
normoxic and hypoxic conditions, the difference in the clearance rate of the dye will reveals suited contrast for tumor-hypoxia
imaging. The optical imaging probe has not been optimized yet but the results presented here exhibit a potential
of PTD-ODD fusion protein as a hypoxia-specific imaging probe.
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