Tumor hypoxia bio-sensor

Hypoxia is tumor specific and represent a target for therapy. Up to 60 % percent of advanced solid tumors are characterized by hypoxia areas (0). Cancer cells present in these hypoxic regions are resistant to both chemotherapy and radiotherapy (1) and must be targeted by hypoxia-selective therapy. These cells are exposed to a very low oxygen tension of pO2 < 10 mmHg, equivalent to < 1.3% O2 in vitro (1).

Besides conferring resistance to cancer treatments, hypoxia also promotes tumor progression and metastasis. Hypoxia up-regulates over 80 genes associated with tumor progression, glycolysis, angiogenesis and metastasis (2) through HIF activity. Patients with a high proportion of hypoxic cell have low survival rate after surgical resection of the primary tumor (3). As 90 % of cancer deaths are attributed to metastatic spreading (3), targeting hypoxic cells could have a deep impact on reducing metastasis and the overall survival.

In order to detect this tumor hypoxic environment, we implemented a hypoxia bio-sensor in the yeast S. cerevisiae.

Design of the hypoxia bio-sensor

The choice of the right promoter is crucial in a bio-sensor design. Yeasts are facultative anaerobes that can naturally detect very low oxygen concentration with the corresponding promoters already present. However, these promoters need an addition fermentable carbon for induction, like ADH2 (4, 5). On the contrary, our system is intended to be used in vivo after encapsulation in alginate beads, without fermentable carbon source. Thus, we cloned the CMV minimal promoter as an inducer of the hypoxia gene reporter. Then, we need a Hypoxia Response Element (HRE) to start transcription factor binding to our CMV minimal promoter. HRE are DNA sequence that enhance transcription activity when the Hypoxia inducing Factor (HIF) binds to it. We assembled 4 HRE sequences ahead of the CMV minimal promoter to increase transcription in presence of hypoxia. The HRE was derived from the human HRE ahead of EPO promoter, as the EPO/HRE was more strongly induced than VEGF/HRE (6) by HIF in presence of hypoxia.

HIF transcription factors are composed of two sub-units. The α-subunits of the HIF transcription factors are degraded by proteasomal pathways during normoxia but are stabilized under hypoxic conditions (6). On the contrary, the beta-subunit of HIF is always expressed and maintained stable in the cytosol. We codon optimized the human HIF-alpha and HIF-beta for yeast and cloned these proteins in yeast S. cerevisiae under control of GAL1, a galactose inducible promoter.