In vivo hypoxia and prostate cancer growth

Background. Solid tumors require a local vascular network that supplies O2 and nutrients for their growth. When cell proliferation exceeds angiogenesis, vasculature might become unable to sustain the O2 needs of tumors. The resulting limitation in O2 supply causes tumors to cope with an environment chronically deficient in O2 (chronic hypoxia, CH). Local over-expression of hypoxia inducible factor 1α (HIF-1α) transactivates compensatory mechanisms aimed at survival, including the angiogenic switch, which provides growth factors for the development of circulation to feed the growing tumor. However, the newly formed vessels are more twisted, tortuous, distorted and irregular than mature vessels, which leads to inefficient and irregular, perhaps pulsing oxygenation. The resulting situation resembles a paradigm whereby CH is interspersed with various reoxygenation events (CHReox). Although hypoxia is known to increase tumor aggressiveness, resistance to radiotherapy, malignant progression and enhanced metastases formation, it remains to be established if the signaling path originating from systemic hypoxia can exacerbate tumorigenesis. Aims. Two are the aims of this thesis. First, testing the hypothesis that systemic hypoxia in vivo up-regulates prostate cancer growth. Second, to get a better insight into the role of HIF-1α, testing a condition whereby HIF-1α expression is perturbed to enable distinguishing the effects due to hypoxia from those due to HIF-1α. To this purpose, CHReox was selected as a non-pharmacological approach to alter HIF-1α expression without use of drugs and substances that may mask the results. Methods. Male athymic nu/nu mice were xenografted with LNCaP cells and exposed for 4 weeks to either CH (10% O2) or CHReox (3 reoxygenation/week for 1 h), with normoxic mice as control. Results. Tumors grew faster in hypoxia than in normoxia, in agreement with higher phosphorylation rate of protein kinase B (Akt), an inhibitor of apoptosis and inducer of cell proliferation. Tumor Hb content was higher in hypoxia than in normoxia, reflecting the higher blood Hb content rather than increased vascularization, in agreement with similar tissue level of VEGF, a mediator of angiogenesis. Although CH and CHReox induced similar patterns of these parameters, expression level of HIF-1α, assessed by Western blot and immunohistochemistry, was markedly higher for CHReox than CH. The inflammatory infiltrate in the tumor mass was less in hypoxia than in normoxia, in the favour of greater mass of tumor cells in hypoxia. However, although the inflammatory infiltrate displayed relevant characteristics of apoptosis, tumor cells were scarcely apoptotic. Conclusion. Systemic hypoxia in vivo increases the rate of prostate LNCaP tumor growth in athymic nude mice, irrespectively of CH or CHReox. Such effect is influenced neither by tumor vascularization nor by VEGF. As HIF-1α expression levels are markedly different in the two hypoxic conditions, yet the tumor growth rate is essentially the same, it appears that in the tested in vivo model the HIF-1α-linked pathways are less prominent that those linked to the PI3K/Akt pathway