Interstitial fluid pressure as an alternate regulator of angiogenesis independent of hypoxia driven HIF-1α in solid tumors.

We previously showed that interstitial fluid pressure (IFP) may be an alternate regulator of angiogenesis in solid tumors. Given the accepted link between hypoxia-induced factor and angiogenesis this study investigated the effect of IFP on hypoxia-inducible factor (HIF-1α) and vascular endothelial growth factor (VEGF) in human osteosarcoma xenografts in SCID mice and in different hypoxic environments. Tumors were grown either at heterotopic (flank) or orthotopic (medullary canal of the proximal tibia) sites in the host animal. Microfluidic probes determined pH, O(2)-saturation, IFP, and peripheral blood flow perfusion continuously. We assessed tumor growth in the orthotopic site (n = 15) by softex radiographs weekly, 3D microCT, histological evaluation, and for molecular responses. An increased cytoplasmic immunohistostaining of cells for HIF-1α (p = 0.03) and VEGF-A (p = 0.004) on the outer periphery was noted compared to the tumor center, with VEGFR2 uniformly stained throughout. This paralleled a raised state of interstitial hypertension (p = 0.007) in the tumor center relative to the peripheral surface but was inconsistent with a state of hypoxia (p = 0.03) in the tumor center. In vitro culture of human osteosarcoma cell lines (HOS, U2OS) and a human osteoblast control at 0- and 20-mmHg of hydrostatic pressure revealed suppression of HIF-1α (p = 0.02) and VEGF-A (p = 0.02) gene expression when IFP was raised, while the effect on VEGFR1 was equivocal. This study proposes an alternative regulatory angiogenic pathway via the influence of IFP on cancer cell function. The identification of a mechanistic cellular link to the physical parameter becomes an important tool to evaluate cancer cell growth within solid tumors.