Endothelial cells increase the radiosensitivity of oropharyngeal squamous carcinoma cells in collagen gel

We assessed the radiosensitivity of human HSCO oropharyngeal squamous carcinoma cells in the presence of paracrine factors produced by human HECV umbilical vein endothelial cells. To this end the cells were embedded in separate collagen gels at the density of 1×10⁶ cells/ml each, and the two gels were placed in a well of a six-well plate, sharing the same medium but without physical contact (two-gel model). The medium was not changed during the observation period to ensure the accumulation of soluble factors. On day 2 of culture the gels were irradiated with 0, 0.5, 1, 2 and 8 Gray (Gy) and on day 7 of culture they were disaggregated and cell survival evaluated by the MTT assay. Results were compared with proper untreated and irradiated control groups. Under these experimental conditions it was found that: (1) HSCO and HECV cells influenced reciprocally their behaviour in the two-gel model, in terms that cell survival was enhanced and inhibited, respectively; (2) as expected, HSCO cells were more radioresistant in collagen gel than in monolayer; (3) on the average the survival of HECV cells was enhanced at low radiation doses, irrespective of whether they were cultured alone or with HSCO cells in the two-gel model and (4) HSCO cells displayed a high radioresistance when irradiated alone at doses from 0.5 to 8 Gy. However, when co-cultured with HECV cells in the two-gel model, they become highly radiosensitive already at the dose of 2 Gy, while none of them survived at the dose of 8 Gy. This radiosensitizing effect was specifically induced by paracrine factors circulating in the medium, supporting the notion that stromal endothelial cells may be essential components of a metabolic circuitry supplying solid tumors with radiosensitizing factors.