Energy Metabolism Alterations in Irradiated Salivary Glands

The salivary gland is a nearby organ in head and neck cancer patients that is damaged by radiation treatment, resulting in hyposalivation and xerostomia. Current treatments for radiation-induced hyposalivation are temporary and do not permanently restore function to the salivary gland, therefore more mechanistic understanding of the damage response is needed to identify therapeutic targets for lasting restoration. The reprogramming of energy metabolism has been observed in cancer and wound healing models to provide the necessary fuel for cell proliferation, but the reprogramming of energy metabolism in the salivary gland in response to radiation has not been investigated. We hypothesize that glycolytic flux increases to fuel compensatory proliferation while oxidative phosphorylation decreases due to mitochondrial dysfunction following radiation. We measured the oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) of irradiated primary acinar cells to assess glycolytic flux and oxidative phosphorylation and ATP production produced from each pathway. We measured protein levels and enzymatic activity of the rate-limiting enzymes in glycolysis and measured lactate concentrations in irradiated parotid gland tissue. To assess mitochondrial function, we measured protein levels of complex I and III subunits of the electron transport chain and measured mitochondrial DNA (mtDNA) copy number and spare respiratory capacity. Lastly, we tested fuel dependency and flexibility of irradiated acinar cells. Our results show an increase in OCR, ECAR, and ATP production rate at 24 hours and 5 days post-radiation with a subsequent decrease at chronic time points. Hexokinase protein levels and activity increase at 3 days post-IR and lactate concentration increases at days 5 and 14 post-radiation. Complex I and III subunit protein levels decrease at days 3 and 5 post-radiation while mitochondrial DNA copy number increases chronically post-radiation. Primary acinar cells become more dependent on long-chain fatty acids for fuel at day 5 post-radiation. These results elucidate the reprogramming of energy metabolism in the salivary gland in response to radiation over time, which may contribute to the radiosensitivity of the gland.