Cellular Senescence and Acinar Cell Survival in Irradiated Salivary Glands

Objectives: The major outcome of radiation treatments leading to salivary gland hypofunction is the permanent loss of the secretory acinar cells. Despite decades of research, the cellular and molecular mechanisms underlying radiation-induced acinar cell loss are unknown. Radiation induces apoptosis and is also correlated with an increase in senescence in the salivary glands. It has been proposed that senescence and accompanying pro-inflammatory products may indirectly drive acinar cell loss in the salivary glands.
Methods: Genetically modified mouse models were used to track secretory acinar cells and their progeny in submandibular glands (SMG) following a single radiation dose of 15 Gy. Immunohistochemistry was used to investigate cell phenotype, proliferation, DNA damage, and cellular senescence. Stimulated saliva secretion was measured at 48 hours, 5, 10, 15 and 20 weeks after irradiation. Quantitative RT-PCR was used to measure expression of senescence markers.
Results: Our data show that clusters of SMG acinar cells, marked by continued expression of Mist1, are detectable, as much as 8 weeks after irradiation. Cells within the clusters express the proliferation marker Ki67, and show evidence of clonal expansion. In agreement with published reports, we observe the appearance of senescence-associated markers in the irradiated glands during this period. Staining for gH2AX, a marker of DNA damage, shows that foci are retained up to 20 weeks after irradiation. Additional markers of senescence and of the senescence-associated secretory phenotype are significantly upregulated in irradiated glands from 5 to 20 weeks post-irradiation.
Conclusions: Our data show a slow, but progressive, loss of secretory acinar cells following salivary gland irradiation. In conjunction, we have measured an increase in cellular senescence in irradiated SMG. The results suggest a role for senescence in the radiation-induced loss of acinar cells.