Characterization of rat parotid and submandibular acinar cell apoptosis in primary culture

Apoptosis is a highly organized cellular process that is critical for maintaining glandular homeostasis. We have used primary rat salivary acinar cells from the parotid and submandibular glands to investigate the critical regulatory events involved in apoptosis. Caspase-3 activity, cleavage of caspase substrates, and deoxyribonucleic acid (DNA) fragmentation were assayed in cells treated with etoposide, a DNA-damaging agent, or brefeldin A (BFA), a Golgi toxin. Dose-response studies showed that the sensitivity of both cell types to etoposide and BFA was similar, with 150 μM etoposide or 1.5 μM BFA inducing maximal caspase activation. However, BFA induced a more robust activation of caspase and DNA fragmentation in both cell types. Similar results were observed when the caspase cleavage of poly(adenosine 5′-diphosphate ribose) polymerase and protein kinase C delta were analyzed by Western blot. Analysis of the kinetics of apoptosis showed that caspase-3 activation was maximal at 8 h of etoposide or BFA treatment in the parotid cells and at 8-18 h in the submandibular cells. A similar time course was observed when DNA fragmentation was assayed, although maximal DNA fragmentation in BFA-treated cells was two- to threefold higher than that observed in etoposide-treated cells. Despite slight kinetic differences, it would appear that the apoptotic cascade is very similar in both primary parotid and submandibular acinar cells. Although limited in their long-term stability in culture, the use of primary, nonimmortalized salivary acinar cultures will also permit the use of specific transgenic animals to further characterize the molecular events involved in the regulation of salivary gland acinar cell apoptosis.