Expression of APE1 endonuclease in primarily cultured aortic valve interstitial cells undergoing spontaneous senescence

Apurinic/apyrimidinic endodeoxyribonuclease 1 (APE1) is the major enzyme involved in the base excision repair pathway working on DNA damages mainly caused by oxidative stress. Namely, APE1 hydrolyses the phosphodiester bond at the abasic sites generated by DNA glycosylases, creating the substrate for DNA polymerase β and DNA ligase IIIa which terminate the reparative process [1]. APE1 also seems to play a role in cell senescence maintaining telomere stability and size in interaction with specifi c telomere-protective proteins [2]. Here, aortic valve interstitial cells (AVICs) isolated from healthy bovine valve leafl ets were cultured under normal conditions for up to 90 days to achieve spontaneous cell senescence. Time-dependent increase in β-galactosidase activity, a marker of cell senescence, was paralleled by a remarkable decrease of APE1-expressing AVICs starting from day 60, as immunocytochemically revealed. Quantitative Western blot analyses also showed a drop of APE1 protein content at day 60, whereas RTPCR analyses revealed a mild increase of the enzyme mRNA over time. Ultrastructurally, AVICs appeared well preserved up to 30-day-long culturing. Conversely, starting from day 60, cells showed non-lysosomal autophagocytosis features mainly consisting of a hypertrophic rough endoplasmic reticulum engulfi ng suff ering mitochondria [3]. Cytoplasm vacuolization due to large organelle degeneration was also clearly appreciable. In conclusion, decreasing APE1 expression over time is supposed to contribute to AVIC decay in a model of spontaneous cell senescence.