Oxidative stress inhibits the proliferation, induces premature senescence and promotes a catabolic phenotype in human nucleus pulposus intervertebral disc cells.

Aged and degenerated intervertebral discs are characterised by a significant increase in the number of senescent cells, which may be associated with the deterioration of this tissue due to their catabolic phenotype. On the other hand, carboxymethyl-lysine has been found to be accumulated with ageing in the proteins of the disc, evidencing the existence of oxidative stress in this tissue. Accordingly, here we investigated the effect of oxidative stress on the physiology of human nucleus pulposus cells. Hydrogen peroxide (H2O2) at subcytotoxic concentrations transiently increased the intracellular levels of reactive oxygen species, activated the p38 MAPK, ERKs, JNKs and Akt signalling pathways and induced the nuclear translocation of NF-κΒ and Nrf2. It also provoked DNA damage and triggered a DNA repair response by activating the ATM-Chk2-p53-p21(WAF1)-pRb pathway, ultimately resulting in a G1 cell cycle delay and the decrease of cells' proliferation. Prolonged exposure to H2O2 led to premature cellular senescence, as characterised by the inhibition of proliferation, the enhanced senescence-associated ß galactosidase staining and the over-expression of known molecular markers, without though a significant decrease in the chromosome telomere length. H2O2-senescent cells were found to possess a catabolic phenotype, mainly characterised by the up-regulation of extracellular matrix-degrading enzymes (MMP-1, -2, -9 and ADAMTS-5) and the down-regulation of their inhibitors (TIMPs), as well as of several proteoglycans, including aggrecan, the major component of the nucleus pulposus. The senescent phenotype could be reversed by N-acetyl-L-cysteine, supporting the use of antioxidants for the improvement of disc physiology and the deceleration of disc degeneration.

Investigation of cell death induced by N-methyl-N-nitrosourea in cell lines of human origin and implication of RNA binding protein alterations.

Methylating agents, a widely used class of anticancer drugs, induce DNA methylation adducts, the most biologically significant being O(6)-methylguanine. The efficacy of these drugs depends on the interplay of three DNA repair systems: base excision repair (BER), methyl-directed mismatch repair (MMR) and direct damage reversal by O(6)-methylguanine-DNA methyltransferase (MGMT). An MGMT-inducible, MMR- and BER-proficient HeLa cell line was treated with different concentrations of N-methyl-N-nitrosourea (MNU), a model S(N)1 methylating agent, analogous to widely used methylating cancer chemotherapeutic drugs, under different expression levels of the repair enzyme (MGMT). MNU induced MGMT-dependent apoptotic cell death. In this particular cellular context, the induction of apoptosis was accompanied by modifications of the RNA binding protein poly(A)polymerase and significant down-regulation of the heterogeneous nuclear ribonucleoprotein (hnRNP) C1/C2. These results implicate alterations of the above mentioned RNA binding proteins in S(N)1 methylating agent-induced cell death and apoptosis, providing a possible perspective regarding their use as biomarkers of tumor resistance/sensitivity to chemotherapy.