RESUMO
Variolin B (VAR-B) is a natural product isolated from the sponge Kirkpatrickia variolosa, found in Antarctica. VAR-B has been shown previously to possess potent pro-apoptotic activity. This study was undertaken to investigate the mechanism of action of chemically synthesised VAR-B and its analogue deoxy-variolin B (dVAR-B). In different human cancer cell lines both compounds inhibited colony formation, caused cell cycle perturbations and induced apoptosis at concentrations ranging from 0.1 to 2 microM. LoVo/Dx cells over-expressing Pgp were equally sensitive as the parental cell line to VAR-B and dVAR-B, indicating that variolins are not substrates of Pgp. Although variolins induced an increase in the levels of p53 with an increase in p21, their cytotoxicities did not appear to be dependent on p53 status as their potency was comparable in cells with wild-type p53, or in sub-lines with inactivated p53. Both VAR-B and dVAR-B prevent the cells from entering S phase, blocking cells in G1 and cause an accumulation of cells in G2. The apoptosis induced by VAR-B and dVAR-B occurs very rapidly in some cell lines (e.g., Jurkat leukaemia cells) and is already evident 4h after the beginning of treatment. Although intercalation of dVAR-B in DNA has been demonstrated, neither VAR-B nor dVAR-B produce detectable breaks in DNA. These results are consistent with the in vitro biochemical assays that also demonstrated that dVAR-B is not topoisomerase I or II poison. Instead, each of these variolins appears to inhibit cyclin-dependent kinases (CDKs) in the muM range. CDK1-cyclin B, CDK2-cyclin A and CDK2/cylin E complexes were inhibited in a range of concentrations lower than those required to inhibit the activity of CDK4/cyclin D or CDK7/cyclin H complexes. In conclusion, these variolins are a new class of CDK inhibitors that activate apoptosis in a p53-independent fashion and thus they may be effective against tumours with p53 mutations or deletions.
