| dc.description.abstracteng | Mutation or functional inactivation of tumour suppressors represents a key event in the transformation of cells and contributes to the development of cancer. P300 and CBP constitute two histone acetyltransferases with tumour suppressor functions that are frequently mutated or functional inactivated in cancer. Accordingly, after chemotherapy-induced DNA damage, for example, p300 and CBP can co-activate the key tumour suppressor proteins p53 and TAp73, thereby contributing to tumour cell apoptosis.
Here we investigated the impact of the co-factor protein iASPP on p300 and TAp73 function, after treatment of tumourigenic cell lines with the chemotherapeutic drug cisplatin. iASPP belongs to the ASPP-family, another class of co-factors that contribute to p53-family-mediated apoptosis induction. Direct interaction of iASPP with p300 and TAp73 has been revealed before; functional consequences of these interactions remain elusive though and are therefore subject of our analyses.
By investigating the consequences of stable iASPP knockdown in tumourigenic cell lines, we found that direct interaction of iASPP and p300 in cisplatin-treated cells led to enhanced protein stability of p300 and TAp73. Correspondingly, iASPP depletion resulted in decreased protein amounts of p300, reduced induction of pro-apoptotic p73 target genes and impaired apoptosis.
BRMS1 represents a recently discovered E3 ubiquitin ligase for p300. Hence, we observed that BRMS1 depletion rescued the degradation of p300 and CBP in cisplatin-treated, iASPP-depleted cells. Therefore, we hypothesize that BRMS1 inhibition constitutes the molecular mechanism underlying the increased protein stability of p300 in the presence of iASPP. Furthermore, we discovered, that malignant melanoma are characterized by down-regulated iASPP expression level. Follow-up studies on melanoma cell lines revealed that low iASPP expression correlated with decreased protein levels of p300 in cisplatin-treated cells. BRMS1 knockdown in some of these cell lines could up-regulate p300 protein level, suggesting that down-regulation of iASPP expression leads to functional inactivation of p300 in melanoma, by allowing BRMS1 activity on p300. Treatment of the cells with the MKP-1 inhibitor BCI, could re-establish p300 level and induce p300-dependent apoptosis by yet unknown mechanisms.
Summing up, iASPP represents a regulator for p300 function. It contributes to p300-dependent apoptosis induction by enhancing its protein stability after DNA damage. Re-establishment of p300 levels in low iASPP-expressing melanoma might represent a novel strategy to overcome chemoresistance. | de |