title: Chemoresistance to paclitaxel in human ovarian xenografts: The role of apoptosis-regulating proteins
Creators: name: Al-Moundhri, Mansour
Publisher: name: University of Glasgow
Year: 1999
Subjects: subject: Pharmacology.

description: Paclitaxel has established an evolving role in the management of ovarian cancer. However, the emergence of resistance to paclitaxel is likely to become an increasing problem, and thus a reason for failure of therapy of ovarian cancer. Understanding of the molecular pathways involved in the development of resistance to paclitaxel may provide suitable targets for novel therapies to help to circumvent resistance to paclitaxel. Three main mechanisms have been shown to be involved in paclitaxel resistance: (a) altered paclitaxel uptake, (b) altered paclitaxel-microtubules interaction, (c) alterations in apoptosis regulating proteins that control the induction of cell death. The realization that most chemotherapeutic agents- including paclitaxel- induce cell death by a genetically programmed process termed apoptosis together with the demonstration that alteration of apoptosis regulating proteins such as bcl-2 family of proteins or p53 may induce chemoresistance to paclitaxel has led to intense research in this area. Almost all these studies were in vitro with an obvious lack of in vivo studies. Hence, these apoptosis regulating proteins were the focus of the present in vivo study as they may either represent oncogenes responsible for acquired resistance, or alternatively potential targets for molecular intervention to circumvent resistance to paclitaxel. The first part of the study compared the alterations in apoptosis regulating proteins between CH1 paclitaxel sensitive xenografts (CH1/TS) and acquired paclitaxel resistant xenografts (CH1/TR). These xenografts were established from a CH1 paclitaxel sensitive cell line treated with paclitaxel which then acquired resistance in vitro. Following the confirmation of the sensitivity pattern of both tumours to paclitaxel in vivo, it was demonstrated that there was no paclitaxel uptake defect in CH1/TR tumours that may contribute to paclitaxel resistance. Flow cytometric and apoptosis studies provided an insight into the mechanism of action of paclitaxel. They confirmed the response pattern with paclitaxel inducing mitotic arrest, G2/M phase arrest, and apoptosis in CH1/TS tumours. In contrast, these parameters were unaltered in the CH1/TR tumours, consistent with its resistant phenotype. There were clear differences in the induction of apoptosis regulating proteins between the paclitaxel sensitive and resistant tumours. Following the treatment of CH1/TS tumours with paclitaxel, there was a significant early induction of p53, and p21 with evidence of inactivation (phosphorylation) of survival promoting (anti-apoptotic) protein bcl-2, and down regulation of anti-apoptotic protein bc1-xl. These changes were not significantly altered in CH1/TR tumours. Subsequently, these xenografts were treated with cisplatin to compare and contrast its effect to that of paclitaxel. The first difference demonstrated was that both CH1/TS and CH1/TR tumours displayed complete and partial response, respectively, to cisplatin. In contrast to paclitaxel, the main cell cycle alterations to cisplatin were accumulation of cells in S phase, and late G2 arrest. These cell cycle alterations were shown to be associated with induction of apoptosis. More importantly, it was observed that cisplatin was able to induce p53 and down regulate bc1-xl in CH1/TR tumours suggesting that these pathway are intact. This suggested that CH1/TR tumours acquired resistance to paclitaxel was not due to inherent defect in the expression of apoptosis regulating proteins, but instead due to an upstream defect possibly at the level of paclitaxel-microtubule interaction. This is consistent with previous in vitro studies that have shown that successful polymerization of microtubules is required for induction of cell death, induction of p53 and p21 and phosphorylation of bcl-2. Taking all these observations into consideration, it could postulated that resistance to paclitaxel in CH1/TR tumours that was reflected in the lack of significant alteration in apoptosis regulating protein was due to unsuccessful microtubule-paclitaxel interaction which is supported by the lack of G2/M arrest and induction of apoptosis. The paclitaxel-microtubule interaction will be the focus of a subsequent study. The second part of this study demonstrated in vivo that the overexpression of a single apoptosis regulating protein resulted in the modulation of response to chemotherapy. Xenografts of CHI paclitaxel sensitive cells overexpressing the anti-apoptotic protein bc1-xl were established and treated with paclitaxel and cisplatin. The overexpression of bc1-xl protein resulted in development of chemoresistance to paclitaxel and cisplatin. This observation was particularly important because it demonstrated that these proteins do control a common pathway controlling cell death, and further underline their importance as possible targets for molecular therapy.