Title

An investigation on role of the ATP-binding cassette B5 (ABCB5) transporter as potential mediator of melanoma resistance to BRAF inhibition

Author Identifiers

Lokeswari P. Tangella
ORCID: 0000-0002-1293-3348

Date of Award

2020

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Medical & Health Sciences

First Advisor

Associate Professor Elin S. Gray

Second Advisor

Professor Ricardo L. Mancera

Third Advisor

Professor Mel Ziman

Abstract

Cutaneous melanoma is a highly metastatic and drug-resistant skin cancer type, responsible for a disproportionate number of skin cancer deaths. Targeted therapies, in the form of BRAF inhibitors (BRAFis), have been effective at treating BRAFV600 mutant melanomas. However, majority of the melanoma patients fail to respond to BRAFis due to intrinsic or acquired resistance within one year of treatment commencement. Multiple mechanisms that contribute to BRAFi resistance in melanoma cells have been identified, as discussed in the review in Chapter 1. Overexpression of ATP-binding cassette (ABC) transporters has been linked to multidrug resistance in numerous cancer types. These transporters expel the anti-cancer drugs out of the cell, thereby decreasing the intracellular concentration of the drug. In melanoma, the ATP-binding cassette B5 transporter (a member of ABC superfamily) has been linked to chemoresistance by drug extrusion. Moreover, overexpression of ABCB5 has been observed in BRAFV600 melanoma cells after short-term BRAFi treatment. In this study we investigated the role of the ABCB5 transporter as potential mediators of resistance to BRAFis by drug expulsion.

In Chapter 2, we showed increased ABCB5 expression in melanoma cell lines after short-term treatment with the BRAFis accompanied by an increased expression of melanocytic signature. Gene expression of fluorescent activated cell sorted melanoma cells into ABCB5high and ABCB5low populations, revealed an increased melanocytic signature in the ABCB5high population. Moreover, analysis of single-cell RNA sequencing (scRNAseq) data of two BRAFV600 melanoma cell lines, A2058 and 451Lu, revealed a strong association between ABCB5 expression and melanocytic signature. Based on these initial observations, the capacity of the ABCB5 transporter to efflux BRAFis was evaluated indirectly through an in-silico approach using molecular docking simulations (Chapter 3 and 4), and directly through in vitro experiments using an ABCB5 overexpressing melanoma BRAFV600 cell line (Chapter 5).

In Chapter 3, a full-length ABCB5 model was generated, based on mouse ATP-binding cassette B1 transporter (ABCB1; Pgp1), a close homologue of ABCB5. Molecular dynamics simulations were performed in 2 model cell membranes and the dominant conformation was identified. Docking simulations of known ABCB5 substrates such as taxanes, anthracyclines, camptothecin and etoposide enabled the identification of at least three putative substrate binding sites in ABCB5. The overlap of these three binding sites with validated binding sites for these chemotherapeutic drugs in Pgp1 corroborate our findings. In Chapter 4, docking simulations revealed at least one overlapping binding site for BRAFis and chemotherapeutic drugs on ABCB5, suggesting that BRAFis could potentially act as a substrate for ABCB5. In Chapter 5, we generated an ABCB5 overexpressing BRAFV600E melanoma cell line. However, no differences in sensitivity to BRAF inhibition was observed as a result of ABCB5 overexpression. Intracellular drug accumulation analyses revealed no reduction in vemurafenib or dabrafenib concentrations, indicating that BRAFis do not act as substrates for ABCB5.

Altogether, our studies suggest that ABCB5 expression is linked to the melanocytic program. However, despite the molecular docking evidence that BRAFis may be substrates of ABCB5, in vitro studies failed to demonstrate direct efflux of BRAFis by ABCB5.

Access Note

Access to this thesis is embargoed until 17 May 2022. Email queries to library@ecu.edu.au

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