The Forkhead Box M1 (FOXM1) transcription factor is an important master
cell cycle regulator that can control a wide spectrum of downstream target genes
involved in apoptosis, metastasis, DNA repair, oxidative stress, G1/S and G2/M cell
cycle transition. Elevated expression or activity of FOXM1 is correlated with poor
prognosis in breast cancer patients.
In this study, I demonstrated that the novel thiazole antibiotics - thiostrepton
selectively induces cell cycle arrest and cell death in breast cancer cells through
down-regulating of FOXM1 at the transcriptional and gene promoter levels.
Furthermore, thiostrepton also demonstrates efficacy in repressing breast cancer cell
migration, metastasis and transformation, which are all downstream functional
attributes of FOXM1. Interestingly, thiostrepton exhibited minimal cytotoxic effect
in untransformed MCF-10A breast epithelial cells, suggesting that under in vitro
conditions thiostrepton represents a novel lead compound for targeted therapy of
breast cancer with minimal toxicity against non-cancer cells.
Prior to establishing the possibility of employing thiostrepton to circumvent
acquired cisplatin resistance, I needed to address the hitherto unknown roles that
FOXM1 might play in acquired cisplatin resistance through its proposed downstream
DNA repair targets - breast cancer associated gene 2 (BRCA2) and X-ray-crosscomplementing
group 1 (XRCC1). I demonstrated that the cisplatin resistant breast
cancer cell line (MCF-7-CISR) showed an elevation of both FOXM1 mRNA and
protein expression levels relative to parental MCF-7 cells. Interestingly, the siRNA knockdown of FOXM1, but not BRCA2 or XRCC1 could reduce the rate of
proliferation in response to cisplatin treatment in the resistant cells. This suggests
FOXM1 could be a better therapeutic target than DNA damage repair genes, as
FOXM1 controls several other critical cellular functions which are important for
cancer cell survival.
Moreover, the mitogenic activation protein kinase (MAPK) - ERK-1/2 has
previously been implicated in conferring acquired cisplatin resistance in ovarian
cancer cells. Hitherto, its role in acquired cisplatin resistance in breast cancer is
unknown. Given that ERK-1/2 is critical in mediating FOXM1 phosphorylation and
translocation prior to the mitotic phase of cell cycle, I hypothesized that an upregulation
of ERK-1/2 could also contribute to cisplatin resistance in breast cancer
cells through enhancing nuclear FOXM1 translocation. Surprisingly, I discovered
that the inhibition ERK-1/2 kinase activity through the use of MEK inhibitors
PD098059 failed to reverse cisplatin sensitivity in MCF-7-CISR cells. Furthermore,
PD098059 failed to inhibit FOXM1 nuclear translocation in MCF-7-CISR cells, but
not in MCF-7s. The uncoupling of ERK-1/2 and FOXM1 in MCF-CISR cells confer
cellular cisplatin resistance independent of ERK mediated mitogenic signals suggests
that the direct targeting of FOXM1 would be a better strategy in circumventing
cisplatin resistant breast cancer. Indeed, thiostrepton exhibited potent cytotoxicity in
MCF-7-CISR cells either as a single agent, or synergistically with cisplatin. Taken
together, the body of pre-clinical research data presented in this thesis has laid
foundations for future work to further validate thiostrepton as a treatment for patients
with breast cancer or acquired chemoresistance through the inhibition of FOXM1
expression.