|dc.description.abstract||Small cell lung cancer (SCLC) is characterised by rapid growth, early metastatic
spread and poor long-term survival. The tumour is initially sensitive to chemotherapy
and thus objective response rates are high. Unfortunately, this response is often
short-lived and SCLC recurs with acquired drug resistance, resulting in early patient
death. Despite intensive chemo- and radiotherapy regimes survival has not improved
significantly in 20 years. Prior research suggests a critical role for the tumour
microenvironment in the pathogenesis of other cancers. Therefore, investigating
interactions between SCLC cells and components of the tumour stroma may identify
novel therapeutic targets.
This thesis demonstrates that extracellular matrix (ECM) proteins present in the
tumour microenvironment protect SCLC cells in vitro from chemo- and radiotherapy
induced cell cycle arrest and apoptosis via cell surface β1 integrins. Pharmacological
and genetic inhibition of phosphoinositol-3 kinase signalling abrogates this effect,
defining a central role for this pathway in SCLC de novo drug resistance.
Furthermore, the protective effect of ECM occurs without alteration in
chemotherapy-induced DNA damage allowing SCLC cells to survive with new
genetic defects. Integrin-mediated drug resistance has been shown to be important in
other tumours and thus development of strategies to inhibit this pathway may yield
new anti-cancer treatments.
The design of targeted agents to down-regulate integrin-ECM interaction requires an
in depth understanding of the intracellular signals that modulate integrin affinity.
Two such pathways are investigated in this thesis. 1) H-Ras, a dominant suppressor
of integrin affinity, acts in part through phosphorylation of Erk. Data presented here
demonstrate that H-Ras also suppresses integrins through a phospholipase-C epsilon
(PLCε)-dependent pathway, thus explaining discrepancies in prior data and
confirming a physiological role for this recently identified phospholipase. 2) The
Notch signalling pathway has been shown to have important roles in both
development and cancer. It is shown here that activation of Notch signalling
increases β1 integrin affinity and can protect SCLC cells from chemotherapyinduced
apoptosis. However the mechanisms appear to be different; Notch-1 modulates integrin activation through the small GTPase R-Ras and Notch-2
promotes SCLC cell survival. These results define a new Notch pathway, a novel
integrin modulator and a potential therapeutic target in SCLC cells.
In addition to ECM proteins, the tumour microenvironment contains immune cells
that may contribute to cancer growth. The cellular composition of the SCLC stroma
is poorly understood. The data presented here indicate that the microenvironment of
SCLC is infiltrated by lymphocytes and macrophages, the degree of which
independently predicts patient survival. This suggests that the host immune system
may be able to suppress SCLC growth. It is well recognised that patients with SCLC
have defects in cellular immunity which correlate with survival. An in vitro coculture
model was used to investigate the underpinning mechanisms, showing SCLC
cells can suppress CD4+ T-cell proliferation and macrophage CD86 expression.
Furthermore, preliminary data suggest a role for a soluble factor released by SCLC
cells that up-regulates CD4+ T-cell production of IL-10.
The work in this thesis implies a complex interaction between SCLC cells, ECM and
immune cells in the tumour microenvironment. Manipulation of these pathways may
have important therapeutic implications. Further investigation is required to
understand the mechanisms of this interplay, which may in part be aided by
prospective analysis of patient tumour samples and an in vivo model of SCLC.||en