Introduction: Obesity is a known risk factor for breast cancer. Sphingosine kinase 1 (SK1) is an oncogenic lipid
kinase that is overexpressed in breast tumours and linked with poor prognosis, however, its role in obesity-driven
breast cancer was never elucidated.
Methods: Human primary and secondary breast cancer tissues were analysed for SK1 and leptin receptor
expression using quantitative real-time polymerase chain reaction (qRT-PCR) assay. Leptin-induced signalling was
analysed in human oestrogen receptor (ER)-positive and negative breast cancer cells using Western blotting,
qRT-PCR and radiolabelling assays.
Results: Our findings show for the first time that human primary breast tumours and associated lymph node
metastases exhibit a strong correlation between SK1 and leptin receptor expression (Pearson R = 0.78 and R = 0.77,
respectively, P <0.001). Both these genes are elevated in metastases of ER-negative patients and show a significant
increase in patients with higher body mass index (BMI). Leptin induces SK1 expression and activation in ER-negative
breast cancer cell lines MDAMB-231 and BT-549, but not in ER-positive cell lines. Pharmacological inhibition and
gene knockdown showed that leptin-induced SK1 activity and expression are mediated by activation of extracellular
signal-regulated kinases 1/2 (ERK1/2) and Src family kinase (SFK) pathways, but not by the major pathways
downstream of leptin receptor (LEPR) - janus kinase 2 (JAK2) and signal transducer and activator of transcription 3
(STAT3). Src-homology 2 domain-containing phosphatase 2 (SHP2) appeared to be key to SK1 activation, and may
function as an adaptor protein between SFKs and LEPR. Importantly, leptin-induced breast cancer cell proliferation
was abrogated by SK1-specific small interfering RNA (siRNA).
Conclusions: Overall, our findings demonstrate a novel SFK/ERK1/2-mediated pathway that links leptin signalling
and expression of oncogenic enzyme SK1 in breast tumours and suggest the potential significance of this pathway
in ER-negative breast cancer.