Investigating the role of Sphingosine Kinase 1 pathway in cancer cell-monocyte interactions

Strong evidence suggests that the tumour microenvironment is inflammatory and that activation of the innate immune system plays a role in cancer progression, therefore targeting the multiple interactions of tumour cells with other cell types within the tumour microenvironment may lead to development of new cancer therapies. Sphingosine kinase (SPHK1) is a tumour-associated enzyme whose over-expression has been linked to patient mortality in many types of cancer. Here I investigate whether activation of the SPHK1 pathway, with a known involvement in inflammatory responses, is a signal transduction component of the tumour-monocyte/macrophage cellular interaction and a key element in inflammation-related cancer progression. Using a co-culture model, this study shows that the presence of monocytes increases cancer cell proliferation, an effect abrogated by knockdown of SPHK1 in cancer cells. Both monocytes and cancer cells showed a transient increase in SPHK1 activity and mRNA expression levels together with an increase in MCP-1 and IL-6 secretion. Silencing of SPHK1 in cancer cells abrogated SPHK1 activation in monocytes and pharmacological inhibition of SPHK1 in monocytes cells decreased monocyte induced-SPHK1 expression in cancer cells. Mechanistically, activation of AKT was observed in cancer cells upon co-culture with monocytes, an effect that was abrogated when cancer cells were pre-treated with siRNA for SPHK1. Moreover, the increase of phospho-AKT, ERK1/2 and NF-KB in monocytes by cancer cells was also reduced by RNAi-mediated knockdown of SPHK1 in cancer cells. My data show that STAT1 can bind to SPHK1 promoter or coding region and may be involved in SPHK1 transcriptional regulation in cancer cells upon monocyte stimulation, however its role still remains unclear as it acts as a transcriptional repressor of SPHK1. Monocytes induced cancer cell chemoprotection via a SPHK1-dependent mechanism, and reduced the inhibitory effect of docetaxel on cancer cell proliferation. Accordingly, increased AKT and ERK1/2 phosphorylation in monocytes were also affected by siRNA targeting of SPHK1 in docetaxel treated cancer cells. Altogether I show for the first time that selective inhibition of SPHK1 in tumour cells can affect their interaction with surrounding cells through the modulation of signal transduction pathways (ERK, PI3K, NF-kB) and cytokine exchange (IL-6, MCP-1 and potentially S1P, GM-CSF, GROα, IL-32 and ICAM-1). SPHK1-mediated increase in proliferation and chemoresistance of cancer cells conferred by monocytes renders this enzyme a promising target for future cancer therapies.Open Acces