Role of erythrocyte Gq protein in tumor metastasis

Erythrocytes are the most abundant cell type in the bloodstream and have, besides oxygen transport, different regulatory functions by releasing signaling molecules under the influence of oxygen deprivation, mechanical stress, or activation by hormones and other mediators. They contain various G protein-coupled receptors and ion channels that regulate multiple downstream signaling pathways and thus enable diverse cellular responses. This suggests that erythrocytes may play an important role in the pathogenesis of several diseases, including tumorigenesis, however, their role in this context has so far not been addressed. Hematogenous dissemination of tumor cells requires the survival of metastatic tumor cells in the bloodstream, and different cell types have been shown to promote tumor cell survival, including neutrophils, monocytes, platelets and endothelial cells. The project aimed to investigate the role of erythrocytes during metastasis and to elucidate the potential underlying molecular mechanisms. Several erythrocyte-specific knockouts of genes encoding Gαs, Gαq, Gα13, Piezo1, Pannexin 1/2 and Ackr1 were tested in murine primary tumor and metastasis models, and reduced metastasis was only found in mice lacking Gαq (Ery-Gαq-KO). Further analysis showed that Ery-Gαq-KO have reduced metastasis as a consequence of reduced tumor cell numbers during early stages of metastatic colonization, even before extravasation into the lung tissue. Reduced numbers of tumor cells in the circulation were associated with increased apoptotic cell death during early stages of hematogenous metastasis. When isolated erythrocytes were stimulated in a Gq-dependent manner, increased levels of S1P were detected in the cell supernatants, an effect that was dependent on ATP-mediated signaling through Gαq. In addition, S1P inhibited tumor cell survival in vitro through activation of prosurvival signaling pathways such as MAPK/ERK and PI3K/Akt. In vivo analysis of tumor cells lacking the S1P receptor 2 showed reduced ability to form metastasis and reduced numbers of tumor cells in lungs during the early stages of hematogenous spreading, resembling the phenotype observed in Ery-Gαq-KO mice. The results show that signal transduction via Gq in erythrocytes plays a crucial role during tumor metastasis by releasing S1P and thereby influencing cell survival of tumor cells in the circulation. This study provides new insights into the previously unknown function of erythrocytes in the development of tumor metastasis. Further studies are required to address the role of S1P and S1PR2 in human cancer cells and further translate this research to understand their role in promoting survival of circulating tumor cells and overall metastasis development in cancer patients. Overall, pharmacological targeting of the S1P-S1PR2 signaling axis in tumor cells and/or Gq-Sphk1-S1P singling in erythrocytes as a therapeutic strategy might be beneficial in preventing metastasis formation.