Multiplex coherent anti-Stokes Raman scattering microspectroscopy detection of lipid droplets in cancer cells expressing TrkB

International audienceFor many years, scientists have been looking for specific biomarkers associated with cancer cells for diagnosis purposes. these biomarkers mainly consist of proteins located at the cell surface (e.g. the TrkB receptor) whose activation is associated with specific metabolic modifications. Identification of these metabolic changes usually requires cell fixation and specific dye staining. MCARS microspectroscopy is a label-free, non-toxic, and minimally invasive method allowing to perform analyses of live cells and tissues. We used this method to follow the formation of lipid droplets in three colorectal cancer cell lines expressing TrkB. MCARS images of cells generated from signal integration of cH 2 stretching modes allow to discriminate between lipid accumulation in the endoplasmic reticulum and the formation of cytoplasmic lipid droplets. We found that the number of the latter was related to the TrkB expression level. This result was confirmed thanks to the creation of a HEK cell line which over-expresses TrkB. We demonstrated that BDNF-induced TrkB activation leads to the formation of cytoplasmic lipid droplets, which can be abolished by K252a, an inhibitor of TrkB. So, MCARS microspectroscopy proved useful in characterizing cancer cells displaying an aberrant lipid metabolism. Cancer is characterized by an uncontrolled cell growth, and tumor cell dissemination enabling the spread of cancer from its site of origin 1. For a number of years, studies have focused on the identification of specific biomarkers associated with cancer processes. Biomarkers comprise many molecular structures such as DNA, mRNA, proteins or metabolites. However, in most cases, cancer biomarkers consist of proteins such as receptors at the cell surface 1,2. Human epidermal growth factor receptor (ErbB) family has been the first receptor family discovered as involved in human cancers. In this family, the most common receptors are EGFR (Epithelial Growth Factor Receptor) and HER2 (Human Epidermal Growth Factor Receptor 2) 3. Other biomarkers have also been discovered which are specific for certain types of cancer. For example, breast cancer can be detected by the presence of biomarkers such as HER2 or BRCA1, while KRAS and TP53 biomarkers are linked to non-small cell lung cancer 4. Aside from distinguishing cancer cells from healthy ones, these biomarkers also allow to determine the stage of the cancer. Neurotrophins have been discovered in the central and peripheral nervous systems, thereby regulating the growth and differentiation of neurons 5. The neurotrophin family is composed of three major receptors, Trk A, B and C, associated with specific ligands: NGF binds to TrkA, BDNF and NT-4 bind to TrkB and NT-3 binds to TrkC 6. When they are expressed in other human tissues, neurotrophins are associated with tumorigenesis, metastases, for example in ovarian, breast, and colorectal cancers 7. The BDNF/TrkB complex is mainly correlated with colorectal cancer (CRC) and is of poor prognosis 8. BDNF binding to TrkB induces an autophosphorylation open 1 PEIRENE, EA 7500