The mechanisms whereby some tumour cells detach from the primary
lesion to colonize distant sites are still largely unknown. Pro-metastatic
events common to the majority of solid tumours might include the
reversible transition of tumour cells from an epithelial to a mesen-
chymal state as well as their interactions with stromal components
or tumour-activated stromal cells. Some tumours also secrete
metastasis-promoting exosomes that contain proteins, mRNAs and
microRNAs to establish a distant pro-metastatic niche 9,13,18,19. However,
whether a subpopulation of metastasis-initiating cells exists among
primary tumour-initiating cells is not clear.
LRCs express lipid metabolism genes
When cell lines and patient-derived cells (PDCs) arising from human
oral carcinomas (Methods) were pulsed with a lipophilic fluorescent
dye (DiD) that non-specifically binds to membranes and is diluted
upon cell division
signature
was associated with chromosomal instability, cell transformation
and neoplasm and genes involved in the cell cycle, as expected from
a proliferative tumour population (Extended Data Fig. 2b, c and
Supplementary Table 1b). On the other hand, the signature of LRCs
included an over-representation of genes associated with lymphatic
metastasis, neoplasm metastasis, response to lipids and lipid metabolic
process (Fig. 1c, d and Extended Data Fig. 2c, d); this was confirmed
in LRCs sorted from a second OSCC tumour line (Detroit-562;
Extended Data Fig. 2e and Supplementary Table 1c, d). We validated
the differential expression of several of these genes by quantitative
PCR with reverse transcription (RT–qPCR) in five biological replicates
(Extended Data Fig. 2f). Notably, 69 genes were found in the dye
signatures from both the SCC-25 and the Detroit-562 cell lines; the
main functions of these genes were related to neoplasm metastasis,
lymphatic metastasis, response to stimulus, lipid distribution and
translocation and response to lipid, underscoring their relevance
in defining the LRC population (Extended Data Fig. 2g and
Supplementary Table 1e).
LRCs expressed genes involved in different aspects of fatty acid
metabolism, including lipid uptake and transport, fatty acid β - and
α -oxidation, lipid biosynthesis and intracellular lipid storage (Extended
Data Fig. 2d). Among the products of these genes, the receptor CD36 is
at the top of the signalling cascade that takes lipids up from the extra-
cellular environment, allowing cells to obtain ATP energy through
lipid β -oxidation
24–26
. As CD36 is a cell surface receptor, we used
it as a surrogate marker to detect and isolate LRCs from tumours,
circumventing fluorescent dyes. Strikingly, LRCs corresponded to the
cells within primary oral lesions derived from all
cell lines and PDCs tested (Extended Data Figs 2h, 3a).
CD36 is essential for metastasis
Overexpression of CD36 in cell lines or PDCs with low metastatic
potential (SCC-25, VDH-00 and JHU-029) greatly increased their
potential to metastasise to lymph nodes, with penetrance increasing
from less than 20% to 75–80% (Extended Data Figs 3b, e–g and 4a).
Lymph node metastases generated by OSCC tumours overexpressing
CD36 were also more than 40 times the size of those generated by
The fact that the identity of the cells that initiate metastasis in most human cancers is unknown hampers the development
of antimetastatic therapies. Here we describe a subpopulation of CD44
bright
cells in human oral carcinomas that do not
overexpress mesenchymal genes, are slow-cycling, express high levels of the fatty acid receptor CD36 and lipid metabolism
genes, and are unique in their ability to initiate metastasis. Palmitic acid or a high-fat diet specifically boosts the metastatic