Collective invasion into adjacent tissue is a hallmark of luminal breast cancer, with about 20% of cases that eventually undergo metastasis. It remained unclear how less aggressive luminal-like breast cancer transit to invasive cancer. Our study revealed that CD44hicancer cells are the leading subpopulation in collective invading cancer cells, which could efficiently lead the collective invasion of CD44lo/follower cells. CD44hi/leading subpopulation showed specific gene signature of a cohort of hybrid epithelial/mesenchymal state genes and key functional co-regulators of collective invasion, which was distinct from CD44lo/follower cells. However, the CD44hi/leading cells, in partial-EMT state, were readily switching to CD44lophenotype along with collective movements and vice versa, which is spontaneous and sensitive to tumor microenvironment. The CD44lo-to-CD44hiconversion is accompanied with a shift of CD44s-to-CD44v, but not corresponding to the conversion of non-CSC-to-CSC. Therefore, the CD44hileader cells are not a stable subpopulation in breast tumors. This plasticity and ability to generate CD44hicarcinoma cells with enhanced invasion-initiating powers might be responsible for the transition from in situ to invasive behavior of luminal-type breast cancer.SignificanceNow, the mechanisms involved in local invasion and distant metastasis are still unclear. We identified a switch of CD44 that drives leader cell formation during collective invasion in luminal breast cancer. We provided evidence that interconversions between low and high CD44 states occur frequently during collective invasion. Furthermore, these findings demonstrated that the CD44hi/leader cells featuring partial EMT are inducible and attainable in response to tumor microenvironment. The CD44locancer cells are plastic that readily shift to CD44histate, accompanied with shifts of CD44s-to-CD44v, thereby increasing tumorigenic and malignant potential. There are many “non-invasiveness” epithelial/follower cells with reversible invasive potential within an individual tumor, that casting some challenges on molecular targeting therapy.
