Calcium oxalate crystals with a bipyramidal morphology and a high tendency to aggregate attach directly to breast cancer cells and affect their phenotype

Various types of crystal can form in our bodies under normal and pathological conditions, with the more harmful of these crystals being studied intensively. However, much less is known about calcium oxalate dihydrate (COD) crystals, which are commonly associated with benign conditions. In general, calcium oxalates are pathological biominerals found in kidney stones and as microcalcifications (MCs) in breast and thyroid cancers. In breast cancer MCs, COD crystals are exclusively associated with benign lesions, and we have recently shown that they suppress breast precancer progression in vitro. Here, we studied the influence of synthetic calcium oxalate crystals – both COD and calcium oxalate monohydrate (COM) – with different morphologies and properties on crystal-cell interactions. We found that the less negatively charged crystals exhibited a greater tendency to aggregate and, as aggregates, attached better to cells, particularly to the cells of an invasive breast cancer cell line. We did not find any association between crystallinity, surface area, and solubilities of the crystals and attachment to cells. The crystal types that did attach efficiently to the cells, namely, COD with a thin or thick bipyramidal morphology, had the greatest impact on cell phenotype and functionally delayed cell growth, most probably through contact inhibition. Notably, in terms of morphology and size, these crystals resembled COD MCs found in benign breast tissues, suggesting that they are involved in a mechanism of tumor suppression.