Calcimycin (A23187) is an ionophore widely used in studies related to calcium dynamics in cells, but its fluorometric potential to reveal intracellular physiology has not been explored. Exploiting the microenvironment-induced changes in its fluorescence, we show that a brief exposure of cells to non-toxic concentrations (≤3μM) of the ionophore results in the characteristic organization of the ionophore forming brightly fluorescent cytoplasmic bodies termed "I-Bodies", which are closely related to stress linked disturbances/changes in calcium homeostasis. "I-Bodies" appear to be Ca(2+) rich intracellular sites formed during stress-induced release of intracellular Ca(2+), causing dysfunction and aggregation of mitochondria, providing scaffold for high density packing of A23187. Formation of "I-Bodies" in cells exposed to ionizing radiation and certain anticancer drugs suggest their potential in revealing alterations in calcium signaling and mitochondrial function during (related to) macromolecular damage-induced cell death. The absence of "I-Bodies" in non-malignant cells and their varying numbers in malignant cells with 5 fold increase in fluorescence imply that they can be potential biomarkers of cancer. Thus, "I-Bodies" are novel indicators of endogenous and induced stress linked to disturbances in calcium homeostasis in cells, with a potential to serve as biomarker of cancer.
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