Calcium and Calpain as Key Mediators of Apoptosis-like Death Induced by Vitamin D Compounds in Breast Cancer Cells

Abstract

Very little is known about the signaling pathways mediating vitamin D-induced cell death. A single family of proteases, the caspases, has until recently been considered the pivotal executioner of all programmed cell death (8). Therefore it is interesting to note that breast cancer cells treated with vitamin D compounds die in the complete absence of effector caspase activation (4). Despite the lack of the caspase activation, dying cells present several characteristics of apoptosis, i.e. rounding, shrinkage, and detachment of cells as well as DNA strand breaks and DNA fragmentation (4, 9, 10). Furthermore, antiapoptotic proteins Bcl-2 and Bcl-X L can rescue breast cancer cells from death induced by the active form of vitamin D or its analogs (4). This apoptosis-like death program appears also independent of cysteine cathepsins and p53 tumor suppressor protein (4, 11). Instead, the elevation in the intracellular free calcium ([Ca 2ϩ ] i ) brought about by vitamin D compounds correlates with the induction of apoptosis in breast cancer cells (9,12,13).Data from studies employing various pharmaceutical modulators of calcium homeostasis have suggested that the elevation in [Ca 2ϩ ] i is a sufficient signal to induce apoptosis in several model systems, even though it may also have the opposite effect in other systems (14). Further supporting the idea that the elevation in [Ca 2ϩ ] i may mediate apoptosis, studies based on modulated expression of calcium-binding proteins, calbindin-D 28k or glucose-regulated proteins GRP78 and GRP94, have shown that Ca 2ϩ buffering can confer protection against various apoptotic stimuli (15-19). The calcium-dependent neutral cysteine proteases, calpains, are frequently activated in apoptosis models involving elevated [Ca 2ϩ ] i (20 -22). Two forms of calpains, -calpain and m-calpain or type I and type II calpain, respectively, are ubiquitously expressed in human cells (23)(24)(25). The active forms of the enzymes consist of a variable large subunit (80 kDa) and a common small subunit (30 kDa). To become active, calpains require an elevation in [Ca 2ϩ ] i , and the autoproteolytic cleavage of the enzymes further enhances their activity. Whereas m-calpain requires Ca 2ϩ at a millimolar range, micromolar concentrations are enough for the activation of -calpain (in vitro; lower in cells). So far no difference in the substrate specificity of the two isozymes has been found. Growing evidence suggests that calpains may play a central role in the execution of apoptosis either upstream or * This work was supported by the Danish Medical Research Council, the Danish Cancer Society (to I. S. M. and M. J.), and by Grant CA67317 from the NCI, National Institutes of Health (to I. N. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. § Present address: Molecular Biology, Novo Nordisk A/S, Bagsvaerd DK 2880, Denmar...

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