Ca2+ channel-forming ORAI proteins: cancer foes or cancer allies?

The ORAI family of ion channel-forming proteins in mammals includes three members, ORAI1, ORAI2 and ORAI3, encoded by homologous genes. Of these proteins the ORAI1 one received major attention as plasma membrane constituent of store-operated calcium entry (SOCE) in non-excitable cells. The functional significance of two other proteins, ORAI2 and ORAI3, is much less defined, although both of them participate to various extends in cell-specific modulation of SOCE as well as in supporting some of the store-independent calcium entry mechanisms. Calcium signaling becomes remodeled in cancer to promote cancer hallmarks - enhanced proliferation, resistance to apoptosis, motility and metastasizing. Although such remodeling commonly involves rearrangements of the whole molecular Ca2+-handling toolkit of the cell (Ca2+ pumps and transporters, Ca2+-binding and storage proteins, Ca2+ entry and release channels, Ca2+-dependent effectors), Ca2+ entry through Orai-based channels is especially important, as its dysregulation may contribute to several cancer hallmarks. The latter depend on the type of Ca2+-permeable channel formed by ORAI-proteins, spatiotemporal characteristics of Ca2+ signal that this channel contributes to, and the type Ca2+-dependent effector(s) targeted by this signal, all of which may be cancer-specific. By participating in global Ca2+ entry, ORAI-based SOCE may also contribute to cytosolic Ca2+ overload of cancer cells thereby playing pro-apoptotic, antineoplastic roles which can potentially be exploited for cancer treatment. This mini review examines various aspects of ORAI proteins in malignant transformation.