Intramitochondrial calcium regulation by the FHIT gene product sensitizes to apoptosis

Abstract

Despite the growing interest in the Fhit tumor suppressor protein, frequently deleted in human cancers, the mechanism of its powerful proapoptotic activity has remained elusive. We here demonstrate that Fhit sensitizes the low-affinity Ca 2؉ transporters of mitochondria, enhancing Ca 2؉ uptake into the organelle both in intact and in permabilized cells, and potentiating the effect of apoptotic agents. This effect can be attributed to the fraction of Fhit sorted to mitochondria, as a fully mitochondrial Fhit (a chimeric protein including a mitochondrial targeting sequence) retains the Ca 2؉ signaling properties of Fhit and the proapoptotic activity of the native protein (whereas the effects on the cell cycle are lost). Thus, the partial sorting of Fhit to mitochondria allows to finely tune the sensitivity of the organelle to the highly pleiomorphic Ca 2؉ signals, synergizing with apoptotic challenges. This concept, and the identification of the molecular machinery, may provide ways to act on apoptotic cell death and its derangement in cancer.calcium signaling ͉ mitochondria ͉ oncosopressor ͉ oxidative stress T he fragile histidine triad (FHIT) gene, isolated by positional cloning, encompasses the most common human fragile site FRA3B at 3p14.2. This chromosomal region is involved in hemizygous and homozygous deletions, and indeed mutations of FHIT were demonstrated in a large variety of human tumors (1-4). FHIT encodes a 17-kDa protein (Fhit) that is abundantly expressed in normal human lung, stomach, kidney, and other epithelial tissues, whereas most tumors and tumor-derived cell lines do not express Fhit or show markedly reduced levels of the protein (5). Fhit knock-out mice are more susceptible to cancer development than their wild-type counterparts (6), and FHIT gene therapy can prevent and reverse tumors in carcinogen-exposed Fhit-deficient mice (7). However, the precise molecular mechanism involved in the antitumor function of FHIT remains largely unclear.Fhit is partly localized in mitochondria, and interaction with Hsp60/ Hsp10 could be important for correct refolding after import and Fhit stability (8). This compartmentalization of Fhit could reveal a transcription-independent regulation of cell fate. Indeed, mitochondria are at the crossroad of numerous apoptotic pathways that synergize in triggeringthemorphologicaltransitionsunderlyingthereleaseofproapoptotic factors into the cytoplasm (9-11). In most cases, Ca 2ϩ acts as a fundamental sensitizing factor, and anti-apoptotic proteins, such as Bcl-2, have been shown to reduce ER Ca 2ϩ levels, and agonistdependent release and mitochondrial loading (12)(13)(14).Different agents induce Ca 2ϩ release from the ER Ca 2ϩ store through the IP 3 R Ca 2ϩ release channel (15). Consequent mitochondrial Ca 2ϩ uptake, via a yet unidentified Ca 2ϩ channel of the inner mitochondrial membrane (the mitochondrial Ca 2ϩ uniporter, MCU), regulates different processes: Aerobic metabolism (16), release of caspase cofactors (17), and feedback control of neighboring ER or plasma ...

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The FHIT gene product: tumor suppressor and genome “caretaker”

“…This interaction results in increased reactive oxygen species production, causing caspase-3 activation and apoptosis. It has also been reported that Fhit can increase calcium release from the mitochondria, offering an additional means of promotion of apoptosis [32]. …”

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