Exploiting mitochondrial targeting signal(s), TPP and bis-TPP, for eradicating cancer stem cells (CSCs)

Abstract

Tri-phenyl-phosphonium (TPP) is a non-toxic chemical moiety that functionally behaves as a mitochondrial targeting signal (MTS) in living cells. Here, we explored the hypothesis that TPP-related compounds could be utilized to inhibit mitochondria in cancer stem cells (CSCs). We randomly selected 9 TPP-related compounds for screening, using an ATP depletion assay. Based on this approach, five compounds were identified as “positive hits”; two had no detectable effect on ATP production. Remarkably, this represents a >50% hit rate. We validated that the five positive hit compounds all inhibited oxygen consumption rates (OCR), using the Seahorse XFe96 metabolic flux analyzer. Interestingly, these TPP-related compounds were non-toxic and had little or no effect on ATP production in normal human fibroblasts, but selectively targeted adherent “bulk” cancer cells. Finally, these positive hit compounds also inhibited the propagation of CSCs in suspension, as measured functionally using the 3D mammosphere assay. Therefore, these TPP-related compounds successfully inhibited anchorage-independent growth, which is normally associated with a metastatic phenotype. Interestingly, the most effective molecule that we identified contained two TPP moieties (i.e., bis-TPP). More specifically, 2-butene-1,4-bis-TPP potently and selectively inhibited CSC propagation, with an IC-50 < 500 nM. Thus, we conclude that the use of bis-TPP, a “dimeric” mitochondrial targeting signal, may be a promising new approach for the chemical eradication of CSCs. Future studies on the efficacy of 2-butene-1,4-bis-TPP and its derivatives are warranted. In summary, we show that TPP-related compounds provide a novel chemical strategy for effectively killing both i) “bulk” cancer cells and ii) CSCs, while specifically minimizing or avoiding off-target side-effects in normal cells. These results provide the necessary evidence that “normal” mitochondria and “malignant” mitochondria are truly biochemically distinct, removing a significant barrier to therapeutically targeting cancer metabolism.

Pharmacological modulation of mitochondrial ion channels

“…These mitochondria‐targeted mtK v 1.3 inhibitors, therefore, can be considered especially promising molecules for the treatment of several types of cancer, provided the tumour cells express high levels of mtK v 1.3. A recent study identified some TPP‐related compounds that might even eradicate cancer stem cells (Ozsvari et al ., ).…”

A mitochondrial based oncology platform for targeting cancer stem cells (CSCs): MITO-ONC-RX

“…Because of the success of our drug repurposing efforts, we also decided to develop a series of new therapeutics, by focusing on key mitochondrial targets [23–26]. These results are briefly summarized in Table 3.…”

“…The use of tri-phenyl-phosphonium (TPP) also appears to be a very promising approach, as it behaves as a chemical mitochondrial targeting signal (MTS) (Figure 7, Lower) [26]. Interestingly, a series of TPP compounds we tested were non-toxic in normal fibroblasts, but were selectively toxic in “bulk” cancer cells and CSCs [26].…”

“…Interestingly, a series of TPP compounds we tested were non-toxic in normal fibroblasts, but were selectively toxic in “bulk” cancer cells and CSCs [26]. Therefore, these TPP compounds appear to be able to metabolically distinguish between “normal cell” mitochondria and “malignant” mitochondria [26]. Since “bulk” cancer cells and CSCs likely have mitochondria with higher membrane potentials, this could explain the selective targeting of cancer cell mitochondria with TPP compounds.…”

Novel mitochondrion‐targeting copper(II) complex induces HK2 malfunction and inhibits glycolysis via Drp1‐mediating mitophagy in HCC

“…In this study, we cited a new copper complex, [Cu(ttpy‐tpp)Br 2 ]Br (expressed as CTB, shown in Figure A), which is obtained by introducing tri‐phenyl‐phosphine (TPP) into copper‐terpyridine complex. TPP is a non‐toxic chemical group that exhibits mitochondrial targeting properties in living cells . TPP can impart delocalized charge and lipophilic properties to the compound, which facilitates mitochondrial accumulation .…”