Specific mtDNA mutations in mouse carcinoma cells suppress their tumor formation via activation of the host innate immune system.

In mammalian species, mitochondrial DNA (mtDNA) with pathogenic mutations that induce mitochondrial respiration defects has been proposed to be involved in tumor phenotypes via induction of enhanced glycolysis under normoxic conditions (the Warburg effects). However, because both nuclear DNA and mtDNA control mitochondrial respiratory function, it is difficult to exclude the possible contribution of nuclear DNA mutations to mitochondrial respiration defects and the resultant expression of tumor phenotypes. Therefore, it is important to generate transmitochondrial cybrids sharing the same nuclear DNA background but carrying mtDNA with and without the mutations by using intercellular mtDNA transfer technology. Our previous studies isolated transmitochondrial cybrids and showed that specific mtDNA mutations enhanced tumor progression as a consequence of overproduction of reactive oxygen species (ROS). This study assessed whether mtDNA mutations inducing ROS overproduction always enhance tumor progression. We introduced mtDNA from senescence-accelerated mice P1 (SAMP1) into C57BL/6J (B6) mice-derived Lewis lung carcinoma P29 cells, and isolated new transmitochondrial cybrids (P29mtSAMP1 cybrids) that overproduced ROS. The inoculation of the cybrids into B6 mice unexpectedly showed that mtDNA from SAMP1 mice conversely induced tumor suppression. Moreover, the tumor suppression of P29mtSAMP1 cybrids in B6 mice occurred as a consequence of innate immune responses of the host B6 mice. Enzyme pretreatment experiments of P29mtSAMP1 cybrids revealed that some peptides encoded by mtDNA and expressed on the cell surface of P29mtSAMP1 cybrids induce increased IL-6 production from innate immune cells (dendritic cells) of B6 mice, and mediate augmented inflammatory responses around the tumor-inoculated environment. These observations indicate presence of a novel role of mtDNA in tumor phenotype, and provide new insights into the fields of mitochondrial tumor biology and tumor immunology.

在哺乳动物物种中，具有致病性突变且会诱导线粒体呼吸缺陷的线粒体DNA（mtDNA）被认为可通过在常氧条件下诱导糖酵解增强（即瓦尔堡效应）而参与肿瘤表型的形成。然而，由于核DNA和mtDNA都控制线粒体呼吸功能，所以很难排除核DNA突变对线粒体呼吸缺陷以及由此产生的肿瘤表型表达的可能影响。因此，利用细胞间mtDNA转移技术产生具有相同核DNA背景但携带含突变和不含突变的mtDNA的转线粒体胞质杂种是很重要的。我们之前的研究分离出了转线粒体胞质杂种，并表明特定的mtDNA突变由于活性氧（ROS）的过量产生而促进了肿瘤进展。本研究评估了诱导ROS过量产生的mtDNA突变是否总是促进肿瘤进展。我们将来自快速衰老小鼠P1（SAMP1）的mtDNA引入源自C57BL/6J（B6）小鼠的Lewis肺癌P29细胞，并分离出了新的转线粒体胞质杂种（P29mtSAMP1胞质杂种），其会过量产生ROS。将这些胞质杂种接种到B6小鼠中，结果出乎意料地显示，来自SAMP1小鼠的mtDNA反而诱导了肿瘤抑制。此外，P29mtSAMP1胞质杂种在B6小鼠中的肿瘤抑制是由于宿主B6小鼠的先天免疫反应所致。对P29mtSAMP1胞质杂种进行的酶预处理实验表明，由mtDNA编码且在P29mtSAMP1胞质杂种细胞表面表达的一些肽会诱导B6小鼠的先天免疫细胞（树突状细胞）增加白细胞介素 - 6（IL - 6）的产生，并介导肿瘤接种环境周围炎症反应增强。这些观察结果表明mtDNA在肿瘤表型中具有一种新的作用，并为线粒体肿瘤生物学和肿瘤免疫学领域提供了新的见解。