Abstract
BackgroundMitochondria are highly dynamic organelles which remain in a continuous state of fission/ fusion dynamics to meet the metabolic needs of a cell. However, this fission/fusion dynamism has been reported to be dysregulated in most cancers. Such enhanced mitochondrial fission is demonstrated to be positively regulated by some activating oncogenic mutations; such as those of KRAS (Kristen rat sarcoma viral oncogene homologue) or BRAF (B- rapidly accelerated fibrosarcoma), thereby increasing tumor progression/ chemotherapeutic resistance and metabolic deregulation. However, the underlying mechanism(s) are still not clear, thus highlighting the need to further explore possible mechanism(s) of intervention. We sought to investigate how BRAFV600E driven CRC (colorectal cancer) progression is linked to mitochondrial fission/fusion dynamics and whether this window could be exploited to target CRC progression.MethodsWestern blotting was employed to study the differences in expression levels of key proteins regulating mitochondrial dynamics, which was further confirmed by confocal microscopy imaging of mitochondria in endogenously expressing BRAFWT and BRAFV600E CRC cells. Proliferation assays, soft agar clonogenic assays, glucose uptake/lactate production, ATP/ NADPH measurement assays were employed to study the extent of carcinogenesis and metabolic reprograming in BRAFV600E CRC cells. Genetic knockdown (shRNA/ siRNA) and/or pharmacologic inhibition of Dynamin related protein1/Pyruvate dehydrogenase kinase1 (DRP1/PDK1) and/or BRAFV600E were employed to study the involvement and possible mechanism of these proteins in BRAFV600E driven CRC. Statistical analyses were carried out using Graph Pad Prism v 5.0, data was analyzed by unpaired t-test and two-way ANOVA with appropriate post hoc tests.ResultsOur results demonstrate that BRAFV600E CRC cells have higher protein levels of mitochondrial fission factor- DRP1/pDRP1S616 leading to a more fragmented mitochondrial state compared to those harboring BRAFWT. This fragmented mitochondrial state was found to confer glycolytic phenotype, clonogenic potential and metastatic advantage to cells harboring BRAFV600E. Interestingly, such fragmented mitochondrial state seemed positively regulated by mitochondrial PDK1 as observed through pharmacologic as well as genetic inhibition of PDK1.ConclusionIn conclusion, our data suggest that BRAFV600E driven colorectal cancers have fragmented mitochondria which confers glycolytic phenotype and growth advantage to these tumors, and such phenotype is dependent at least in part on PDK1- thus highlighting a potential therapeutic target.
Highlights
Unlike other cellular organelles, mitochondria continuously change their morphology through balanced fission/fusion events- a phenomenon referred to as mitochondrial dynamics [1, 2]
We evaluated as to what extent endogenous BRAFV600E CRC cells lead to a fragmented mitochondrial phenotype compared to BRAFWT
Since mitochondrial morphologies are regulated by various fission/fusion dynamins, mainly dynamin related protein 1 (DRP1)/mitofusins 1 & 2 (Mfn1 & 2) respectively, our interest was to investigate whether such variation in mitochondrial morphologies was related to pro-fission or fusion mediators alone or both
Summary
Mitochondria continuously change their morphology through balanced fission/fusion events- a phenomenon referred to as mitochondrial dynamics [1, 2] This mitochondrial dynamics is regulated by small GTPases—DRP1 regulating fission while as optic atrophy factor 1 (OPA1) regulating inner membrane fusion, and mitofusin 1 & 2 (Mfn1 & 2) controlling outer membrane fusion [8]. We sought to investigate how BRAFV600E driven CRC (colorectal cancer) progression is linked to mitochondrial fission/fusion dynamics and whether this window could be exploited to target CRC progression
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