Adipocyte vesicles: ‘all-in-one’ packages that stimulate tumor mitochondrial metabolism and dynamics

 

Background

Obesity is a rising global public health issue, with well-known associations to health problems like cardiovascular diseases and diabetes. However, evidence continues to emerge that obesity is a risk factor for pathologies that you might not expect, such as melanoma, the deadliest form of skin cancer.

Invasive melanomas are often found in close proximity to adipose tissue under the skin. Adipocytes have been shown to respond to tumour signals by secreting fatty acids (FAs), which are used by tumour cells as a source of energy. Previous work from the authors of this preprint showed that this process involves the secretion of extracellular vesicles (EVs) by adipocytes, which carry nucleic acids, proteins and lipids to melanoma cells to promote their migration. These EVs are more numerous, and more effective in promoting melanoma cell migration in obese patients⁽¹⁾. However, the precise mechanisms of how EV cargo components reprogram metabolism to increase motility, the stage of melanoma when this might start, and how this is all enhanced in obesity remains unclear. In this preprint, Clement, Lazar and colleagues followed up their previous research to attempt to answer these questions.

 

Key findings

To confirm a direct link between adipocyte-derived EVs and increased fatty acid oxidation (FAO) in melanoma cells, the authors established a technique using SILAC (Stable Isotope Labelling of Amino Acids in Cell culture). Heavy amino acids were added to 3T3-F442A preadipocytes during differentiation. EVs produced by these adipocytes were isolated and added to melanoma cell culture. Subsequent mass-spec analysis showed 30% of EV proteins were effectively transferred to melanoma cells, and these were predominantly enzymes involved in mitochondrial FAO, FA storage/transport and oxidative phosphorylation.

How might obesity change the contents and behaviour of EVs? Interestingly, the team found that FAO in melanoma cells was increased after exposure to EVs derived from obese patients compared to their leaner counterparts. They first hypothesised that obese adipocyte EVs might carry more FAO enzyme machinery than leaner controls, but proteomic analysis of EVs from lean and obese mice showed no changes in FAO-associated enzyme abundance.

Therefore, the authors next investigated whether FAs – the substrate of FAO enzymes – were especially increased in obese adipocyte EVs. They first cultured adipocytes with BODIPY FL C₁₆ to fluorescently label FAs, isolated EVs and added them to melanoma cell culture as before. They confirmed that fluorescent FAs were successfully transferred into melanoma cell lines. EVs secreted from obese mice contained higher levels of lipids compared to lean mice, and melanoma cells cultured with EVs isolated from adipocytes of mice fed a high fat diet had a greater lipid content than those fed a normal diet. Therefore, the authors concluded that extra FAs, delivered by EVs from obese adipose tissue contribute towards the changes in melanoma cell metabolism.

Next the team moved on to investigating how this uptick in FAO after exposure to obese EVs translates into increased melanoma cell migration. It is already known that increases in tumour lipids and FAO can initiate redistribution of mitochondria, where FAO takes place, towards cell protrusions in order to fuel actin polymerisation⁽²⁾. Here, the authors found that increased FAO after EV treatment caused melanoma cells to elongate, and mitochondria redistribute from perinuclear to more cytoplasmic locations. Interestingly, after treatment with obese-derived EVs, this movement to cell extremities was enhanced for both mitochondria, as well as lipids.

In the final part of the manuscript, the authors wanted to see whether their in vitro work could be clinically relevant in melanoma patients. Unfortunately, body weight is not listed with melanoma tumour data in the TCGA database, however genes associated with FAO enzymes as well as mitochondrial fission/fusion genes were found to correlate with poor overall survival. Moreover, the FAO and lipid content of several melanoma cell lines correlated with their metastatic potential.

To summarise, Clement, Lazar and colleagues show that adipocytes can influence metabolism and migration of melanoma cells via EVs, without prior signalling from the tumour. In obese patients, more FAs within EVs are transferred to melanoma cells, which further increases FAO in mitochondria. This increased mitochondrial activity leads to their redistribution to cell extremities to provide a ready source of energy for invasive behaviours.

 

Why I chose this preprint

In attending conferences and keeping up to date with melanoma literature, I’ve been seeing more and more research linking obesity to melanoma, which to me seemed quite far out of the left field at first! However, this preprint offers a mechanistic insight for how adipose and melanoma could cooperate and thereby helps highlight how far-reaching the effects of obesity might be. This also opens up further questions of how we need to think more about how tumour microenvironments might differ from patient to patient, and the best therapeutic strategies to use to target these processes in melanoma.

 

Questions for the authors

1. You show that mitochondria are redistributed towards the extremities of melanoma cells after EV treatment, dependent on mitochondrial fission. In HFD EV-treated melanoma cells, do you see noticeably greater numbers of mitochondria compared NFD-treated?
2. What do you think happens at the molecular level between increases in FAO and changes to mitochondrial location and activity? Do you think FAO metabolites cause changes at the transcriptional or epigenetic level, or is there some sort of signalling event that encourages mitochondria and lipid droplets to move to the edges of tumour cells?
3. Do you have any plans to investigate if melanoma is influenced by obesity in in vivo animal models? It would be interesting to see how far away (and at what stage of melanoma) adipocyte EVs can travel to significantly change tumour metabolism.
4. Although this is different to obesity, female animals are more likely to have higher proportions of body fat than males. Do you see any gender-specific effects of melanoma outcome in your TCGA analyses, or do you know if there are differences in outcomes in female animal models of melanoma?

 

Further reading

1. Lazar I, et al (2016) Adipocyte Exosomes Promote Melanoma Aggressiveness through Fatty Acid Oxidation: A Novel Mechanism Linking Obesity and Cancer. Cancer research 76: 4051-4057
2. Altieri DC (2017) Mitochondria on the move: emerging paradigms of organelle trafficking in tumour plasticity and metastasis. British journal of cancer 117: 301-305

 

 

Tags: fat, metabolism, skin cancer

doi: https://doi.org/10.1242/prelights.11360

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