however, isg expression was not upregulated in the mitocepted fibroblasts, demonstrating that they are not notably affected by cancer cell mitochondria. by contrast, several classical caf markers were upregulated in the mitotracker - high population and in mitocepted fibroblasts, suggesting that coculture with cancer cells alters the expression of different sets of genes in fibroblasts through distinc t mechanisms. in a search for the mechanistic underpinning of the mitochondrial transfer, we identified miro2. this rho guanosine triphosphatase links mitochondria to the cellular trafficking machinery and is responsible for intracellular mitochondrial positioning. it also has important cell - autonomous functions that are important for cancer cell proliferation and migration, as also shown in this study. these features and its overexpression in invasive cancer cells at the edge of different tumors make it an interesting target for cancer treatment. this is not restricted to skin cancer because we also observed mitochondrial transfer from vulvar, breast and pancreatic cancer cells to fibroblasts, which promoted a caf phenotype. given the high expression of miro2 in metastatic prostate cancer, it will be of interest to study the role of intercellular mitochondrial transfer in metastasis. in conclusion, we show that cancer cells transfer their mitochondria to fibroblasts and thereby reprogram them into protumorigenic cafs. we also discovered the mechanism underlying this transfer and identified miro2 as a potential target for cancer treatment. these findings offer promising therapeutic opportunities for skin cancer and also for malignancies with high mortality, such as pancreatic cancer, which has a large stromal component. lastly, the data obtained in this study suggest that mitochondrial transfer from epithelial to stromal cells is an important mechanism of cell – cell communication, which may also be relevant for development, homeostasis and tissue repair and for the pathogenesis of nonmalignant diseases. the work performed in this study complies with all relevant ethical regulations. mouse maintenance and experimentation were approved by the veterinary authorities of zurich ( kantonales veterinaramt zurich, 32060, 35555, 36338 and 33866 ). human skin and tumor samples, which were used for the isolation of primary cells, were obtained anonymously from the department of dermatology, university hospital of zurich ( in the context of the biobank project ). written informed consent for use in research was obtained from all donors ( in case of foreskin from the parents ). all experiments with human samples were approved by the local and cantonal research ethics committees ( kantonale ethikkommission zurich, basec no. 2017 - 00684 ), adhering to the declaration of helsinki principles. nod scid ( nod. cb17 - prkdc / ncrcrl ) mice were bred in the eth zurich epic facility and kept under specific pathogen - free conditions in a 12 h dark – light cycle at 21 – 23 °c and 40 – 60 % humidity. they received food and water ad libitum. hacat, hacat - ras cells and scc13 cells were provided by p. boukamp. a431 cells were from merck ( 85090402 ). lifeact cell lines were generated by transduction with the lentiviral vector rlv - ubi - lifeact – rfp - tag ( vitaris ). hacat cells are spontaneously immortalized but nontumorigenic human keratinocytes. hacat - ras cells were obtained by transfection of hacat cells with a c - ha – ras oncogene. scc13 cells were derived from a human cutaneous scc. the metastatic mda - mb - 231 tripl e - negative breast cancer cell line and the metastatic panc1 pancreatic cancer cell line were obtained from the american type culture collection ( htb - 26, crl - 1469 ). lm2 cells, a lung metastatic variant of mda - mb - 231, were kindly provided by j. massague. cell lines expressing fluorescent mitochondrial proteins were generated by lentiviral transduction. primary fibroblasts expressing tom20 – gfp were generated by lentiviral transduction with plenti - x1 - blast - gfp - tom20 - mts, provided by j. corn ( eth zurich ). immortalized mouse fibroblasts were isolated from pdgfrα – egfp transgenic mice and spontaneously immortalized by serial passaging. authentication of hacat, hacat - ras, scc13 and a431 cells was performed by microsynth using highly polymorphic short tandem repeat loci ( most recently in february 2025 ). absence of mycoplasma was confirmed monthly using the pcr mycoplasma test kit i / c ( promokine ) or the mycostrip kit ( invivogen ). cells were cultured in dmem supplemented with 10 % fbs and 1 % penicillin – streptomycin ( complete dmem ), unless indicated otherwise. hpks from skin of adult healthy volunteers or from the edges of skin sccs of adult participants ( diag nosed by an experienced dermatopathologist ) were from h. - d. beer ( university hospital zurich ). foreskin hpfs were obtained from foreskin of healthy boys. human skin and tumor samples, which were used for the isolation of primary cells, were obtained anonymously from the department of dermatology, university hospital of zurich ( in the context of the biobank project ). written informed consent for use in research was obtained from all donors ( in case of foreskin from the parents ). all experiments with human samples were approved by the local and cantonal research ethics committees ( kantonale ethikkommission zurich, basec no. 2017 - 00684 ), adhering to the declaration of helsinki principles. hpks were cultured in keratinocyte serum - free medium with epidermal growth factor and bovine pituitary extract ( all from thermo fisher scientific ). hpfs were cultured in complete dmem. cancer cells were transfected with sirnas ( microsynth ag ) using lipofect amine rnaimax ( thermo fisher scientific ) and incubated for 24 – 72 h or retransfected after 72 h and incubated for an additional 24 h. the following sirnas were used: connexin 26 sirna ( 72 h ): 5 ′ - cccaguuguuagauuaagatt - 3 ′ miro1 sirna ( 72 h + 24 h ): 5 ′ - uaaccaaaucgucgaagcacagucctt - 3 ′ miro2 sirna ( 24 h ): 5 ′ - gcguggaguguucggccaatt - 3 ′ sec3 sirna ( 72 h ): 5 ′ - agaugaauaccaagaguua - dtdt - 3 ′ sec5 sirna ( 72 h ): 5 ′ - gggugauuaugaugugguudtdt - 3 ′ trak1 sirna ( 72 h ): 5 ′ - ggaaacgaugagcggaguatt - 3 ′ trak2 sirna ( 72 h ): 5 ′ - ggauagauaugcacugaaatt - 3 ′ negative control sirna: 5 ′ - agguaguguaaucgccuug - 3 ′ first, 1 μg of prk5 - myc - miro2 expression vector ( addgene, 47891 ) or empty vector were used for transfection using lipofect amine 2000 ( 11668019, thermo fisher scientific ). dna, lipofect amine and opti - mem reduced - serum medium ( 31985062, thermo fisher scientific ) were incubated at room temperature for 20 min. cells were incubated with the mixture for 6 h, followed by incubation in complete dmem for 24 h. xenograft skin tumorigenesis assays were performed as described by intradermal injection of 2 × 10 cancer cells or 10 cancer cells together with 10 fibroblasts into the ear of male nod scid mice at the age of 8 – 12 weeks. breast and pancreatic cancer xenografts were established by orthotopic injection of lm2 breast cancer cells into the mammary fat pad ( glands 2 – 3 ) of adult female nod scid mice or by direct injection of panc1 cells into the pancreas of adult mice. the maximal tumor size permitted by the ethics committee ( 1 - cm diameter for skin cancer, 2. 8 - cm volume for breast cancer ) or the end point for wellbeing ( hunching, piloerection or decreased activity for pancreatic cancer ) was never reached in our experiments. spheroid assays were performed using the experimental parameters proposed by the mispheroid consortium. a total of 2, 000 cancer cells in 20 μl of cm from hpfs, which were cultured in complete dmem, were placed on the lids of 6 - cm culture plates using a hanging - drop method. then, 5 ml of pbs was added to the bottom. equal numbers of hacat or cancer cells and fibroblasts were seeded to reach 80 – 100 % confluency.