Mammary tumorigenesis in feral Mus cervicolor popaeus.

A pedigreed breeding population of feral Mus cervicolor popaeus with a high incidence of mammary tumors, arising between 6 and 14 months of age, is described. These mice were chronically infected with a type B retrovirus which is distantly related to the mouse mammary tumor virus (MMTV) of inbred strains of Mus musculus. MMTV-induced mammary tumors in inbred mice frequently (80%) contained an insertion of the viral genome into the int-1 or int-2 loci of the tumor cellular genome. These two cellular genetic loci were also altered by viral insertion in 11 of 20 M. cervicolor popaeus mammary tumor cellular DNAs tested. Results of our study of mammary tumorigenesis in feral mice demonstrate that viral-induced rearrangement and activation of the int loci are not limited to the genetic background of inbred mice selected for highly infectious MMTV and a high incidence of mammary tumors.     

Overexpression of cyclooxygenase-2 is sufficient to induce tumorigenesis in transgenic mice

The cyclooxygenase (COX)-2 gene encodes an inducible prostaglandin synthase enzyme that is overexpressed in adenocarcinomas and other tumors. Deletion of the murine Cox-2 gene in Min mice reduced the incidence of intestinal tumors, suggesting that it is required for tumorigenesis. However, it is not known if overexpression of Cox-2 is sufficient to induce tumorigenic transformation. We have derived transgenic mice that overexpress the human COX-2 gene in the mammary glands using the murine mammary tumor virus promoter. The human Cox-2 mRNA and protein are expressed in mammary glands of female transgenic mice and were strongly induced during pregnancy and lactation. Female virgin Cox-2 transgenic mice showed precocious lobuloalveolar differentiation and enhanced expression of the beta-casein gene, which was inhibited by the Cox inhibitor indomethacin. Mammary gland involution was delayed in Cox-2 transgenic mice with a decrease in apoptotic index of mammary epithelial cells. Multiparous but not virgin females exhibited a greatly exaggerated incidence of focal mammary gland hyperplasia, dysplasia, and transformation into metastatic tumors. Cox-2-induced tumor tissue expressed reduced levels of the proapoptotic proteins Bax and Bcl-x(L) and an increase in the anti-apoptotic protein Bcl-2, suggesting that decreased apoptosis of mammary epithelial cells contributes to tumorigenesis. These data indicate that enhanced Cox-2 expression is sufficient to induce mammary gland tumorigenesis. Therefore, inhibition of Cox-2 may represent a mechanism-based chemopreventive approach for carcinogenesis.     

HER-2/neu x aromatase double transgenic mice model: the effects of aromatase overexpression on mammary tumorigenesis

A majority of breast cancers are hormone-responsive, and require estrogen for growth, and respond to hormonal therapy that blocks estrogen receptor action. Breast tumors with low levels of or completely lacking estrogen receptor fail to respond to antiestrogen therapy yet require estrogen for tumor initiation. To address the importance of local estrogen in oncogene-mediated breast tumorigenesis, we have crossed MMTV-aromatase with MMTV-HER2/neu and examined the incidence of breast cancer in double transgenic mice in comparison with parental strains. Double transgenic mice show normal mammary development and express both transgenes at similar levels to that of parental strains. Tumor incidence in double transgenic mice (<5%) decreased compared to HER2/neu mice (>65%). In addition to a significant decrease in tumorigenesis, these mice expressed ER as well as high levels of ERβ along with decreased levels of cyclin D1 and phosphorylated pRb among other changes. Furthermore, experiments using THC (ER-agonist and ERβ-antagonist) clearly demonstrate the critical role of ERβ in HER2/neu-mediated tumorigenesis. These studies provide the first genetic evidence that estrogen receptor, mainly ERβ than ER and its dependent changes play an important role in regulating mammary tumorigenesis. These findings provide further evidence for development and testing of novel therapeutic approaches based on selective regulation of estrogen receptors (ER and β)-dependent actions for the treatment and prevention of breast cancers.     

Inappropriate P-cadherin expression in the mouse mammary epithelium is compatible with normal mammary gland function

Cadherins comprise a family of cell-cell adhesion proteins critical to the architecture and function of tissues. Expression of family members E-, N-, and P-cadherin is regulated in a spatial and temporal fashion in the developing and adult organism. Using in vivo and in vitro experimental systems, perturbation of cadherin expression by genetic deletion, overexpression, mutant dominant-negative constructs, and, to a lesser degree, expression of an inappropriate cadherin have all been shown to alter embryogenesis, tissue architecture, and cell behavior. Here we studied how expression of an inappropriate cadherin affects the adult mouse mammary gland. Human P-cadherin was expressed in mammary epithelial cells under control of the mouse mammary tumor virus (MMTV) promoter, and the effect on mammary gland behavior was studied. Typically, E-cadherin is expressed by mammary epithelial cells, whereas P-cadherin is found in myoepithelial cells and cap cells of the ductal terminal end bud. However, breast cancers frequently express P-cadherin, even though they are thought to arise from epithelial cells, and it is a marker of poor prognosis. We developed two independent transgenic mouse lines that exhibited high levels of P-cadherin protein expression in the mammary epithelium. P-cadherin was detected in most, but not all, luminal epithelial cells, and was appropriately localized to cell-cell borders. It was detected in the mammary glands of virgin, pregnant, lactating, post-lactation, and aged parous female mice. Despite the robust and widespread expression of an inappropriate cadherin, no effect was observed on mammary gland morphogenesis, architecture, lactation, or involution in transgenic mice compared to wild-type mice. No mammary tumors formed spontaneously in either wild-type or transgenic mice. Moreover, mammary tumors induced by the neu oncogene, which was introduced by a breeding strategy, showed no differences between mice with or without hP-cadherin. Surprisingly, however, none of the tumors expressed hP-cadherin protein. Together, our studies show no apparent effect on adult mammary gland or tumor behavior by inappropriate expression of P-cadherin in normal mammary epithelial cells.     

Comparison of background factors relevant to mammary tumorigenesis between DDD/1-Mtv-2/Mtv-2 congenic and DDD/1 mice

To elucidate the possible participation of the Mtv-2 gene in the background factors relevant to mammary tumorigenesis, DDD/1-Miv-2/Mtv-2 (DDD-Mtv-2) congenic and DDD/1 background mice were compared for endocrine and immune organs, mammary gland development, expression of mouse mammary tumor virus (MMTV)-gp 52 antigen, hyperplastic alveolar nodules (HAN) and mammary tumor incidence. The congenic strain had been established by introducing Mtv-2 from GRS/AJms (GR) into DDD/1 mice by repeated 12 backcrosses. Body, thymus, spleen, uterus, ovary, adrenal and pituitary weights, histology of the ovary and mammary gland development showed no differences ascribable to Mtv-2 between both strains of mice at 4,6 and 12 months of age. In contrast, MMTV-gp 52 antigen expression, HAN and mammary tumor occurred in DDD-Mtv-2 but not in DDD/1 mice. These results showed that the Mtv-2 gene stimulates mammary tumorigenesis by constitutional production of endogenous MMTV which transforms mammary epithelial cells but not by influencing the background factors relevant to mammary tumorigenesis. Furthermore, the expression of Mtv-2 appeared to be lower on the DDD/1 than on the GR background.     

Coexpression of MMTV/v-Ha-ras and MMTV/c-myc genes in transgenic mice: synergistic action of oncogenes in vivo

We have derived and mated separate strains of transgenic mice that carry either the v-Ha-ras or the c-myc gene driven by the mouse mammary tumor virus (MMTV) promoter/enhancer. Mice carrying the MMTV/v-Ha-ras transgene manifest two distinct disturbances of cell growth. The first, a benign hyperplasia of the Harderian lacrimal gland, is diffuse, involves the entire gland, and likely requires only the abnormal action of the v-Ha-ras gene. The second involves the focal development of malignancies of mammary, salivary, and lymphoid tissue and likely requires additional somatic events. When the MMTV/v-Ha-ras and MMTV/c-myc strains are crossed to yield hybrid mice, their joint action results in a dramatic and synergistic acceleration of tumor formation. Since these tumors arise stochastically and are apparently monoclonal in origin, additional somatic events appear necessary for their full malignant progression, even in the presence of activated v-Ha-ras and c-myc transgenes.     

Nonuniform expression of a mouse mammary tumor virus-driven int-2/Fgf-3 transgene in pregnancy-responsive breast tumors.

We have developed transgenic mice in which expression of the mouse int-2/Fgf-3 gene is regulated by a single long terminal repeat from mouse mammary tumor virus. Such mice contain and transmit a replica of the activated int-2/Fgf-3 allele present in a spontaneous mammary tumor from a BR6 mouse. Although free of infectious mouse mammary tumor virus and with a different genetic background, the transgenic mice develop pregnancy-responsive mammary epithelial proliferations that are similar to the early stages of tumorigenesis in the BR6 strain. Histological examination revealed that most of these tumors showed pronounced tubular and acinar structures, features usually associated with morphological differentiation. In some cases, the tumors were locally invasive, causing disruption of the dermis which manifested itself as local hair loss. In situ hybridization showed that patterns of transgene expression in the abnormal glands were markedly nonuniform. In contrast, mouse mammary tumor virus-induced neoplasms showed more uniform expression of int-2/Fgf-3, as did the urogenital epithelial proliferations that occur among males of this transgenic line. These data suggest that mammary tumors in virally infected animals may depend primarily on autocrine stimulation by the int-2/Fgf-3 gene product, whereas both autocrine and paracrine mechanisms may contribute to tumors and hyperplasias found in transgenic animals.     

Establishment of La-tPA/G-CSF dual transgenic mice and expression in their mammary gland

Expression vectors of human granulocyte colony stimulating factor (G-CSG) and long acting tissue plasminogen activator (La-tPA) in mammary gland were constructed using promoters of mouse whey acid protein gene (WAP) and sheep β-lactoglobulin gene (BLG) with sizes of 2.6 and 5 kb respectively. Two kinds of transgenic mice of G-CSF and La-tPA were produced with microinjection. The expression of G-CSF and La-tPA was achieved in mammary glands of transgenic mice, respectively. In order to establish dual transgenic mice of La-tPA/G-CSF, transgenic mice carrying G-CSF and La-tPA gene characterized with specific expression in mammary gland were mated. La-tPA/G-CSF dual transgenic mice were screened out from the hybrid offspring by Once-PCR. The co-expression of La-tPA and G-CSF in mammary gland of the dual transgenic mice was confirmed by the milk assayed and Northern blot analysis. Some parameters about the dual transgenic mice indicated that there were fewer litters than that of normal mice. The ratio of du     

Mouse mammary tumor virus c-rel transgenic mice develop mammary tumors

Amplification, overexpression, or rearrangement of the c-rel gene, encoding the c-Rel NF-kappaB subunit, has been reported in solid and hematopoietic malignancies. For example, many primary human breast cancer tissue samples express high levels of nuclear c-Rel. While the Rev-T oncogene v-rel causes tumors in birds, the ability of c-Rel to transform in vivo has not been demonstrated. To directly test the role of c-Rel in breast tumorigenesis, mice were generated in which overexpression of mouse c-rel cDNA was driven by the hormone-responsive mouse mammary tumor virus long terminal repeat (MMTV-LTR) promoter, and four founder lines identified. In the first cycle of pregnancy, the expression of transgenic c-rel mRNA was observed, and levels of c-Rel protein were increased in the mammary gland. Importantly, 31.6% of mice developed one or more mammary tumors at an average age of 19.9 months. Mammary tumors were of diverse histology and expressed increased levels of nuclear NF-kappaB. Analysis of the composition of NF-kappaB complexes in the tumors revealed aberrant nuclear expression of multiple subunits, including c-Rel, p50, p52, RelA, RelB, and the Bcl-3 protein, as observed previously in human primary breast cancers. Expression of the cancer-related NF-kappaB target genes cyclin D1, c-myc, and bcl-xl was significantly increased in grossly normal transgenic mammary glands starting the first cycle of pregnancy and increased further in mammary carcinomas compared to mammary glands from wild-type mice or virgin transgenic mice. In transient transfection analysis in untransformed breast epithelial cells, c-Rel-p52 or -p50 heterodimers either potently or modestly induced cyclin D1 promoter activity, respectively. Lastly, stable overexpression of c-Rel resulted in increased cyclin D1 and NF-kappaB p52 and p50 subunit protein levels. These results indicate for the first time that dysregulated expression of c-Rel, as observed in breast cancers, is capable of contributing to mammary tumorigenesis.     

neu/ERBB2 cooperates with p53-172H during mammary tumorigenesis in transgenic mice.

Thirty percent of human breast cancers have amplification of ERBB2, often in conjunction with mutations in p53. The most common p53 mutation in human breast cancers is an Arg-to-His mutation at codon 175, an allele that functions in a dominant oncogenic manner in tumorigenesis assays and is thus distinct from loss of p53. Transgenic mice expressing mouse mammary tumor virus-driven neu transgene (MMTV-neu) develop clonal mammary tumors with a latency of 234 days, suggesting that other events are necessary for tumor development. We have examined the role of mutations in p53 in tumor development in these mice. We have found that 37% of tumors arising in these mice have a missense mutations in p53. We have directly tested for cooperativity between neu and mutant p53 in mammary tumorigenesis by creating bitransgenic mice carrying MMTV-neu and 172Arg-to-His p53 mutant (p53-172H). In these bitransgenic mice, tumor latency is shortened to 154 days, indicating strong cooperativity. None of the nontransgenic mice or the p53-172H transgenic mice developed tumors within this time period. Tumors arising in the p53-172H/neu bitransgenic mice were anaplastic and aneuploid and exhibited increased apoptosis, in distinction to tumors arising in p53-null mice, in which apoptosis is diminished. Further experiments address potential mechanisms of cooperativity between the two transgenes. In these bitransgenic mice, we have recapitulated two common genetic lesions that occur in human breast cancer and have shown that p53 mutation is an important cooperating event in neu-mediated oncogenesis.     

Haploid loss of bax leads to accelerated mammary tumor development in C3(1)/SV40-TAg transgenic mice: reduction in protective apoptotic response at the preneoplastic stage.

The dramatic increase in apoptosis observed during the development of preneoplastic mammary lesions is associated with a significant elevation in Bax expression in C3(1)/SV40 large T antigen (TAg) transgenic mice. The significance of Bax expression during tumor progression in vivo was studied by generating double-transgenic mice carrying the C3(1)/TAg transgene and mutant alleles for bax. C3(1)/TAg transgenic mice carrying mutant bax alleles exhibited accelerated rates of tumor growth, increased tumor numbers, larger tumor mass and decreased survival rates compared with mice carrying wild-type bax. Accelerated tumorigenesis associated with the bax+/- genotype did not require the loss of function of the second bax allele. Thus, haploid insufficiency of bax is enough to accelerate tumor progression, suggesting that the protective effect of Bax is dose-dependent. While levels of apoptosis in the preneoplastic lesions, but not carcinomas, were reduced in bax+/- or bax-/- mice compared with bax+/+ mice, rates of cellular proliferation in mammary lesions were similar among all bax genotypes. These data demonstrate that bax is a critical suppressor of mammary tumor progression at the stage of preneoplastic mammary lesion development through the upregulation of apoptosis, but that this protective effect is lost during the transition from preneoplasia to invasive carcinoma.     

Targeted c-myc gene expression in mammary glands of transgenic mice induces mammary tumours with constitutive milk protein gene transcription

We investigated the consequences of augmented c-myc gene expression in the mammary gland of transgenic mice. For this purpose we directed the expression of a mouse c-myc transgene to the differentiating mammary epithelial cells by subjecting the protein coding region to the 5' regulatory sequences of the murine whey acidic protein gene (Wap). Analogous to the expression pattern of the endogenous Wap gene, the Wap-myc transgene is abundantly expressed in the mammary gland during lactation. The tissue-specific and hormone-dependent expression of the Wap-myc transgene results in an 80% incidence of mammary adenocarcinomas. As early as two months after the onset of Wap-myc expression, tumours occur in the mammary glands of the transgenic animals. The tumours express not only the Wap-myc transgene, but also the endogenous Wap and beta casein genes. The expression of the milk protein genes becomes independent of the lactogenic hormonal stimuli and persists even in transplanted nude mouse tumours.     

Rescue of an MMTV transgene by co-integration reveals novel locus control properties of the ovine β-lactoglobulin gene that confer locus commitment to heterogeneous tissues

In an attempt to enhance the frequency and level of expression of a poor-performing MMTV-driven transgene, we co-integrated this construct with the ovine -lactoglobulin (BLG) gene in transgenic mice. Seven lines of transgenic mice possessing co-integrated BLG and MMTV-RZ5 transgenes were compared with 12 lines of mice that possessed only the MMTV-RZ5 construct. Co-integration enhanced the frequency of expression in the mammary gland from two out of 12 lines for the MMTV-RZ5 transgene alone, to five out of seven when co-integrated with BLG. Surprisingly, co-integration also resulted in co-expression of the two transgenes in the salivary gland, lung and spleen in addition to the mammary gland. Furthermore, both transgenes were expressed in virgin animals, and throughout pregnancy and lactation, suggesting that the developmental regulation of the locus follows that of the MMTV-promoter. These findings represent a novel locus control property of the ovine BLG gene that confer s commitment of the locus to the mammary gland, but also to a range of heterogeneous tissues possibly defined by the second promoter at the locus     

Tet-on诱导表达c-myc和SV40Tag的转基因小鼠肿瘤模型

目的研究Tet-on诱导表达c-myc和SV40Tag小鼠肿瘤模型的肿瘤发生和基因表达情况,探讨c-myc基因的作用。方法用pTRE2-c-myc单阳性转基因小鼠和Tet-on、pTRE2-SV40Tag双阳性转基因小鼠交配,后代检测得到Tet-onp、TRE2-SV40Tag、pTRE2-c-myc三阳性转基因小鼠,经强力霉素诱导一段时间以后,观察肿瘤的发生;通过RT-PCR、病理组织切片和磁共振等方法对肿瘤的发生部位和时相进行研究。结果Tet-on、pTRE2-SV40Tag、pTRE2-c-myc三阳性转基因小鼠①经诱导后发生肿瘤,且发瘤率和发瘤时间高于和短于Tet-on、pTRE2-SV40Tag双阳性转基因小鼠;②c-myc和SV40Tag基因在表达部位上有所不同。结论c-myc和SV40Tag基因同时表达与SV40Tag基因单独表达时相比,肿瘤发生明显增强,提示c-myc基因与肿瘤的发生有着密切关系。     

Effect of N-cadherin misexpression by the mammary epithelium in mice

N-cadherin is not typically expressed by epithelial cells. However, it is detected in breast cancers and increases tumor cell migration and invasion in vitro. To explore its misexpression, we generated transgenic mice with N-cadherin in the mammary epithelium. Mammary glands appeared normal and no tumors arose spontaneously. To investigate N-cadherin misexpression in mammary tumors, neu was overexpressed through breeding. Tumors developed in +/neu and N-cadherin/neu mice, although few tumors in bitransgenic mice expressed N-cadherin, and they did not differ from N-cadherin-negative tumors.     

Sustained mammary gland-directed, ponasterone A-inducible expression in transgenic mice.

The ability to regulate temporal- and spatial-specific expression of target genes in transgenic mice will facilitate analysis of gene function and enable the generation of murine models of human diseases. The genetic analysis of mammary gland tumorigenesis requires the development of mammary gland-specific transgenics, which are tightly regulated throughout the adult mammary epithelium. Analysis of genes implicated in mammary gland tumorigenesis has been hampered by mosaic transgene expression and the findings that homozygous deletion of several candidate genes (cyclin D1, Stat5A, prolactin receptor) abrogates normal mammary gland development. We describe the development of transgenic mouse lines in which sustained transgene expression was inducibly regulated, both specifically and homogeneously, in the adult mammary gland epithelium. Transgenes encoding RXRalpha and a chimeric ecdysone receptor under control of a modified MMTV-LTR, which targets mammary gland expression, were used. These transgenic 'receptor' lines were crossed with transgenic 'enhancer' lines in which the ecdysone/RXR binding site induced ligand-dependent expression of transgenic beta-galactosidase. Pharmacokinetic analysis of a highly bioactive ligand (ponasterone A), identified through screening ecdysteroids from local plants, demonstrated sustained release and transgene expression in vivo. This transgenic model with both tightly regulated and homogeneous transgene expression, which was sustained in vivo using ligands readily extracted from local flora, has broad practical applicability for genetic analysis of mammary gland disease.     

Prevention of spontaneous breast carcinoma by prophylactic vaccination with dendritic/tumor fusion cells

Genetically modified mice with spontaneous development of mammary carcinoma provide a powerful tool to study the efficacy of tumor vaccines, since they mimic breast cancer development in humans. We used a transgenic murine model expressing polyomavirus middle T oncogene and mucin 1 tumor-associated Ag to determine the preventive effect of a dendritic/tumor fusion cell vaccine. The MMT (a transgenic murine model) mice developed mammary carcinoma between the ages of 65-108 days with 100% penetrance. No spontaneous CTL were detected. However, prophylactic vaccination of MMT mice with dendritic/tumor fusion cells induced polyclonal CTL activity against spontaneous mammary carcinoma cells and rendered 57-61% of the mice free of the disease at the end of experiment (180 days). Furthermore, the level of CTL activity was maintained with multiple vaccinations. The antitumor immunity induced by vaccination with dendritic/tumor fusion cells reacted differently to injected tumor cells and autochthonous tumor. Whereas the injected tumor cells were rejected, the autochthonous tumor evaded the attack and was allowed to grow. Collectively these results indicate that prophylactic vaccination with dendritic/tumor fusion cells confers sufficient antitumor immunity to counter the tumorigenesis of potent oncogenic products. The findings in the present study are highly relevant to cancers in humans.     

Early role of CD4+ Th1 cells and antibodies in HER-2 adenovirus vaccine protection against autochthonous mammary carcinomas

HER-2 is an oncogenic tumor-associated Ag that is overexpressed in several human tumors including breast and ovarian cancer. The efficacy and mechanism of a HER-2-expressing recombinant adenoviral vaccine to protect against tumorigenesis was examined using HER-2 transgenic (BALB-neuT) mice, which develop spontaneous breast tumors in all 10 mammary glands, and also using a transplantable mouse tumor model. Vaccination beginning at 6-8 wk of age (through 19 wk of age) prevented development of spontaneous mammary tumors even after 50 wk, whereas the animals in the control groups had tumors in all mammary glands by 25 wk. Such long-term protection after the last boost has not been achieved previously in this transgenic mouse in which the oncogene is continuously spawning tumorigenesis. Using beta(2)-microglobulin-knockout, IFN-gamma-knockout, and B cell-deficient mice, CD4(+) and CD8(+) cell depletion, and Ab transfer studies, we show that induction of anti-HER-2/neu Abs are both necessary and sufficient for protection, and the IgG2a isotype is most effective. In contrast, CD8(+) T cells are not necessary at all, and CD4(+) T cells are necessary for only 36-48 h after immunization to provide help for B cells but not as effector cells. Equal protection in immunized mice deficient in FcgammaRI/III excluded an FcR-mediated mechanism. Anti-HER-2 serum not only inhibited growth of mammary tumor cell lines expressing HER-2 in vitro but also protected mice from tumors in vivo, suggesting a direct action of Ab on the tumor cells. Such a vaccine may provide Ab-mediated protection against HER-2-expressing breast cancers in humans.     

Requirement for the simian virus 40 small tumor antigen in tumorigenesis in transgenic mice.

To examine the role of simian virus 40 (SV40) large T and small t antigens in tumorigenesis in animals, we generated transgenic mice which expressed either both the SV40 large T and small t antigens or the SV40 large T antigen alone under the control of the mouse mammary tumor virus long terminal repeat. The mouse mammary tumor virus long terminal repeat directs the expression of transgenes in ductal epithelial cells of several organs, including the mammary gland, lung, and kidney, and in lymphoid cells. The mice which expressed both the T and t tumor antigens developed lung and kidney adenocarcinomas, while those which expressed large T alone did not. Both types of mice developed malignant lymphomas with similar frequencies and latency periods. Our results show that the SV40 small t antigen cooperates with the large T antigen in inducing tumors in slowly dividing epithelial cells in the lung and kidney.     

Influence of fatty acid‐free diet on mammary tumor development and growth rate in HER‐2/neu transgenic mice

Numerous investigations have found a relationship between higher risk of cancer and increased intake of fats, while results of clinical studies of fat reduction and breast cancer recurrence have been mixed. A diet completely free of fats cannot be easily administered to humans, but experimental studies in mice can be done to determine whether this extreme condition influences tumor development. Here, we examined the effects of a FA‐free diet on mammary tumor development and growth rate in female FVB‐neu proto‐oncogene transgenic mice that develop spontaneous multifocal mammary tumors after a long latency period. Mice were fed a fatty acid‐free diet beginning at 112, 35, and 30 days of age. In all these experiments, tumor appearance was delayed, tumor incidence was reduced and the mean number of palpable mammary tumors per mouse was lower, as compared to standard diet‐fed mice. By contrast, tumor growth rate was unaffected in mice fed the fatty acid‐free diet. Plasma of mice fed the fatty acid‐free diet revealed significantly higher contents of oleic, palmitoleic and 20:3ω9 acids and lower contents of linoleic and palmitic acids. In conclusion, these findings indicate that a FA‐free diet reduces tumor incidence and latency but not tumor growth rate, suggesting that a reduction in dietary FAs in humans may have a protective effect on tumorigenesis but not on tumors once they appear. J. Cell. Physiol. 228: 242–249, 2013. © 2012 Wiley Periodicals, Inc.