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Previous studies have shown that stearate (C18:0), a dietary long-chain saturated fatty acid, inhibits breast cancer cell neoplastic progression; however, little is known about the mechanism modulating these processes. We demonstrate that stearate, at physiological concentrations, inhibits cell cycle progression in human breast cancer cells at both the G1 and G2 phases. Stearate also increases cell cycle inhibitor p21CIP1/WAF1 and p27KIP1 levels and concomitantly decreases cyclin-dependent kinase 2 (Cdk2) phosphorylation. Our data also show that stearate induces Ras– guanosine triphosphate formation and causes increased phosphorylation of extracellular signal-regulated kinase (pERK). The MEK1 inhibitor, PD98059, reversed stearate-induced p21CIP1/WAF1 upregulation, but only partially restored stearate-induced dephosphorylation of Cdk2. The Ras/mitogen-activated protein kinase/ERK pathway has been linked to cell cycle regulation but generally in a positive way. Interestingly, we found that stearate inhibits both Rho activation and expression in vitro. In addition, constitutively active RhoC reversed stearate-induced upregulation of p27KIP1, providing further evidence of Rho involvement. To test the effect of stearate in vivo, we used the N-Nitroso-N-methylurea rat breast cancer carcinogen model. We found that dietary stearate reduces the incidence of carcinogen-induced mammary cancer and reduces tumor burden. Importantly, mammary tumor cells from rats on a stearate diet had reduced expression of RhoA and B as well as total Rho compared with a low-fat diet. Overall, these data indicate that stearate inhibits breast cancer cell proliferation by inhibiting key check points in the cell cycle as well as Rho expression in vitro and in vivo and inhibits tumor burden and carcinogen-induced mammary cancer in vivo.
Stearate (C18:0), a long-chain saturated fatty acid, has been reported to inhibit human breast cancer cell proliferation in vitro (1,2) and in vivo (3). This effect contrasts increased cell proliferation observed in vitro with n-6 fatty acids such as linoleate and oleate (2,4). The molecular basis for the inhibition of breast cancer cell proliferation by stearate is not known.
The epidermal growth factor receptor (EGFR) is frequently upregulated in human cancers including those thought to arise from the colon, head and neck, breast, pancreas, lung, kidney, ovary, brain and urinary bladder (5). Overexpression of EGFR in breast cancers is associated with a more aggressive clinical course suggesting that it has an important growth regulatory function (6,7). The stimulation of EGFR with EGF regulates the proliferation, motility and differentiation of cells through activation of several intracellular signal transduction cascades, including the Ras/Erk and Rho/cyclin kinase inhibitor signaling pathways (8). The Ras superfamily of guanosine triphosphatases (GTPases) is a master regulator of many aspects of cell behavior. There are at least 60 small molecular weight, monomeric GTPases in mammalian cells and they have been generally divided into five groups Ras, Rho, RAb, Arf and Ran. They function as switches in signal transduction pathways that regulate such important functions as cell growth, differentiation and survival (9). In cancers with wild-type Ras, such as seen in most breast cancers, growth factor overexpression frequently leads to activation of the Ras/extracellular signal-regulated kinase (ERK) signaling pathway suggesting that Ras makes an important contribution to the development of these human cancers (10). In breast cancer, there is upregulated signaling through multiple pathways, and molecules implicated include growth factor receptors and other tyrosine kinases, Ras regulators commonly found to be overexpressed, the Ras protein itself, as well as downstream effectors (10). Members of both the Ras and Rho subfamilies are known to affect cell proliferation. Over the last decade, it has been generally accepted that Ras and Rho signaling pathways cross talk in such a way as to favor transformation and cell proliferation (11,12). The present studies support these data and further show that stearate induces breast cancer cell cycle inhibition largely in G1 as well as inhibiting carcinogen-induced mammary cancer and Rho both in vitro and in vivo.
Antibodies used and their sources were: Ras (clone RAS10 Mouse IgG2a) from Oncogen (Boston, MA), p27KIP1 (clone F-8 mouse IgG1), cyclin-dependent kinase 2 (Cdk2, rabbit polyclonal IgG) from Santa Cruz Biotechnology (Santa Cruz, CA), p21CIP1/WAF1 (clone SX118 mouse IgG1) from BD Biosciences PharMingen (San Diego, CA), phosphorylated Cdk2 [pCdk2(Thr160)] and phosphorylated p44/42 ERK [pERK1(Thr202)/pERK2(Tyr204)] from Cell Signaling (Beverly, MA). 2’-amino-3’-methoxyflavone (PD98059) and RNase inhibitor were purchased from Promega Corporation (Madison, MI). Stearic acid (stearate), diatomaceous earth, propidium iodide, RNase and protease inhibitor cocktail were obtained from Sigma–Aldrich Chemical Co. (St Louis, MO). Antirabbit or antimouse antibodies labeled with horseradish peroxidase and enhanced chemiluminescence reagents were from Amersham, Pharmacia Biotech (Piscataway, NJ). All other chemicals were of reagent grade.
Hs578T human breast cancer cells (ATCC, HTB-126) were maintained according to the manufacture's recommendations, in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum, 10 μg/ml insulin and penicillin/streptomycin.
The concentration of stearate used to treat the Hs578T cells was 50 μM. When EGF was used, the concentration was 1 nM EGF. Before the treatment with stearate or EGF, cells were first starved for 24–48 h. Stearate was loaded onto fatty acid-free bovine serum albumin (BSA) according to the method reported by Spector et al. (13); briefly, stearate (0.5 g) was dissolved in chloroform (100 ml) and mixed well with 10 g diatomaceous earth in a 1 l flask. The mixture was stirred and dried under nitrogen until powder. BSA is a physiological carrier of fatty acids and was used to avoid the introduction of organic solvents to solutions coming into contact with cells. Fatty acid-free BSA (1 g) was dissolved in 100 ml with Dulbecco's modified Eagle's medium without phenol red and mixed with 3 g of the stearate/diatomaceous earth mixture with stirring for 45 min. The stearate/BSA solution was filtered through a 0.45 μm filter, and adjusted to pH 7.4. The concentration of stearate in the solution was detected with the NEFA C Kit from Wako Chemicals GmbH (Neuss, Germany). All experimental data on Hs578T cells were controlled using fatty acid-free BSA control solutions that were put through the same preparatory procedure described for the stearate/BSA solution except for the fact that no fatty acid was added.
Constitutively active mutant 3xHA epitope-tagged (N-terminus) RhoA, RhoB and RhoC proteins were purchased from the University of Missouri-Rolla, cDNA Resource Center (Rolla, MO). Hs578T cells (105) were cultured in a 35 mm culture dish with complete medium until they were 50–80% confluent. No antibiotics were provided during the 24 h before transfection. The transfectionwas done according to the manufacturer's instructions for use of the FuGENE 6 Transfection Reagent (Roche, Indianapolis IN).
To analyze cellular DNA content, confluent Hs578t cells were harvested, fixed in ice-cold 70% ethanol for 30 min and then resuspended in citrate buffer (4 mM sodium citrate) containing 50 μg/ml of propidium iodide and 100 μg/ml of RNase. After a 20 min incubation at room temperature, cells were run on FACScan flow cytometry. Data were analyzed using the ModFit LT workshop program (BD Immunocytometry System, San Jose, CA).
Ras and Rho activation assay kits were purchased from Millipore (Billerica, MA). The activation assay followed the protocol of the manufacture. Briefly, after cells were treated and the lysates prepared, 1 mg protein (supernatant) was incubated with Rhotekin Rho-binding domain (25 μg)–agarose and then Raf-1/Ras binding domain (10 μg)–agarose beads at 4°C for 45 min. The beads were washed three times with lysis buffer B. Bound Ras–GTP and Rho–GTP proteins were detected by immunoblot using Ras and Rho antibodies.
Cells were treated as described above, and lysed with lysis buffer. The supernatants of the lysates or the immunoprecipitates were loaded with Laemmli sample buffer on 10% sodium dodecyl sulfate–ployacrylamide gel electrophoresis gels after boiling at 100°C for 5 min. Proteins were then transferred to a polyvinylidene difluoride membrane. The membranes were blocked overnight at 4°C with blocking buffer containing 5% non-fat dried milk powder in Tris-buffered saline-T (25 mM Tris, 140 mM NaCl, 2.7 mM KCl, 0.05% Tween-20, pH 8.0), incubated with primary antibody in blocking buffer at room temperature for 1 h and incubated with antirabbit or antimouse antibodies labeled with horseradish peroxidase (1:5000) in blocking buffer under the same conditions, and then washed three times for 10 min in Tris-buffered saline-T. The polyvinylidene difluoride membranes were washed and developed using enhanced chemiluminescence reagents.
Total RNA was extractedand purified with TRIZOL Reagent (GIBCO Invitrogen, Carlsbad, CA). The first-strand complementary DNA (cDNA) synthesis was achieved using a commercially available kit (New England BioLabs, Beverly, MA) according to the protocol from the manufacturer. Briefly, 1 μg of total RNA was reverse-transcribed using M-MuLV reverse transcriptase (25 U) and dT23VN primer (5 μM) in a final volume of 25 μl.
Quantitative real-time reverse-transcription polymerase chain reaction (RT–PCR) cDNA samples were diluted to appropriate concentrations and used for a real-time RT–PCR assay in a volume of 25 μl, containing 2 μl DNA template, 12.5 μl SYBR® Green PCR Master Mix (Applied Biosystems, Foster City, CA) and 0.4 μM each specific primer. The gene expression of p21CIP1/WAF1 and p27KIP1 were determined by real-time quantitative RT–PCR and the house keeping gene 18S ribosomal RNA (rRNA) was used as an internal control to normalize the variable RNA loading in each sample. Sequences of primer sets were as follows: human p21CIP1/WAF1 (sense 5′-GGC GGG CTG CAT CCA-3′; antisense 5′-AGT GGT GTC TCG GTG ACA AAG TC-3′), human p27KIP1 (sense 5′-CGG TGG ACC ACG AAG AGT TAA-3′; antisense 5′-GGC TCG CCT CTT CCA TGT C-3′) and 18S rRNA (sense 5′-CGC CGC TAG AGG TGA AAT TCT-3′; antisense 5′-CGA ACC TCC GAC TTT CGT TCT-3′). All pairs of primers were designed by the Primer Express program (Applied Biosystems) based on sequence information from the GenBank database. After annealing, at 50°C, for 2 min and an initial denaturation at 95°C, for 10 min, 40 repetitive cycles were carried out with denaturing, at 95°C, for 15 s, and annealing, at 60°C, for 60 min, using a GeneAmp® 5700 Sequence Detection System (Applied Biosystems). The comparative cycle threshold (CT) method was used to analyze the data generated from relative values of the amount of target cDNA. CT represents the number of cycles for the amplification of target cDNA to reach a fixed threshold and correlates with the amount of the starting material present. The fluorescence intensity corresponding to the CT was used to quantitate the target cDNA in the mixture of samples with 103-fold dilutions, employing the standard curve for each target gene. Sequence Detector Software (Applied Biosystems) was used to extract the data of the quantitative real-time RT–PCR. The calculated result represents the relative expression levels of target genes compared with its expression in the control group after the value of target genes was normalized to 18S rRNA expression levels.
All animal protocols were approved by the University of Alabama at Birmingham, Institutional Animal Care and Use Committee. Ninety-five female Sprague Dawley rats (Harlan) obtained at 21 days of age were used for the in vivo experiments. The animals were housed two to three rats per cage, and had free access to food and drinking water. The animals were randomly assigned to one of three of the following diets made by Harlan Teklad: (i) low-fat diet (8.5% fat), (ii) stearate diet (17% fat by weight) and (iii) safflower oil diet (17% fat by weight). The weight and food intake were monitored three times a week. At ~50 days of age, the animals were injected with 50 mg/kg N-Nitroso-N-methylurea (NMU). The size of NMU-induced tumors were measured weekly after 42 days post-injection. The experiment was ended 100 days post-injection. Tumor samples were preserved in a 10% paraformaldehyde solution.
Microdissection of specimens for PCR analysis was done at the University of Alabama at Birmingham Laser Microdissection Laboratory. Briefly frozen sections were fixed in 70% ethanol and stained with hematoxylin and eosin. Tumor cells were microdissected from the sections using a laser capture microdissection system with an infrared diode laser (PixCell II System, Arcturus Engineering, Mountain View, CA). Total RNA was extractedand purified with RNaqueous Micro Kit (Applied Biosystems/Ambion, Austin, TX). The first-strand cDNA synthesis was achieved using the iScript cDNA Synthesis Kit (Bio-Rad, Hercules, CA).
In total, 30–40% confluent Hs578T cells were treated with stearate (50 μM) for 48 h after starvation for 24 h. Total RNA was extracted and reverse-transcribed as described in RT–PCR for p21CIP1/WAF1 and p27KIP1.
The PCR assay was performed in a volume of 50 μl, containing 4 μl DNA template, 45 μl Platinum PCR Supermix (Invitrogen) and 0.2 μM each specific primer. PCR specificity and efficiency were improved by using hot start PCR with 3 min pre-denaturation, at 95°C, and 30 cycles of denaturation (95°C, 30 s), annealing (52°C, 30 s) and 1 min extension (72°C). The PCR products (20 INS> μl) were analyzed by means of 1% agarose in tris-acetate ethylenediaminetetraacetic acid gel electrophoresis and visualized by ethidium bromide staining under ultraviolet; digital images were analyzed by means of a FUJI Medical System (FUJIFILM) and the bands quantified by Quantity Software (FUJIFILM). tris-buffered saline was used to normalize the variable RNA loading in each sample. The calculated result represents the relative expression levels of target genes compared with its expression in the control group after the value of target genes was normalized to GAPDH expression levels.
The sequences of the forward and reverse primers used were as follow: rat RhoA (sense 5′-CAG CAA GGA CCA GTT CCC AGA-3′; antisense 5′-TGC CAT ATC TCT GCC TTC TTC AGG-3′), rat RhoB (sense 5′-GCG TGC GGC AAG ACG TGC CTG-3′; antisense 5′-TCA TAG CAC CTT GCA GCA GTT-3′), rat RhoC (sense 5′-GCC TAC AGG TCC GGA AGA AT-3′; antisense 5′-GCA CCA ACC TAG TTC CCA GA-3′), human RhoC (sense 5′-TGCCTCCTCATCGTCTTCA-3′; antisense 5′-GCCTCAGGTCCTTCTTATTCC-3′), and rat GAPDH (sense 5′-CCA TCA CTG CCA CTC AGA AGA C-3′; antisense 5′-TAC CCT GAG CCA TGT AGG-3′). All pairs of primers were designed by the Primer Express program (Applied Biosystems) based on sequence information from GenBank database.
As shown in Figure 1, stearate treatment increased the number of cells in Go/G1 and G2/M and reduced cells in the S phase. These effects persisted even in the presence of complete media and addition of exogenous EGF for up to 16 h. These data suggest a cell cycle-mediated inhibition of proliferation of Hs578T human breast cancer cells.
All major transitions of the eukaryotic cell cycle (G0/G1, G1/S and G2/M) are controlled by the activity of Cdks (14). The activity of Cdks is carefully regulated by the formation of heterodimeric complexes of Cdks with their positive regulatory subunit (cyclins) and negative regulators, including p21CIP1/WAF1 and p27KIP1 (14,15). Both p21CIP1/WAF1 and p27KIP1 have been implicated in G1 arrest and high levels of p21CIP1/WAF1 can also lead to G2 arrest. We reasoned that stearate may increase the expression of p21CIP1/WAF1 and/or p27KIP1.
We found that stearate increases the protein level of p21CIP1/WAF1 and p27KIP1 (Figure 2A). Consistent with these data, pCdk2 was decreased with stearate treatment (Figure 2A). These results indicate that decreased activation of Cdk2 in response to stearate, in combination with increased p21CIP1/WAF1 and p27KIP1, probably halt cell cycle progression from G0/G1 to the S phase and also G2/M progression.
We then investigated whether stearate induced a transcriptional response in p21CIP1/WAF1 and/or p27KIP1. Figure 2B shows that the increased protein level of p21CIP1/WAF1 was due to increased gene expression whereas increased protein level of p27KIP1 was not, suggesting that p27KIP1 degradation might be inhibited by stearate.
GTP loading of Ras plays a crucial role in cell cycle progression and the downstream activation of ERK (16). We evaluated whether stearate influences Ras and ERK activities. We demonstrated that stearate increases the binding of GTP to Ras (Figure 3A) with or without EGF. We also found that phosphorylation of ERK increased between 8 and 16 h post-stearate treatment and that this was sustained up to 24 h after stearate treatment (Figure 3B).
To investigate the role of the ERK signaling pathway in the stearate-induced cell cycle arrest, we examined the effects of a specific inhibitor of MEK1, PD98059, on regulation of protein levels and phosphorylation of cell cycle-related molecules. As shown in Figure 3C, PD98059 blocked ERK phosphorylation induced by stearate and EGF, indicating that ERK activation by stearate is MEK1-dependent and, therefore, likely linked to Ras activation. Addition of PD98059 to cells following stearate and EGF treatment reverses the upregulation of p21CIP1/WAF1, indicating that the p21CIP1/WAF1 response to stearate is dependent on ERK signaling. However, PD98059 did not reverse the increases in p27KIP1 and only partially reversed the decrease in pCdk2 in stearate-treated cells, indicating that stearate-induced changes in p27KIP1 are independent of ERK signaling. This raised the possibility that other signaling pathways linked with p27KIP1 and pCdk2 may coexist.
Expression of Rho family molecules has been reported in breast, lung, pancreas, colon carcinomas and in testicular germ cell tumors (17–21). The consequences of activated Ras–ERK signaling depend on Rho activity (12,22). Thus Rho activity was examined in this next set of experiments.
In Figure 4A, EGF increased Rho–GTP formation at 2 and 16 h, which returned to approximately basal levels at 24 h. Stearate decreased Rho–GTP at all time points tested, especially at 16 and 24 h post-EGF stimulation, compared with controls.
Rho messenger RNA (mRNA) expression was also examined. We found that neither EGF nor stearate affected the mRNA expression over 24 h. However, when cells were treated with stearate for 48 h, the mRNA expression of RhoA, RhoC and total Rho significantly decreased, whereas RhoB remained unchanged (Figure 4B). These data indicate that while stearate activates Ras, it simultaneously inhibits Rho activation and on longer exposure, Rho mRNA expression, indicating an inhibition of Ras–Rho cross talk. The decreased Rho mRNA expression may contribute to a further reduction in Rho activation after 48 h.
In order to identify the role of Rho activity in the regulation of the cell cycle regulatory protein p21CIP1/WAF1 and p27KIP1, we transfected Hs578T cells with constitutively active RhoA, RhoB and RhoC, and treated the cells with/without stearate for 6 h. An immunoblot of p21CIP1/WAF1 and p27KIP1 showed that constitutively active RhoC reverses the effect of stearate on p27KIP1, but not that on p21CIP1/WAF1 (Figure 4C and D). These data indicate that the upregulation of p27KIP1 in response to stearate is dependent on RhoC inhibition.
In order to determine whether stearate inhibits Ras–Rho cross talk in vivo and its effects on breast cancer carcinogenesis in terms of cell transformation, we used the NMU rat mammary cancer carcinogen model. We found that dietary stearate significantly reduced the incidence of mice with tumors that developed over 15 weeks compared with the low fat diet, as did the safflower oil diet (Figure 5A). However, the average number of tumors per rat was only significantly decreased in the stearate diet compared with the low-fat diet (Figure 5B). In addition, tumor burden as defined by average tumor weight per rat was significantly decreased in the stearate diet compared with the low-fat diet (P < 0.001), with the safflower oil diet not reaching significance compared with the low fat (P = 0.057, Figure 5C). When the tumors were classified into four categories by a diagnostic pathologist using the method of Chan et al. (23), intraductal proliferations, tubular adenoma, ductal carcinoma in situ and adenocarcinoma, we found that compared with the low-fat group, the average number of tumors per animal in the stearate group decreased in all the categories (Figure 5D); however, there were no significant differences found between dietary groups in this analysis. Importantly, the mRNA expression of RhoA, RhoB and total Rho of microdissected tumor cells were significantly decreased in both stearate and safflower groups (Figure 6) confirming stearate inhibition of Rho expression in vitro (Figure 4B).
In summary, stearate inhibits breast cancer cell cycle in G1 and to a lesser extent G2 while at the same time increasing cell cycle inhibitors p21CIP1/WAF1 and p27KIP1 and decreasing phosphorylation of Cdk2. Stearate also decreased Rho activation and expression in vitro and Rho expression in vivo while decreasing NMU-induced mammary cancer incidence and tumor burden.
Long-chain saturated fatty acids are a major component of dietary fat. We and others have reported that long-chain saturated fatty acids and especially stearate inhibit proliferation of breast cancer cells (1–3). However, the molecular and cellular mechanism for this inhibition is not clear. Our present studies indicate that stearate arrested cell cycle progression from G1 to S and to a lesser extent from G2 to M. These results differ from several studies demonstrating stearate-induced cell death/apoptosis (24–26). This may be due to differences in the concentration of stearate used, time of exposure and cell type. Typically, the minimum concentration of stearate used in studies demonstrating decreased cell viability was 100 μM, which is double the concentration used in our experiments and other manuscripts use even higher, non-physiologic, concentrated preparations. In one study, treating human ovarian granulosa cells with 50 μM stearate (or palmitate) for 3 days demonstrated no decrease in cell viability (24). Recent studies indicate both a time and concentration dependence of stearate to induce apoptosis in human breast cancer cells and that this effect is specific for breast cancer cells compared with non-cancer breast cells (27). The stearate concentration (50 μM) used in our present study was generally maintained for 6 h and represents a high normal physiological exposure with respect to concentration and time (28,29). Thus, the experiments herein indicate an early stage of stearate exposure that precedes apoptosis.
Cell cycle entry and progression rely on the precisely controlled expression and activation of cell cycle-related enzymes, termed Cdks, cyclins and cyclin-dependent kinase inhibitors. The activity of Cdks is controlled by cyclin-binding interactions, regulated phosphorylation and association with cyclin kinase inhibitors (14). p21CIP1/WAF1 is a broad spectrum cell cycle inhibitor involved in G1 to S and G2 to M phase transitions (30) and increased p21CIP1/WAF1 would be expected to inhibit both cdc2 and cyclin E/Cdk2 complexes. p27KIP1 is also known to inhibit G1 progression via Cdk2 inhibition (15,31,32). Mitogens stimulate elimination of p27KIP1 by decreased translation and increased ubiquitin-directed degradation (33). EGF increased p27KIP1 degradation in Hs578T human breast cancer cells; however, stearate prevented EGF-induced p27KIP1 degradation from reaching control cell levels. It has been proposed that inhibition of p27KIP1 degradation results in elevated levels of p27KIP1 and inhibition of G1 progression (12,22,32). Thus, upregulation of both p21CIP1/WAF1 and p27KIP1 as observed in our experiments are linked with cell cycle arrest caused by stearate.
Cell transformation by oncogenic Ras has been shown to require the function of Rho. Rho GTPases such as RhoA, Rac1 and Cdc42 have been shown to be required for Ras-induced cell transformation (34–36). Subsequent studies indicate that Ras mobilizes not only the Raf–mitogen-activated protein kinase–ERK-mediated kinase signaling cascade but also the PI-3-kinase and RalGDS pathways for complete cell transformation (37). Exactly how Rho functions in the PI-3-kinase and RalGDS signaling pathways is not clear; however, it has been proposed that Rho signaling involves these two pathways in Ras transformation (38).
It is known that transformed cells have elevated levels of activated Rho that inhibit the expression of p21CIP1/WAF1 and induce cyclin D1 thereby promoting cell proliferation (22,39). There is evidence indicating that palmitoylation and possibly acylation by stearate can increase Ras activity by promoting Ras association with the plasma membrane (48,49). However, the mechanism whereby stearate inhibits Rho activity and expression is not yet known. One possibility of how Rho activity is inhibited by stearate is via inhibition of the translocation of p190 Rho–GAP to detergent insoluble membranes in response to Ras (40). Our data in Figure 4C and D are consistent with this hypothesis. We used constitutively active Rho's that are not affected by p190 Rho–GAP and showed that constitutively activated Rho B and C were both able to at least partially reverse the effects of stearate on p21CIP1/WAF1 and p27KIP1 protein concentration.
In our experiments, although stearate increased Ras activity, it decreased Rho activation and mRNA expression. This may be the key as to how stearate inhibits cancer cell cycle progression. RhoA is known to stimulate p27KIP1 degradation by inducing cyclin E/Cdk2 activity (32,33). Thus, blocking Rho activity and mRNA expression would be expected to lead to a decrease in both cyclin E/Cdk2 activity and p27KIP1 degradation that is exactly what happened with stearate treatment. Although in vitro data on Hs578T cells demonstrated that both RhoA and RhoC mRNA expression are inhibited by stearate, only constitutively active RhoC and to a lesser extent B inhibit the effect of stearate on cell cycle proteins p27KIP1 and p21CIP1/WAF1. Interestingly, it was reported that upregulation of RhoC plays an important role in inflammatory breast cancer (41), as well as in other malignant neoplasms including those thought to arise in the urinary bladder, ovary, pancreas and skin (16,42–44). Although we did not see a decrease in RhoC in the NMU model of mammary cancer, it is possible that RhoC does not play a major role in the development of this particular cancer. Consistent with this hypothesis is the fact that RhoC seems to be involved in aggressive forms of breast cancer and the NMU model develops a type of cancer that slowly progresses and did not demonstrate metastasis in our hands. It further indicates that RhoA and B may play important roles both in carcinogen-induced mammary cell transformation and cell proliferation. Although Ras mutations are important in ~30% of cancers (9), the incidence of Ras point mutations in primary breast cancers is rare (<5%) (10). Nevertheless in breast cancer there is upregulated Ras signaling through growth factor receptors and other tyrosine kinases or Ras regulators commonly overexpressed, the Ras protein itself or downstream effectors (10). In the NMU rat model ~80–90% of tumors are Ras dependent making it an ideal model to confirm our in vitro data that stearate inhibits Rho and thus cell transformation and tumor growth in vivo (45,46). Although this study focuses tightly on dietary stearate, the cell cycle, Rho and Ras, a consideration of other mechanistic studies concerning fatty acids and cancer provides better perspective of how the present studies may fit in this field. Dietary fat has been suggested to promote the development of cancer via altering cellular membrane structure (47). Membrane lipid structure can affect membrane-bound proteins thereby influencing intracellular signaling. Importantly, stearate is preferentially incorporated into phospholipids such as phosphatidylinositol (48). One of the other signaling systems that may be affected by dietary stearate is the protein kinase C (PKC) pathway. PKC is activated by phospholipases including phosphoinositide phospholipase C which when activated produces diacylglycerol, a co-activator of classical PKCs and 1,4,5-trisphosphate that stimulates the release of intracellular calcium another co-activator of classical PKCs. Others have suggested that palmitate incorporation into diacylglycerol rather than triacylglycerol is associated with apoptosis of MDA-MB-231 breast cancer cells (49). More recently, we have investigateda possible role of PKC in stearate-induced apoptosis of breast cancer cells in vitro and found that stearate appears to work specifically via a diacylglycerol/PKC/caspase-3-mediated pathway (27). Although potentially this is a very important finding, it should be kept in mind that this mechanism has not yet been demonstrated in vivo with dietary stearate. In addition, although it may be related to the reduction of tumor burden seen in this and one other study (50), PKC pathways have also been shown to be involved in tumorigenesis. In fact, it has been suggested that Ras and PKC may cooperate during transformation (51). Investigation of a dietary stearate-induced link between PKC and Ras in the NMU carcinogen model was not explored in the present study but is a logical future study. Interestingly, in a transgenic model of colon cancer, transformation was found to mediated by a PKCβII–Ras–PKCι–RAC1 (a Rho family member)–mitogen-activated protein kinase pathway that was found to be highly sensitive to a mitogen-activated protein kinase inhibitor (51). Thus, it is possible that a PKC/diacylglycerol/Rho signaling pathway mediates the effects of dietary stearate on carcinogenesis; however, much work remains to prove this.
Nevertheless, results of the in vivo studies herein support our in vitro findings via inhibition of mammary tumor burden and carcinogenesis. Although one study has also found that stearate inhibits carcinogenesis using the NMU model (3), they injected iodostearic acid subcutaneously rather than give highly purified stearate in dietary form. The present study not only provides molecular insights as to how stearate is working but also shows for the first time that dietary stearate inhibits carcinogenesis. Recent studies have also indicated that dietary stearate inhibits breast cancer tumor and metastasis burden in an orthotopic nude mouse model (50). These studies indicate that dietary stearate may be a preventative agent but is there evidence to support this role? Many case control and cohort studies have been performed in different countries to determine the correlation of dietary fat intake and breast cancer risk. Five meta-studies have summarized these results over the years and their results are conflicting. The link between total dietary fat intake and human breast cancer seems weak and may be related to menopausal hormone use (52). With respect to saturated fatty acids, three of these meta-studies found no association between saturated fatty acids and breast cancer (53–55) whereas two studies did (56,57). Data obtained by actually measuring individual fatty acid composition of adipose tissue, erythrocyte membranes, serum and plasma provide quantitative measurement independent of energy intake, and reflects bioavailable and post-absorptive amounts of fat consumed. This eliminates inadequacies of food frequency questionnaires, food composition tables and nutrient databases. A meta-analysis of these data and the risk of breast cancer (3 cohort and 7 case–control studies with 2031 breast cancer cases and 2334 controls) indicate that in cohort studies, stearate was not associated with increased risk of breast cancer whereas palmitate was (58). They also demonstrate that in a cohort of post-menopausal women both stearate and the stearate/oleate ratio were negatively associated with breast cancer risk. This is consistent with a protective effect of stearate with respect to the risk of breast cancer. In this meta-analysis, no significant associations were derived from the case–control studies. Since the meta-analysis report, a case–control study looking at red blood cell fatty acids and breast cancer found that stearate did not have a positive association with breast cancer whereas palmitate did (59) similar to the results found from the cohort studies mentioned above. The only other such study since the meta-analysis was done found no relations between breast cancer risk and any fatty acids of erythrocyte membranes (60). Overall, these studies suggest that stearate is either neutral or may be protective for breast cancer and thus do not contraindicate a possible role for stearate in preventing breast cancer especially in post-menopausal women.
A limitation of these studies is that the in vitro experiments were only done on one cell line. The choice of Hs578t cells was made because initially we were interested in studying EGFR expressing breast cancer cells since the presence of the EGFR is associated with poorer outcomes. In addition, there remains a great need for therapies/prevention of this type of breast cancer exactly because it is so aggressive. Nevertheless, it is clear that stearate has affects on cancer cells other than basal breast cancer cells. We have published the effects of stearate on HT1080 (human fibrosarcoma) and PC3 and DU145 (human prostate cancer) cells (61). Interestingly, the basal type non-tumorigenic but EGF-responsive breast cancer cell line MCF10A was not affected by stearate whereas Hs578t, MDA-MB-435 and MDA-MB-231 cells were (27). Since estrogen suppresses the expression of the EGFR (62), we have yet to investigate estrogen-responsive (ER+) cell lines. Nevertheless, it is possible that stearate has similar effects on other cancer cell lines and we hope to be able to characterize the effects of stearate on other breast cancer cell lines in future studies.
In summary, our in vitro results demonstrate that stearate inhibits breast cancer cell cycle largely in G1, as well as inhibiting Rho expression and activity in vitro and expression in vivo. In vivo results further showed that dietary stearate inhibited the incidence and tumor burden of NMU-induced mammary cancer. These studies raise the possibility of stearate inhibiting Ras/Rho signaling and demonstrate the effectiveness of dietary stearate as a preventative agent for mammary cancer carcinogenesis.
National Institute of Health (RO1-CA81236, R21-AT002922 and R21-AT001636) to R.W.H.
We thank Dr. Clinton Grubbs for his expert help with the NMU rat model. The contents of the work are solely the responsibility of the authors and do not necessarily represent the official views of the National Center for Complementary and Alternative Medicine or the National Institutes of Health.
Conflict of Interest Statement: None declared.