Characteristics and functional relevance of apolipoprotein-A1 and cholesterol binding in mammary gland tissues and epithelial cells

Cholesterol in milk is derived from the circulating blood through a complex transport process involving the mammary alveolar epithelium. Details of the mechanisms involved in this transfer are unclear. Apolipoprotein-AI (apoA-I) is an acceptor of cellular cholesterol effluxed by the ATP-binding cassette (ABC) transporter A1 (ABCA1). We aimed to 1) determine the binding characteristics of (125)I-apoA-I and (3)H-cholesterol to enriched plasma membrane vesicles (EPM) isolated from lactating and non-lactating bovine mammary glands (MG), 2) optimize the components of an in vitro model describing cellular (3)H-cholesterol efflux in primary bovine mammary epithelial cells (MeBo), and 3) assess the vectorial cholesterol transport in MeBo using Transwell(®) plates. The amounts of isolated EPM and the maximal binding capacity of (125)I-apoA-I to EPM differed depending on the MG's physiological state, while the kinetics of (3)H-cholesterol and (125)I-apoA-I binding were similar. (3)H-cholesterol incorporated maximally to EPM after 25±9 min. The time to achieve the half-maximum binding of (125)I-apoA-I at equilibrium was 3.3±0.6 min. The dissociation constant (KD) of (125)I-apoA-I ranged between 40-74 nmol/L. Cholesterol loading to EPM increased both cholesterol content and (125)I-apoA-I binding. The ABCA1 inhibitor Probucol displaced (125)I-apoA-I binding to EPM and reduced (3)H-cholesterol efflux in MeBo. Time-dependent (3)H-cholesterol uptake and efflux showed inverse patterns. The defined binding characteristics of cholesterol and apoA-I served to establish an efficient and significantly shorter cholesterol efflux protocol that had been used in MeBo. The application of this protocol in Transwell(®) plates with the upper chamber mimicking the apical (milk-facing) and the bottom chamber corresponding to the basolateral (blood-facing) side of cells showed that the degree of (3)H-cholesterol efflux in MeBo differed significantly between the apical and basolateral aspects. Our findings support the importance of the apoA-I/ABCA1 pathway in MG cholesterol transport and suggest its role in influencing milk composition and directing cholesterol back into the bloodstream.

Cholesterol Transport and Regulation in the Mammary Gland.

The milk-producing alveolar epithelial cells secrete milk that remains after birth the principal source of nutrients for neonates. Milk secretion and composition are highly regulated processes via integrated actions of hormones and local factors which involve specific receptors and downstream signal transduction pathways. Overall milk composition is similar among mammalian species, although the content of individual constituents such as lipids may significantly differ from one species to another. The milk lipid fraction is essentially composed of triglycerides, which represent more than 95 % of the total lipids in human and commercialized bovine milk. Though sterols, including cholesterol, which is the major milk sterol, represent less than 0.5 % of the total milk lipid fraction, they are of key importance for several biological processes. Cholesterol is required for the formation of biological membranes especially in rapidly growing organisms, and for the synthesis of sterol-based compounds. Cholesterol found in milk originates predominantly from blood uptake and, to a certain extent, from local synthesis in the mammary tissue. The present review summarizes current knowledge on cellular mechanisms and regulatory processes determining intra- and transcellular cholesterol transport in the mammary gland. Cholesterol exchanges between the blood, the mammary alveolar cells and the milk, and the likely role of active cholesterol transporters in these processes are discussed. In this context, the hormonal regulation and signal transduction pathways promoting active cholesterol transport as well as potential regulatory crosstalks are highlighted.

Nutrient Transport in the Mammary Gland: Calcium, Trace Minerals and Water Soluble Vitamins.

Milk nutrients are secreted by epithelial cells in the alveoli of the mammary gland by several complex and highly coordinated systems. Many of these nutrients are transported from the blood to the milk via transcellular pathways that involve the concerted activity of transport proteins on the apical and basolateral membranes of mammary epithelial cells. In this review, we focus on transport mechanisms that contribute to the secretion of calcium, trace minerals and water soluble vitamins into milk with particular focus on the role of transporters of the SLC series as well as calcium transport proteins (ion channels and pumps). Numerous members of the SLC family are involved in the regulation of essential nutrients in the milk, such as the divalent metal transporter-1 (SLC11A2), ferroportin-1 (SLC40A1) and the copper transporter CTR1 (SLC31A1). A deeper understanding of the physiology and pathophysiology of these transporters will be of great value for drug discovery and treatment of breast diseases.

Cholesterol transporters in lactating and nonlactating human mammary tissue

In mammals milk is the principal nutrient for neonates at birth. The basic milk composition is similar between different mammals, but the content of individual constituents such as lipids may differ significantly from one species to another. The milk fat fraction is mainly composed of triglycerides which account for more than 95% of the lipids found in human and bovine milk. Though sterols and in particular cholesterol, the predominant milk sterol, represent less than 0.5% of the total milk lipid fraction, they are of ultimate importance for biological processes such as the formation of biological membranes or as precursors for steroid hormone synthesis. Cholesterol found in milk originates either from blood uptake or from local synthesis. This chapter provides an overview of cholesterol exchanges between the blood, the mammary tissue and the milk. The current knowledge on the expression, localization and function of candidate cholesterol transporters in mammary tissues of human, murine and bovine origin is summarized. Different mechanisms of how cholesterol can be transferred via the mammary tissue into milk, and which active cholesterol transporters are likely to play a role in this process will be discussed.

Colostrogenesis: candidate genes for IgG1 transcytosis mechanisms in primary bovine mammary epithelial cells.

Bovine colostrogenesis is distinguished by the specific transfer of IgG1 from the blood to mammary secretions. The process has been shown to be initiated by hormones and occurs during the last weeks of pregnancy when steroid concentrations of estradiol (E2 ) and progesterone (P4 ) are highly elevated. Rodent intestinal uptake of immunoglobulin G is mediated by a receptor termed Fc fragment of IgG, Receptor, Transporter, alpha (FcGRT) and supported by light chain Beta-2-Microglobulin (β2M). We hypothesized that steroid hormone treatments (E2 and P4 ) of bovine mammary epithelial cells in vitro would induce up-regulation of IgG1 transcytosis candidate gene mRNA expression suggesting involvement in IgG1 transcytosis. Two different primary bovine mammary epithelial cell cultures were cultured on plastic and rat tail collagen and treated with hormonal combinations (steroids/lactogenic hormones). Evaluated mRNA components were bLactoferrin (bLf: a control), bFcGRT, β2M, and various small GTPases; the latter components are reported to direct endosomal movements in eukaryotic cells. All tested transcytosis components showed strong expression of mRNA in the cells. Expression of bFcGRT, bRab25 and bRhoB were significantly up-regulated (p < 0.05) by steroid hormones. bRab25 and bRhoB showed increased expression by steroid treatments, but also with lactogenic hormones. Analysis for the oestrogen receptor (ER) mRNA was mostly negative, but 25% of the cultures tested exhibited weak expression, while the progesterone receptor (PR) mRNA was always detected. bRab25 and bRhoB and likely bFcGRT are potential candidate genes for IgG1 transcytosis in bovine mammary cells.

Effects of induced energy deficiency on lactoferrin concentration in milk and the lactoferrin reaction of primary bovine mammary epithelial cells in vitro.

A dietary energy restriction to 49% of total energy requirements was conducted with Red Holstein cows for three weeks in mid-lactation. At the last day of the restriction phase, primary bovine mammary epithelial cells (pbMEC) of eight restriction (RF) and seven control-fed (CF) cows were extracted out of one litre of milk and cultured. In their third passage, an immune challenge with the most prevalent, heat-inactivated mastitis pathogens Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was conducted. Lactoferrin (LF) was determined on gene expression and protein level. An enzyme-linked immunosorbent assay (ELISA) was developed to determine LF in milk samples taken twice weekly throughout the animal trial, beginning on day 20 pp (post-partum) until day 150 pp, in cell culture total protein and in cell culture supernatant. Milk LF increased throughout the lactation and decreased significantly during the induced energy deficiency in the RF group. At the beginning of realimentation, LF concentration increased immediately in the RF group and reached higher levels than before the induced deficit following the upward trend seen in the CF group. Cell culture data revealed higher levels (up to sevenfold up-regulation in gene expression) and significant higher LF protein concentration in the RF compared to the CF group cells. A further emphasized effect was found in E. coli compared to S. aureus exposed cells. The general elevated LF levels in the RF pbMEC group and the further increase owing to the immune challenge indicate an unexpected memory ability of milk-extracted mammary cells that were transposed into in vitro conditions and even displayed in the third passage of cultivation. The study confirms the suitability of the non-invasive milk-extracted pbMEC culture model to monitor the influence of feeding experiments on immunological situations in vivo.

Function of Natural Internal Mammary-to-Coronary Artery Bypasses and Its Effect on Myocardial Ischemia.

BACKGROUND The function of naturally existing internal mammary (IMA)-to-coronary artery bypasses and their quantitative effect on myocardial ischemia are unknown. METHODS AND RESULTS The primary end point of this study was collateral flow index (CFI) obtained during two 1-minute coronary artery balloon occlusions, the first with and the second without simultaneous distal IMA occlusion. The secondary study end point was the quantitatively determined intracoronary ECG ST-segment elevation. CFI is the ratio of simultaneously recorded mean coronary occlusive pressure divided by mean aortic pressure both subtracted by mean central venous pressure. A total of 180 pairs of CFI measurements were performed among 120 patients. With and without IMA occlusion, CFI was 0.110±0.074 and 0.096±0.072, respectively (P<0.0001). The difference of CFI obtained in the presence minus CFI obtained in the absence of IMA occlusion was highest and most consistently positive during left IMA with left anterior descending artery occlusion and during right IMA with right coronary artery occlusion (ipsilateral occlusions): 0.033±0.044 and 0.025±0.027, respectively. This CFI difference was absent during right IMA with left anterior descending artery occlusion and during left IMA with right coronary artery occlusion (contralateral occlusions): -0.007±0.034 and 0.001±0.023, respectively (P=0.0002 versus ipsilateral occlusions). The respective CFI differences during either IMA with left circumflex artery occlusion were inconsistently positive. Intracoronary ECG ST-segment elevations were significantly reduced during ipsilateral IMA occlusions but not during contralateral or left circumflex artery occlusions. CONCLUSION There is a functional, ischemia-reducing extracardiac coronary artery supply via ipsilateral but not via contralateral natural IMA bypasses. CLINICAL TRIAL REGISTRATION URL http://www.clinicaltrials.gov. Unique identifier: NCTO1676207.

Metabolic adaptations and changes of the mammary immune response during beta-hydroxybutyrate administration

Elevation of ketone bodies occurs frequently after parturition during negative energy balance in high yielding dairy cows. Previous studies illustrated that hyperketonemia interferes with metabolism and it is assumed that it impairs the immune response. However, a causative effect of ketone bodies could not be shown in vivo before, because spontaneous hyperketonemia comes usually along with high NEFA and low glucose concentrations. The objective was to study effects of beta-hydroxybutyrate (BHBA) infusion and an additional intramammary lipopolysaccharide (LPS) challenge on metabolism and immune response in dairy cows. Thirteen dairy cows received intravenously either a BHBA infusion (group BHBA, n=5) to induce hyperketonemia (1.7 mmol/L), or an infusion with a 0.9 % saline solution (Control, n=8) for 56 h. Infusions started at 0900 on day 1 and continue up to 1700 two days later. Two udder quarters were challenged with 200 μg Escherichia coli-LPS 48 h after the start of infusion. Blood samples were taken one week and 2 h before the start of infusions as reference samples and hourly during the infusion. Liver and mammary gland biopsies were taken one week before the start of the infusion, 48 h after the start of the infusion, and mammary tissues was additionally taken 8 h after LPS challenge (56 h after the start of infusions). Rectal temperature (RT) and somatic cell count (SCC) was measured before and 48 h after the start of infusions and hourly during LPS challenge. Blood samples were analyzed for plasma glucose, BHBA, NEFA, triglyceride, urea, insulin, glucagon, and cortisol concentration. The mRNA abundance of factors related to potential adaptations of metabolism and immune system was measured in liver and mammary tissue biopsies. Differences between blood constituents, RT, SCC, and mRNA abundance before and 48 h after the start of infusions, and differences between mRNA abundance before and after LPS challenges were tested for significance by GLM of SAS procedure with treatment as fixed effect. Area under the curve was calculated for blood variables during 48 h BHBA infusion and during the LPS challenge, and additionally for RT and SCC during the LPS challenge. Most surprisingly, both plasma glucose and glucagon concentration decreased during the 48 h of BHBA infusion (P<0.05). During the 48 h of BHBA infusion, serum amyloid A mRNA abundance in mammary gland was increased (P<0.01), and haptoglobin (Hp) mRNA abundance tended to increase in cows treated with BHBA compared to control group (P= 0.07). RT, SCC, and candidate genes related to immune response in the liver were not affected by BHBA infusion. However, during LPS challenge the expected increase of both plasma glucose and glucagon concentration was much less pronounced in the animals treated with BHBA (P<0.05) and also SCC increased much less pronounced in the animals infused with BHBA (P<0.05) than in the controls. An increased BHBA infusion rate to maintain plasma BHBA constant could not fully compensate for the decreased plasma BHBA during the LPS challenge which indicates that BHBA is used as an energy source during the immune response. In addition, BHBA infused animals showed a more pronounced increase of mRNA abundance of IL-8, IL-10, and citrate synthase in the mammary tissue of LPS challenged quarters (P<0.05) than control animals. Results demonstrate that infusion of BHBA affects metabolism through decreased plasma glucose concentration which is likely related to a decreased release of glucagon during hyperketonemia and during additional inflammation. It also affects the systemic and mammary immune response which may reflect the increased susceptibility for mastitis during spontaneous hyperketonemia. The obviously reduced gluconeogenesis in response to BHBA infusion may be a mechanism to stimulated the use of BHBA as an energy source instead of glucose, and/or to save oxaloacetate for the citric acid cycle instead of gluconeogenesis and as a consequence to reduce ketogenesis.

Overexpression of EphB4 in the mammary epithelium shifts the differentiation pathway of progenitor cells and promotes branching activity and vascularization

Postnatally, the mammary gland undergoes continuous morphogenesis and thereby is especially prone to malignant transformation. Thus, the maintenance of the epithelium depends on a tight control of stem cell recruitment. We have previously shown that epithelial overexpression of the EphB4 receptor results in defective mammary epithelial development and conferred a metastasizing tumor phenotype on experimental mouse mammary tumors accompanied by a preponderance of progenitor cells. To analyze the effect of EphB4 overexpression on mammary epithelial cell fate, we have used Fluorescence Activated Cell Sorting (FACS) analyses to quantify epithelial sub-populations and repopulation assays of cleared fat pads to investigate their regenerative potential. These experiments revealed that deregulated EphB4 expression leads to an augmentation of bi-potent progenitor cells and to a shift of the differentiation pathway towards the luminal lineage. The analyses of the ductal outgrowths indicated that EphB4 overexpression leads to enforced branching activity, impedes ductal differentiation and stimulates angiogenesis. To elucidate the mechanisms forwarding EphB4 signals, we have compared the expression profile of defined cell populations between EphB4 transgene and wild type mammary glands concentrating on the wnt signaling pathway and on genes implicated in cell migration. With respect to wnt signaling, the progenitor cell population was the most affected, whereas the stem cell-enriched population showed the most pronounced deregulation of migration-associated genes. Thus, the luminal epithelial EphB4 signaling contributes, most likely via wnt signaling, to the regulation of migration and cell fate of early progenitors and is involved in the determination of branching points along the ductal tree.

The mammary gland in small ruminants: major morphological and functional events underlying milk production – a review

The importance of small ruminants to the dairy industry has increased in recent years, especially in developing countries, where it has a high economic and social impact. Interestingly and despite the fact that the mammary gland is the specialised milk production organ, very few authors studied the modifications occurring in the mammary gland through the lactation period in production animals, particularly in the small ruminants, sheep (Ovis aries) and goat (Capra hircus). Nevertheless, understanding the different mammary gland patterns throughout lactation is essential to improve dairy production. In addition, associating these patterns with different milking frequencies, lactation number or different diets is also of high importance, directly affecting the dairy industry. The mammary gland is commonly composed of parenchyma and stroma, which includes the ductal system, with individual proportions of each changing during the different periods and yields in a lactation cycle. Indeed, during late gestation, as well as during early to mid-lactation, mammary gland expansion occurs, with an increase in the number of epithelial cells and lumen area, which leads to increment of the parenchyma tissue, as well as a reduction of stroma, corresponding macroscopically to the increase in mammary gland volume. Throughout late lactation, the mammary gland volume decreases owing to the regression of the secretory structure. In general, common mammary gland patterns have been shown for both goats and sheep throughout the several lactation stages, although the number of studies is limited. The main objective of this manuscript is to review the colostrogenesis and lactogenesis processes as well as to highlight the mammary gland morphological patterns underlying milk production during the lactation cycle for small ruminants, and to describe potential differences between goats and sheep, hence contributing to a better description of mammary gland development during lactation for these two poorly studied species.

Continuous milking of dairy cows disrupts timing of peak IgG concentration appearance in mammary secretions

The length of the dry period in commercial dairy production is under close scrutiny. While the main concern is the composition and volume of milk produced, the evaluation of colostrum quality under these new paradigms has suggested a decline in IgG concentrations, while some reports indicate no change. Colostrum quality has been defined as an adequate concentration (>50 mg/ml) of immunoglobulin in the secretions to provide the newborn with maximal disease resistance. We investigated the appearance of IgG in mammary pre- and post partum secretions in cows without a dry period (continuously milked, Dry0) and compared the secretions with cows that experienced a dry period of 60 d (Dry60). Blood was collected during the experimental period and plasma analysed for progesterone (P4) and prolactin (Prl). Approximately -6 d relative to parturition, the Dry0 animals exhibited increased concentration of IgG in their secretions to an average of ∼35 mg/ml that remained rather constant through subsequent pregnancy and following parturition. Dry0 cows were producing an average IgG concentration in parturition colostrum of 44·2±17·6 mg/ml that was not different than that of controls (66·86±16·8 mg/ml). However, Dry0 cows exhibited high variation, different peak times (day) of IgG concentration including times that occurred both pre and post parturition. IgG mass of the Dry0 cows remained rather constant pre- and post partum and did not show the same declining mass following parturition that was shown for the Dry60 cows. The change in plasma P4 and Prl were shown to have no timing effect on colostrum IgG concentration.

Effect of intramammary administration of prednisolone on the blood-milk barrier during the immune response of the mammary gland to lipopolysaccharide

OBJECTIVE: To investigate effects of intramammary administration of prednisolone on the immune response of mammary glands in cows. ANIMALS: 5 lactating Red Holsteins. PROCEDURES: Cows received a different intramammary infusion in each mammary gland (10 mg of prednisolone, 100 μg of lipopolysaccharide [LPS], 100 μg of LPS and 10 mg of prednisolone, or saline [0.9% NaCl] solution). Milk samples were collected before (time 0) and 3, 6, 9, 12, 24, and 36 hours after treatment. Somatic cell count (SCC), lactate dehydrogenase (LDH) activity, and concentrations of serum albumin (SA) and tumor necrosis factor (TNF)-α in milk and mRNA expression of TNF-α, interleukin (IL)-8, and IL-1β in milk somatic cells were analyzed. RESULTS: Saline solution or prednisolone did not change SCC, LDH activity, and SA and TNF-α concentrations in milk and mRNA expression of TNF-α, IL-1β, and IL-8 in milk somatic cells. The SCC and TNF-α concentration in milk increased similarly in glands infused with LPS, independent of prednisolone administration. However, the increase of LDH activity and SA concentration in milk after LPS infusion was diminished by prednisolone administration. The mRNA expression of TNF-α, IL-8, and IL-1β in milk somatic cells increased after LPS infusion and was unaffected by prednisolone. CONCLUSIONS AND CLINICAL RELEVANCE: Intramammary administration of prednisolone did not induce an immune response and did not change mRNA expression of TNF-α, IL-8, and L-1β during the response to intramammary administration of LPS. However, prednisolone reduced disruption of the blood-milk barrier. This could influence the severity and cure rate of mastitis.

The Inflammatory Response of Primary Bovine Mammary Epithelial Cells to Staphylococcus aureus Strains Is Linked to the Bacterial Phenotype

Staphylococcus aureus is a major mastitis-causing pathogen in dairy cows. The latex agglutination-based Staphaurex test allows bovine S. aureus strains to be grouped into Staphaurex latex agglutination test (SLAT)-negative [SLAT(-)] and SLAT-positive [SLAT(+)] isolates. Virulence and resistance gene profiles within SLAT(-) isolates are highly similar, but differ largely from those of SLAT(+) isolates. Notably, specific genetic changes in important virulence factors were detected in SLAT(-) isolates. Based on the molecular data, it is assumed that SLAT(+) strains are more virulent than SLAT(-) strains. The objective of this study was to investigate if SLAT(-) and SLAT(+) strains can differentially induce an immune response with regard to their adhesive capacity to epithelial cells in the mammary gland and in turn, could play a role in the course of mastitis. Primary bovine mammary epithelial cells (bMEC) were challenged with suspensions of heat inactivated SLAT(+) (n = 3) and SLAT(-) (n = 3) strains isolated from clinical bovine mastitis cases. After 1, 6, and 24 h, cells were harvested and mRNA expression of inflammatory mediators (TNF-α, IL-1β, IL-8, RANTES, SAA, lactoferrin, GM-CSF, COX-2, and TLR-2) was evaluated by reverse transcription and quantitative PCR. Transcription (ΔΔCT) of most measured factors was induced in challenged bMEC for 6 and 24 h. Interestingly, relative mRNA levels were higher (P<0.05) in response to SLAT(+) compared to SLAT(-) strains. In addition, adhesion assays on bMEC also showed significant differences between SLAT(+) and SLAT(-) strains. The present study clearly shows that these two S. aureus strain types cause a differential immune response of bMEC and exhibit differences in their adhesion capacity in vitro. This could reflect differences in the severity of mastitis that the different strain types may induce.

Induced hyperketonemia affects the mammary immune response during lipopolysaccharide challenge in dairy cows

Metabolic adaptations during negative energy and nutrient balance in dairy cows are thought to cause impaired immune function and hence increased risk of infectious diseases, including mastitis. Characteristic adaptations mostly occurring in early lactation are an elevation of plasma ketone bodies and free fatty acids (nonesterified fatty acids, NEFA) and diminished glucose concentration. The aim of this study was to investigate effects of elevated plasma β-hydroxybutyrate (BHBA) at simultaneously even or positive energy balance and thus normal plasma NEFA and glucose on factors related to the immune system in liver and mammary gland of dairy cows. In addition, we investigated the effect of elevated plasma BHBA and intramammary lipopolysaccharide (LPS) challenge on the mammary immune response. Thirteen dairy cows were infused either with BHBA (HyperB, n=5) to induce hyperketonemia (1.7 mmol/L) or with a 0.9% saline solution (NaCl, n=8) for 56 h. Two udder quarters were injected with 200 μg of LPS after 48 h of infusion. Rectal temperature (RT) and somatic cell counts (SCC) were measured before, at 48 h after the start of infusions, and hourly during the LPS challenge. The mRNA abundance of factors related to the immune system was measured in hepatic and mammary tissue biopsies 1 wk before and 48 h after the start of the infusion, and additionally in mammary tissue at 56 h of infusion (8h after LPS administration). At 48 h of infusion in HyperB, the mRNA abundance of serum amyloid A (SAA) in the mammary gland was increased and that of haptoglobin (Hp) tended to be increased. Rectal temperature, SCC, and mRNA abundance of candidate genes in the liver were not affected by the BHBA infusion until 48 h. During the following LPS challenge, RT and SCC increased in both groups. However, SCC increased less in HyperB than in NaCl. Quarters infused with LPS showed a more pronounced increase of mRNA abundance of IL-8 and IL-10 in HyperB than in NaCl. The results demonstrate that an increase of plasma BHBA upregulates acute phase proteins in the mammary gland. In response to intramammary LPS challenge, elevated BHBA diminishes the influx of leukocytes from blood into milk, perhaps by via modified cytokine synthesis. Results indicate that increased ketone body plasma concentrations may play a crucial role in the higher mastitis susceptibility in early lactation.

BRCA2-Deficient Sarcomatoid Mammary Tumors Exhibit Multidrug Resistance.

Pan- or multidrug resistance is a central problem in clinical oncology. Here, we use a genetically engineered mouse model of BRCA2-associated hereditary breast cancer to study drug resistance to several types of chemotherapy and PARP inhibition. We found that multidrug resistance was strongly associated with an EMT-like sarcomatoid phenotype and high expression of the Abcb1b gene, which encodes the drug efflux transporter P-glycoprotein. Inhibition of P-glycoprotein could partly resensitize sarcomatoid tumors to the PARP inhibitor olaparib, docetaxel, and doxorubicin. We propose that multidrug resistance is a multifactorial process and that mouse models are useful to unravel this. Cancer Res; 75(4); 732-41. ©2014 AACR.