Capacity of adipose tissue to promote growth and metastasis of a murine mammary carcinoma: effect of estrogen and progesterone

Adipocyte-secreted factors synergistically promote mammary tumorigenesis through induction of anti-apoptotic transcriptional programs and proto-oncogene stabilization

Mammary epithelial cells are embedded in a unique extracellular environment to which adipocytes and other stromal cells contribute. Mammary epithelial cells are critically dependent on this milieu for survival. However, it remains unknown which adipocyte-secreted factors are required for the survival of the mammary epithelia and what role these adipokines play in the process of ductal carcinoma tumorigenesis. Here, we take a systematic molecular approach to investigate the multiple ways adipocytes and adipokines can uniquely influence the characteristics and phenotypic behavior of malignant breast ductal epithelial cells. Microarray analysis and luciferase reporter assays indicate that adipokines specifically induce several transcriptional programs involved in promoting tumorigenesis, including increased cell proliferation (IGF2, FOS, JUN, cyclin D1), invasive potential (MMP1, ATF3), survival (A20, NFkappaB), and angiogenesis. One of the key changes in the transformed ductal epithelial cells associated with the cell cycle involves the induction of NFkappaB (five-fold) and cyclin D1 (three-fold). We show that by regulating the transcription of these molecules, the synergistic activity of adipocyte-derived factors can potentiate MCF-7 cell proliferation. Furthermore, compared to other stromal cell-secreted factors, the full complement of adipokines shows an unparalleled ability to promote increased cell motility, migration, and the capacity for angiogenesis. Adipocyte-secreted factors can affect tumorigenesis by increasing the stabilization of pro-oncogenic factors such as beta-catenin and CDK6 as a result of a reduction in the gene expression of their inhibitors (i.e. p18). An in vivo coinjection system using 3T3-L1 adipocytes and SUM159PT cells effectively recapitulates the host-tumor interactions in primary tumors. Type VI collagen, a soluble extracellular matrix protein abundantly expressed in adipocytes, is further upregulated in adipocytes during tumorigenesis. It promotes GSK3beta phosphorylation, beta-catenin stabilization, and increased beta-catenin activity in breast cancer cells and may critically contribute towards tumorigenesis when not counterbalanced by other factors.

Minireview: The adipocyte--at the crossroads of energy homeostasis, inflammation, and atherosclerosis

Adipose tissue evolved to efficiently store energy for times of caloric restriction. The large caloric excess common in many Western diets has negated the need for this thrifty function, leaving adipose tissue ill-equipped to handle this increased load. An excess of adipose tissue increases risk for a number of conditions including coronary artery disease, hypertension, dyslipidemias, type 2 diabetes, and even cancer. Indeed, the ability of the adipocyte to function properly when engorged with lipid can lead to lipid accumulation in other tissues, reducing their ability to function and respond normally. The role of adipose tissue as an endocrine organ capable of secreting a number of adipose tissue-specific or enriched hormones, known as adipokines, is gaining appreciation. The normal balance of these adipose tissue secretory proteins is perturbed in obesity. Paradoxically, the lack of normal adipose tissue, as seen in cases of lipodystrophy and lipoatrophy, is also associated with pathologic sequelae similar to what is seen with obesity. The pathologic findings associated with lack of adipose tissue, largely due to inability to properly store lipids, may also be due to a lack of adipokines. In this review, we highlight the role of adipose tissue as an endocrine organ focusing on some of the recent advances in the identification and pharmacological characterization of adipokines as well as their regulation in the context of obesity and insulin-resistant states.

Constitutive activation of met kinase in non-small-cell lung carcinomas correlates with anchorage-independent cell survival

Lung cancer is currently the most frequent cause of cancer death in North America. Hepatocyte growth factor (HGF) and its receptor Met are frequently over-expressed in non-small-cell lung carcinomas (NSCLC), but their potential role in tumor progression is not clearly known. To assess the role of HGF/Met signaling in lung carcinomas, we have examined the expression, activation status, and function of Met in NSCLC cell lines (n = 7), established from primary tumors or pleural fluids of cancer patients. We observed Met expression in three NSCLC cell lines, two of which exhibited constitutive tyrosine-phosphorylation of Met, and Met kinase activity. In addition, the observed constitutive activation of Met was sustained under anchorage-independent conditions, and correlated with phosphatidyl inositol 3-kinase-dependent cell survival. Immunoreactive HGF-like protein was secreted by two Met-positive and two Met-negative NSCLC cell lines. However HGF activity, as determined by the ability to induce cell scattering and tyrosine-phosphorylation of Met in reporter cell lines, was detected in conditioned medium from only one Met-negative NSCLC cell line: none of the conditioned media from Met-expressing NSCLC cell lines showed detectable HGF activity. Thus, constitutive activation of Met in NSCLC cell lines may occur at least in part through intracrine, or HGF-independent mechanisms. Interestingly, additional paracrine stimulation with exogenous recombinant HGF was required for DNA synthesis and correlated with increased activation of ERK1/2 in all Met-positive NSCLC cell lines, regardless of the basal activation status of Met. These findings indicate that a medium level of constitutive activation of Met occurs in some NSCLC cell lines, and correlates with survival of detached carcinoma cells; whereas additional paracrine stimulation by recombinant HGF is required for DNA synthesis. Thus constitutive and paracrine activation of Met may provide complementary signals that promote survival and proliferation, respectively, during tumor progression of NSCLC.

Stromal effects on mammary gland development and breast cancer

Breast cancer manifests itself in the mammary epithelium, yet there is a growing recognition that mammary stromal cells also play an important role in tumorigenesis. During its developmental cycle, the mammary gland displays many of the properties associated with breast cancer, and many of the stromal factors necessary for mammary development also promote or protect against breast cancer. Here we review our present knowledge of the specific factors and cell types that contribute to epithelial-stromal crosstalk during mammary development. To find cures for diseases like breast cancer that rely on epithelial-stromal crosstalk, we must understand how these different cell types communicate with each other.

Opposite effects of androgens and estrogens on adipogenesis in rat preadipocytes: evidence for sex and site-related specificities and possible involvement of insulin-like growth factor 1 receptor and peroxisome proliferator-activated receptor gamma2

To investigate the role of sex steroid hormones in adipose tissue development and distribution, we have studied the effect of various sex steroids (testosterone, dihydrotestosterone (DHT), and 17beta-estradiol) in vitro, on the proliferation and differentiation processes in rat preadipocytes from deep (epididymal and parametrial) and superficial (femoral sc) fat deposits. All added steroids failed to affect the growth rate of preadipocytes from male rats when determined from day 1 to day 4 after plating, whether FCS was present or not in the culture medium. In contrast, in preadipocytes from female rats, we observed a positive effect (x2) of 17beta-estradiol (0.01 microM) on the proliferative capacities of sc but not parametrial preadipocytes. When preadipocytes were exposed to testosterone or DHT (0.1 microM) during the differentiation process, the glycerol 3-phosphate dehydrogenase activity was significantly decreased in epididymal preadipocytes only. When preadipocytes from male rats were exposed to 17beta-estradiol (0.01 microM), the differentiation capacities of preadipocytes were not modified. However, in parametrial preadipocytes from ovariectomized female rats, 17beta-estradiol significantly increased (x1.34) the glycerol 3-phosphate dehydrogenase activity. In differentiated preadipocytes that had been exposed to sex steroids, expression of peroxisome proliferator-activated receptor gamma2 was up-regulated by 17beta-estradiol but not by androgens. As described in other cell types, sex steroids modulate insulin growth factor 1 receptor (IGF1R) expression in preadipocytes. Indeed, IGF1R levels were either enhanced by 17 beta-estradiol (0.01 microM) in sc preadipocytes from female ovariectomized rats or decreased by DHT (0.01 microM) in epididymal preadipocytes. These effects were reversed by simultaneous exposure to androgen or estrogen receptor antagonists. In conclusion, this study demonstrates that, in rat preadipocytes kept in primary culture and chronically exposed to sex hormones, androgens elicit an antiadipogenic effect, whereas estrogens behave as proadipogenic hormones. Moreover, our results suggest that these opposite effects could be related to changes in IGF1R (androgens and estrogens) and peroxisome proliferator-activated receptor gamma2 expression (estrogens).

Detection and regulation of leptin receptor mRNA in ovine mammary epithelial cells during pregnancy and lactation

Adipocyte-epithelial cell interactions and their secretions are critical determinants of mammary gland development. In this present study, we examined the possible involvement of leptin and its receptors in the process of mammogenesis/lactogenesis. We demonstrated by reverse transcription and polymerase chain reaction analysis that long and short forms of leptin receptors were expressed in the ovine mammary gland during pregnancy and lactation. Furthermore, quantitative determinations, via ribonuclease protection assays, provided evidence that the level of leptin receptor expression was greatest during mid-pregnancy when active growth of the mammary gland is initiated. Location of the leptin receptors, as determined by in situ hybridization analysis, revealed that leptin receptor transcripts were expressed specifically in mammary epithelial cells. These data provide evidence that leptin, with its receptors, could be an important mediator in regulating mammary gland growth and development.

Regulation of mammary gland growth and morphogenesis by the mammary fat pad: a species comparison

The growth and morphogenesis of mammary parenchyma varies substantially between species and is regulated by an array of systemic and local factors. Central to this regulation is the mammary fat pad, a matrix of adipose and connective tissue capable of mediating hormone action and synthesizing an array of growth regulatory molecules. In this article we highlight differences between the morphological development of the mammary parenchyma in rodents, humans, and ruminant dairy animals, placing emphasis on differences in the cellular composition and structure of the mammary fat pad. While a great deal remains to be understood about the ability of stroma to locally regulate mammary development, the significance of its contribution is becoming increasingly apparent. The actions of several steroid and peptide hormones appear to be mediated by an array of growth factors, proteases and extracellular matrix components synthesized by constituents of the mammary fat pad. Further, mammary adipose tissue represents a significant store of lipid which, by itself and through its derivatives, could influence the growth of mammary epithelium in diverse ways. This review describes the integral role of the mammary fat pad during mammogenesis, emphasizing the point that species differences must be addressed if local growth and morphogenic mechanisms within the mammary gland are to be resolved.

c-Src kinase activity is required for hepatocyte growth factor-induced motility and anchorage-independent growth of mammary carcinoma cells

Overexpression and amplification of hepatocyte growth factor (HGF) receptor (Met) have been detected in many types of human cancers, suggesting a critical role for Met in growth and development of malignant cells. However, the molecular mechanism by which Met contributes to tumorigenesis is not well known. The tyrosine kinase c-Src has been implicated as a modulator of cell proliferation, spreading, and migration; these functions are also regulated by Met. To explore whether c-Src kinase is involved in HGF-induced cell growth, a mouse mammary carcinoma cell line (SP1) that co-expresses HGF and Met and a nonmalignant epithelial cell line (Mv1Lu) that expresses Met but not HGF were used. In this study, we have shown that c-Src kinase activity is constitutively elevated in SP1 cells and is induced in response to HGF in Mv1Lu cells. In addition, c-Src kinase associates with Met following stimulation with HGF. The enhanced activity of c-Src kinase also correlates with its ability to associate with Met. Expression of a dominant negative double mutant of c-Src (SRC-RF), lacking both kinase activity (K295R) and a regulatory tyrosine residue (Y527F), in SP1 cells significantly reduced c-Src kinase activity and strongly blocked HGF-induced motility and colony growth in soft agar. In contrast, expression of the dominant negative c-Src mutant had no effect on HGF-induced cell proliferation on plastic. Taken together, our data strongly suggest that HGF-induced association of c-Src with Met and c-Src activation play a critical role in HGF-induced cell motility and anchorage-independent growth of mammary carcinomas and further support the notion that the presence of paracrine and autocrine HGF loops contributes significantly to the transformed phenotype of carcinoma cells.

Autocrine secretion of TGF-beta 1 and TGF-beta 2 by pre-adipocytes and adipocytes: a potent negative regulator of adipocyte differentiation and proliferation of mammary carcinoma cells

We have developed an in vitro system to examine the influence of adipocytes, a major mammary stromal cell type, on the growth of a murine mammary carcinoma, SP1. Previously, we have shown that 3T3-L1 adipocytes release a mitogenic factor, hepatocyte growth factor, which strongly stimulates proliferation of SP1 cells. We now show that 3T3-L1 pre-adipocytes secrete active inhibitory molecules which inhibit DNA synthesis in SP1 cells. In addition, latent inhibitory activity is present in conditioned media (CM) from both pre-adipocytes and adipocytes, and is activated following acid treatment. CM also inhibited DNA synthesis in Mv1Lu wild type epithelial cells, but not DR27 mutant epithelial cells which lack TGF-beta type II receptor. Inhibitory activity of CMs was partially abrogated by neutralizing anti-TGF-beta1 and anti-TGF-beta2 antibodies, and was removed following ultrafiltration through membranes of 10,000 Mr but not 30,000 Mr pore size. These results show that the inhibitory effect on DNA synthesis is mediated by TGF-beta1-like and TGF-beta2-like molecules. In addition, acid-treated CM as well as purified TGF-beta inhibited differentiation of pre-adipocytes. Untreated pre-adipocyte CM, but not mature adipocyte CM, spontaneously inhibited adipocyte differentiation. Together, these findings indicate that pre-adipocytes spontaneously activate their own secreted TGF-beta, whereas mature adipocytes do not, and suggest that activation of TGF-beta has a potent negative regulatory effect on adipocyte differentiation and tumor growth. Thus, TGF-beta may be an important modulator of tumor growth and adipocyte differentiation via both paracrine and autocrine mechanisms. These findings emphasize the importance of adipocyte-tumor interactions in the regulation of tumor microenvironment.

The role of mammary stroma in modulating the proliferative response to ovarian hormones in the normal mammary gland

Postnatal mammary gland development is highly dependent on the ovarian steroids, estrogen and progesterone. However, evidence from both in vitro and in vivo studies indicates that steroid-induced development occurs indirectly, requiring stromal cooperation in epithelial proliferation and morphogenesis. Stromal cells appear to influence epithelial cell behavior by secretion of growth factors and/or by altering the composition of the extracellular matrix in which epithelial cells reside. This review will discuss the requirement for stromal tissue in modulating proliferative responses to ovarian hormones during postnatal development and the potential role of the EGF, IGF, HGF and FGF3 growth factor families. Additionally, the roles of extracellular matrix proteins, including fibronectin, collagens and laminin, will be summarized.

The mammary fat pad

The mammary fat pad is essential for development of the mammary epithelium, providing signals that mediate ductal morphogenesis and, probably, alveolar differentiation. The "cleared" fat pad is often used as a transplantation site. Considering the crucial role of the fat pad, its properties have received relatively little attention from researchers in the field. Some of the questions whose investigation is pertinent to understanding both normal mammary development and carcinogenesis are outlined in this commentary in the spirit of stimulating enquiry into this important subject. It is clear from a brief perusal of the available literature that until studies are specifically designed to clearly differentiate between functional effects of the fibrous and the adipose stroma, more substantive information about their differential effects on mammary development and tumorigenesis will not be forthcoming.

Outgrowth of BT-474 human breast cancer cells in immune-deficient mice: a new in vivo model for hormone-dependent breast cancer

The effect of co-inoculation of basement membrane matrix, Matrigel and two human breast cancer cell lines, BT-474 and SK-BR-3, was tested in immune-deficient mice. Both cell lines strongly overexpress c-ErbB-2 protein, whereas only BT-474 is reported to be oestrogen receptor positive. Co-inoculation of Matrigel and BT-474 cells but not of Matrigel and SK-BR-3 cells resulted in tumour formation in bg-nu-xid mice. Oestrogen supplementation greatly enhanced tumorigenicity, but did not seem to be an absolute requirement. In vivo, BT-474 cells grow as a poorly differentiated adenocarcinoma with a doubling time of 9.4 +/- 1.1 days after inoculation into the neck region. A high proliferative activity appears to be compensated by a relatively high rate of cell loss, as BT-474 tumours contain many cells with the typical morphology of apoptotic cell death. Wild-type p53, known to participate in the induction of apoptosis, is absent from the tumours, whereas Bcl-2, known to inhibit apoptosis, is expressed at intermediate levels. BT-474 tumours tend to metastasise to the regional lymph nodes and are capable of forming micrometastatic lesions in the lung. Flow cytometrical analysis of DNA ploidy demonstrated no change in tumours compared with the cell line. Immunohistochemical and flow cytometrical detection of a number of hormone and growth factor receptors, transcription factors, cell adhesion molecules and proteins involved in proliferation and cell death demonstrated no major changes in ploidy and phenotype of tumours compared with the cell line. High expression of the cell-surface molecules c-ErbB-2 and episialin make it a potentially useful model for research in immune therapy.

Role of hepatocyte growth factor in breast cancer: a novel mitogenic factor secreted by adipocytes

Stromal cells can dramatically affect the growth and metastatic capability of breast carcinoma cells. Growth factors, considered to be important mediators of this process, act as either mitogenic or mito-inhibitory regulators. We have developed an in vitro coculture system to examine the influence of adipocytes, a dominant mammary stromal cell type, on the growth of a murine mammary carcinoma, SP1. Previously, we have reported that conditioned medium (CM) from 3T3-L1 adipocytes can promote in vitro growth of SP1 cells. We now show that the major mitogenic signal derived from 3T3-L1 adipocyte CM is mediated by hepatocyte growth factor (HGF). Neutralizing antibody against HGF at 15 micrograms/ml completely abrogated mitogenic activity of 3T3-L1 CM. Furthermore, heparin, an inhibitor of biological activity of HGF, inhibited the mitogenic activity of 3T3-L1 CM. Western blot analysis also confirmed the presence of HGF in 3T3-L1 CM. Although basic fibroblast growth factor (bFGF) and insulin-like growth factor I (IGF-I) were mitogenic for SP1 cells, neutralizing antibodies against IGF-I, bFGF, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) did not inhibit the mitogenic activity of 3T3-L1 CM. Immunoprecipitation and immunoblotting of HGF receptor/c-met showed that c-met is expressed at high level in SP1 cells, and is phosphorylated following HGF ligation. Together, our present data demonstrate that 3T3-L1 adipocytes secrete HGF, which stimulates SP1 cell growth by a paracrine mechanism. Furthermore, the mitogenic effect of 3T3-L1 CM requires HGF receptor ligation and activation of tyrosine kinase signaling cascades in SP1 cells. These results highlight the importance of stromal-tumor cell interactions and suggest that HGF secreted by adipocytes may be a key regulator of mammary tumor growth.

Anti-beta 1 integrin IgG inhibits pulmonary macrometastasis and the size of micrometastases from a murine mammary carcinoma

In the present report, we investigated the possible importance of beta 1 integrins in the growth and metastasis of a murine mammary carcinoma, SP1, and a metastatic variant, SP1-3M in vivo. CBA/J female mice bearing SP1 tumor transplants were injected with anti-beta 1 integrin IgG or control nonimmune IgG (200 micrograms per mouse; i.p.) every two days. Animals received anti-CD4 antibody (100 micrograms per mouse) at time zero to suppress immunity against rabbit IgG. Outgrowth of macroscopic metastases from SP1, but not from SP1-3M primary tumors, was markedly inhibited in animals receiving anti-beta 1 integrin IgG but not nonimmune IgG. To assess the stage(s) in the metastatic cascade affected, we examined the number and diameter of micrometastatic nodules in treated and untreated groups. The diameter of micrometastases was significantly reduced in SP1-tumor-bearing mice treated with anti-beta 1 integrin IgG compared to control IgG, although the number of nodules per cm2 of lung sections examined remained unchanged. No change in the number or size of micrometastases in SP1-3M tumor-bearing mice was observed. No difference in the binding, or complement-mediated and antibody-dependent cell-mediated cytotoxicity of anti-beta 1 integrin IgG with SP1 and SP1-3M cells was detected. The results suggest that under these conditions anti-beta 1 integrin inhibits metastatic tumor growth in lung tissue, but has minimal effect on intravasation, adhesion to target organs and extravasation.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

Mechanisms involved in the metastasis of cancer to bone

The metastasis of cancer to bone is a frequent outcome of common malignancies and is often associated with significant morbidity due to osteolysis. Bone metastasis is also selective in that a disproportionately small number of malignancies account for the majority of tumors which spread to bone. While the mechanisms of bone destruction have been studied, those responsible for the site-specific nature of bone metastasis are poorly understood. As a metastatic target, bone is unique in that it is continuously being remodelled under the influence of local and systemic growth factors, many of which are embedded in the bone matrix. This review summarizes evidence for the hypothesis that the formation of metastatic tumors in bone is the consequence of a unique microenvironment where metastatic cells can alter the metabolism of bone, thereby regulating the release of soluble bone-derived growth factors as a consequence of bone resorption. These, in turn, can modulate the malignant phenotypic properties of receptive cells. Transforming growth factor-beta is one factor which can promote the growth and motility of Walker 256 cells, a rat cell line with a propensity to metastasize spontaneously to bone.