Transforming growth factor-beta and breast cancer: Tumor promoting effects of transforming growth factor-beta

Transgenic mice expressing a dominant-negative mutant type II transforming growth factor-beta receptor exhibit impaired mammary development and enhanced mammary tumor formation

We have previously shown that expression of a dominant-negative type II transforming growth factor-beta receptor (DNIIR) in mammary epithelium under control of the MMTV promoter/enhancer causes alveolar hyperplasia and differentiation in virgin mice. Here we show that MMTV-DNIIR female mice have accelerated mammary gland differentiation during early pregnancy with impaired development during late pregnancy and lactation followed by delayed postlactational involution. Mammary tumors, mostly carcinoma in situ, developed spontaneously in the MMTV-DNIIR mice with a long median latency (27.5 months). Crossbreeding to MMTV-transforming growth factor (TGF)-alpha mice to obtain mice expressing both transgenes resulted in mammary tumor formation with a much shorter latency more similar to those expressing only the MMTV-TGF-alpha transgene (<10 months median latency). The major difference in mammary tumors arising in MMTV-TGF-alpha compared to bigenic MMTV-DNIIR/MMTV-TGF-alpha was the marked suppression of tumor invasion by DNIIR transgene expression. Invading carcinoma cells in both MMTV-DNIIR and bigenic animals showed loss of DNIIR transgene expression as determined by in situ hybridization. The data indicate that signaling from endogenous TGF-betas not only plays an important role in normal mammary gland physiology but also can also suppress the early stage of tumor formation and contribute to tumor invasion once carcinomas have developed.

Effect of glutamine on glutathione, IGF-I, and TGF-beta 1

BACKGROUND

Our previous results have showed that oral glutamine (GLN) supplementation decreased carcinogenesis in 7,12-dimethylbenz[a]antracene (DMBA) breast cancer model. We also have found that GLN raises blood glutathione (GSH) levels in an implantable breast cancer model. The process of tumor growth was accompanied by depressed GSH production and increased levels of insulin-like growth factor-I (IGF-I) and transforming growth factor beta1 (TGF-beta 1). GSH is counter-regulatory to IGF-I. We therefore hypothesized that in DMBA model of breast cancer, the increased GSH levels seen with oral GLN would be associated with lowered levels of IGF-I &TGF-beta(1).

METHODS

Time-dated pubertal Sprague-Dawley rats were gavaged at time 0 with 1 g/kg/day glutamine (GLN) (n = 18), isonitrogenous Freamine (FA) (n = 18), or water (H(2)O) (n = 18). Rats were further randomized on day 7 to 100 mg/kg DMBA or oil. After 14 days, the animals were sacrificed and blood GSH, IGF-1, TGF-beta 1, breast tissue, and gut mucosa GSH levels were measured.

RESULTS

Oral GLN increased significantly blood, breast tissue, and gut mucosa levels of GSH in both DMBA and control groups in comparison with the control groups not treated with GLN. At the same time, the levels of blood IGF-I and TGF-beta 1 decreased significantly in both DMBA-treated and control groups. DMBA did not significantly affect any of these levels. CONCLUSIONS ;Oral GLN increased GSH levels and lowered IGF-I and TGF-beta 1 in a range that is considered clinically significant. However, the effect of GLN in maintaining normal gut GSH production in the presence of DMBA was much more significant. Inconsistent with our hypothesis, reduction in IGF and TGF-beta 1 levels did not correlate with DMBA's effect on gut GSH production.

The tumor microenvironment: a potential arbitrator of the tumor suppressive and promoting actions of TGFbeta

Transforming growth factor beta (TGFbeta) members are secreted in biologically inactive complexes that must be activated in order to enable binding to their cell surface receptors. Interestingly, many of the proteins that can activate TGFbeta have been implicated in either suppressing or promoting tumorigenesis. Included among these are matrix proteins (thrombospondin-1), receptors (integrins alphanubeta6 and alphanubeta8) and proteases (matrix metalloproteases and plasmin). These proteins cannot only activate TGFbeta, but can also modulate cell responsiveness to TGFbeta. In this section, we review data highlighting the complexity and bidirectionality of TGFbeta matrix interactions within the tumor microenvironment, and propose that these dynamic interactions are a critical spatial and temporal determinant of the effects of TGFbeta on tumorigenesis.

A role for TGF-beta in estrogen and retinoid mediated regulation of the nuclear receptor coactivator AIB1 in MCF-7 breast cancer cells

AIB1 (amplified in breast cancer 1) is a nuclear receptor coactivator gene amplified and overexpressed in breast cancer. However, the mechanisms by which AIB1 is regulated are unclear. Here we show that 17beta-estradiol represses AIB1 mRNA and protein expression in MCF-7 human breast cancer cells primarily by suppressing AIB1 gene transcription. Estrogen levels present in fetal calf serum are sufficient to maintain AIB1 mRNA and protein at low basal levels, and this repression is reversed by the addition of antiestrogens or all-trans retinoic acid. Interestingly, cycloheximide inhibition experiments revealed that secondary protein synthesis was necessary to induce AIB1 expression by antiestrogens and retinoids. Experiments with TGF-beta and TGF-beta blocking antibodies demonstrated that this growth factor modulates AIB1 expression and showed that the antiestrogen and retinoid induction of AIB1 gene expression is mediated at least in part through TGF-beta. These data reveal a mechanism of estrogen-induced down-modulation of the overall hormone sensitivity of cells through feedback inhibition of coactivator gene expression. These data also suggest that antiestrogens can shift the sensitivity of cells to non-estrogenic proliferative signaling by increasing cellular levels of AIB1. This effect may play a role in breast cancer progression and resistance to drug treatment.

Blockade of TGF-beta inhibits mammary tumor cell viability, migration, and metastases

TGF-betas are potent inhibitors of epithelial cell proliferation. However, in established carcinomas, autocrine/paracrine TGF-beta interactions can enhance tumor cell viability and progression. Thus, we studied the effect of a soluble Fc:TGF-beta type II receptor fusion protein (Fc:TbetaRII) on transgenic and transplantable models of breast cancer metastases. Systemic administration of Fc:TbetaRII did not alter primary mammary tumor latency in MMTV-Polyomavirus middle T antigen transgenic mice. However, Fc:TbetaRII increased apoptosis in primary tumors, while reducing tumor cell motility, intravasation, and lung metastases. These effects correlated with inhibition of Akt activity and FKHRL1 phosphorylation. Fc:TbetaRII also inhibited metastases from transplanted 4T1 and EMT-6 mammary tumors in syngeneic BALB/c mice. Tumor microvessel density in a mouse dorsal skin window chamber was unaffected by Fc:TbetaRII. Therefore, blockade of TGF-beta signaling may reduce tumor cell viability and migratory potential and represents a testable therapeutic approach against metastatic carcinomas.

Latent transforming growth factor-beta activation in mammary gland: regulation by ovarian hormones affects ductal and alveolar proliferation

Transforming growth factor-beta1 (TGF-beta 1) is a pluripotent cytokine that can inhibit epithelial proliferation and induce apoptosis, but is also widely implicated in breast cancer progression. Understanding its biological action in mammary development is critical for understanding its role in cancer. TGF-beta 1 is produced as a latent complex that requires extracellular activation before receptor binding. To better understand the spatial and temporal regulation of its action during mammary gland development, we examined the pattern of activation in situ using antibodies selected to distinguish between latent and active TGF-beta. Activation was highly restricted. TGF-beta 1 activation was localized primarily to the epithelium, and within the epithelium it was restricted to luminal epithelial cells but absent from either cap or myoepithelial cells. Within the luminal epithelium, we noted a further restriction. During periods of proliferation (ie, puberty, estrus and pregnancy), which are stimulated by ovarian hormones, TGF-beta 1 activation decreased in some cells, consistent with preparation for proliferation. Paradoxically, other cells simultaneously increase TGF-beta 1 immunoreactivity, which suggests that TGF-beta 1 differentially restrains epithelial subpopulations from responding to hormonal signals to proliferate. These data suggest that endogenous TGF-beta 1 activation and thus activity are regulated by ovarian hormones. To determine the specific consequences of TGF-beta 1 activity, we manipulated TGF-beta 1 levels in vivo using Tgfbeta 1 knockout mice and undertook tissue recombination experiments with heterozygous tissue. In Tgfbeta 1 heterozygous mice, which have <10% wild-type levels of TGF-beta1, ductal development during puberty and alveolar development during pregnancy were accelerated, consistent with its role as a growth inhibitor. The proliferative index of Tgfbeta 1+/- epithelium was increased approximately twofold in quiescent tissue and fourfold in proliferating tissue but both ducts and alveoli were grossly and histologically normal. To test whether epithelial TGF-beta1 was critical to the proliferative phenotype, Tgfbeta 1+/+ and +/- epithelium were transplanted into +/+ mammary stroma. The outgrowth of Tgfbeta 1+/- epithelium was accelerated in wild-type hosts, indicating that the phenotype was intrinsic to the epithelium. Moreover, proliferation was 15-fold greater in Tgfbeta 1+/- than wild-type mice after ovariectomy and treatment with estrogen and progesterone, suggesting that TGF-beta 1 acts in an autocrine or juxtacrine manner to regulate epithelial proliferation. Together these data indicate that ovarian hormones regulate TGF-beta 1 activation, which in turn restricts proliferative response to hormone signaling.

Altered p27(Kip1) phosphorylation, localization, and function in human epithelial cells resistant to transforming growth factor beta-mediated G(1) arrest

p27(Kip1) is an important effector of G(1) arrest by transforming growth factor beta (TGF-beta). Investigations in a human mammary epithelial cell (HMEC) model, including cells that are sensitive (184(S)) and resistant (184A1L5(R)) to G(1) arrest by TGF-beta, revealed aberrant p27 regulation in the resistant cells. Cyclin E1-cyclin-dependent kinase 2 (cdk2) and cyclin A-cdk2 activities were increased, and p27-associated kinase activity was detected in 184A1L5(R) cells. p27 from 184A1L5(R) cells was localized to both nucleus and cytoplasm, showed an altered profile of phosphoisoforms, and had a reduced ability to bind and inhibit cyclin E1-cdk2 in vitro when compared to p27 from the sensitive 184(S) cells. In proliferating 184A1L5(R) cells, more p27 was associated with cyclin D1-cdk4 complexes than in 184(S). While TGF-beta inhibited the formation of cyclin D1-cdk4-p27 complexes in 184(S) cells, it did not inhibit the assembly of cyclin D1-cdk4-p27 complexes in the resistant 184A1L5(R) cells. p27 phosphorylation changed during cell cycle progression, with cyclin E1-bound p27 in G(0) showing a different phosphorylation pattern from that of cyclin D1-bound p27 in mid-G(1). These data suggest a model in which TGF-beta modulates p27 phosphorylation from its cyclin D1-bound assembly phosphoform to an alternate form that binds tightly to inhibit cyclin E1-cdk2. Altered phosphorylation of p27 in the resistant 184A1L5(R) cells may favor the binding of p27 to cyclin D1-cdk4 and prevent its accumulation in cyclin E1-cdk2 in response to TGF-beta.

Mechanisms of tumour invasion and metastasis: emerging targets for therapy

The progression of a tumour from one of benign and delimited growth to one that is invasive and metastatic is the major cause of poor clinical outcome in cancer patients. The invasion and metastasis of tumours is a highly complex and multistep process that requires a tumour cell to modulate its ability to adhere, degrade the surrounding extracellular matrix, migrate, proliferate at a secondary site and stimulate angiogenesis. Knowledge of the process has greatly increased and this has resulted in the identification of a number of molecules that are fundamental to the process. The involvement of these molecules has been shown to relate not only to the survival and proliferation of the tumour cell but, also to the processes of tumour cell adhesion, migration, and the tumour cells ability to degrade and escape the primary site as well as play a role in angiogenesis. These molecules may provide important therapeutic targets that represent the ability to target specific steps in the process of invasion and metastasis and provide additional therapies. The review focuses on representative key targets in each of these processes and summarises the state of play in each case.

Transforming growth factors-beta are not good biomarkers of chemopreventive efficacy in a preclinical breast cancer model system

Using a carcinogen-initiated rat model of mammary tumorigenesis, we tested the hypothesis that transforming growth factor (TGF)-betas are useful biomarkers of chemopreventive efficacy in the breast. The chemopreventive agents tested were tamoxifen and the retinoids 9-cis-retinoic acid (9cRA) and N-(4-hydroxyphenyl)retinamide (4-HPR), because both antiestrogens and retinoids have previously been shown to upregulate TGF-betas in vitro. Despite demonstrable chemopreventive efficacy in this model, none of these agents, alone or in combination, had any significant impact on the expression of TGF-betas in the mammary ductal epithelium or periductal stroma as determined by immunohistochemistry. These data suggest that TGF-betas are not likely to be useful biomarkers of chemopreventive efficacy in a clinical setting.

TGF-beta signaling in mammary gland development and tumorigenesis

Ligands of the TGF-beta superfamily are unique in that they signal through transmembrane receptor serine-threonine kinases, rather than tyrosine kinases. The receptor complex couples to a signal transduction pathway involving a novel family of proteins, the Smads. On phosphorylation, Smads translocate to the nucleus where they modulate transcriptional responses. However, TGF-betas can also activate the mitogen-activated protein kinase (MAPK)4 pathway, and the different biological responses to TGF-beta depend to varying degrees on activation of either or both of these two pathways. The Smad pathway is a nexus for cross-talk with other signal transduction pathways and for modulation by many different interacting proteins. Despite compelling evidence that TGF-beta has tumor suppressor activity in the mammary gland, neither TGF-beta receptors nor Smads are genetically inactivated in human breast cancer, though receptor expression is reduced. Possible reasons are discussed in relation to the dual role of TGF-beta as tumor suppressor and oncogene.

Transforming growth factor-beta and breast cancer: Cell cycle arrest by transforming growth factor-beta and its disruption in cancer

Altered responsiveness to extracellular signals and cell cycle dysregulation are hallmarks of cancer. The cell cycle is governed by cyclin-dependent kinases (cdks) that integrate mitogenic and growth inhibitory signals. Transforming growth factor (TGF)-beta mediates G1 cell cycle arrest by inducing or activating cdk inhibitors, and by inhibiting factors required for cdk activation. Mechanisms that lead to cell cycle arrest by TGF-beta are reviewed. Loss of growth inhibition by TGF-beta occurs early in breast cell transformation, and may contribute to breast cancer progression. Dysregulation of cell cycle effectors at many different levels may contribute to loss of G1 arrest by TGF-beta. Elucidation of these pathways in breast cancer may ultimately lead to novel and more effective treatments for this disease.