Insulin-like growth factor-I (IGF-I) has gained broad recognition as an important survival factor for epithelial cells in numerous tissues. The IGF-I receptor signaling pathway is deregulated in the ...majority of carcinomas, and such deregulation has also been reported to be tightly associated with enhanced tumor progression and metastasis. One of the key proteins that transduces IGF-I signals and is phospho-activated downstream of the IGF-I receptor, is the non-receptor serine/threonine kinase proto-oncogene protein kinase B (PKB, also known as Akt). This kinase serves as a major molecular node to control the function of many cell survival and death proteins through phosphorylation-mediated protein modification. The end result of the activation of Akt is enhanced cell survival and proliferation, pre-requisites for malignant transformation. Recent studies show that IGF-I signals cross-talk at multiple levels with various components of the TGF-β signaling pathway, which depending on context may function either as tumor suppressor or as tumor promoter. Thus, a better understanding of how the IGF-I and TGF-β signaling pathways are mutually interconnected is likely to unveil novel targets for the therapeutic intervention of many cancers.
The anterior chamber of the eye is an immunologically privileged site. Over the past 15 years, numerous laboratories documented that the privileged status of this unique site is mediated by ...multifactorial immunoregulatory processes. Among the participating factors is the aqueous humor that circulates in the anterior chamber and is in contact with most of the tissues in the anterior segment of the eye. Recently, it was found that normal aqueous humor is a powerful inhibitor of antigen-driven T lymphocyte activation, but it spares other important functional properties of T cells. The spectrum of immune inhibitory properties resembles some of the activities of transforming growth factor-beta (TGF-beta), a polypeptide cytokine. Because of this similarity, the authors tried to determine if TGF-beta is present in aqueous humor and whether this cytokine could account for the lymphocyte inhibitory activity of this biologic fluid. They found TGF-beta in aqueous humor by dot-blot analysis. Using the CCL-64 mink lung epithelial cell bioassay for this compound, TGF-beta bioactivity was shown in aqueous humor from several different species, including human. In rabbit and human aqueous humor, most of the biologic activity was due to TGF-beta 2 (80-90%). Dose-response curves generated by using purified porcine TGF-beta showed that aqueous humor contained sufficient concentrations of TGF-beta to account for the observed inhibition in several assays for T cell activation and proliferation. Partial purification of the lymphocyte inhibitor in rabbit aqueous humor by size exclusion in high-performance liquid chromatography demonstrated that several lymphocyte inhibitory fractions contained TGF-beta bioactivity. Finally, neutralizing antisera to TGF-beta 2 were able to reverse most of the lymphocyte inhibitory activity of aqueous humor. It was concluded that TGF-beta was present in high concentration in normal aqueous humor and that this cytokine contributed to the immunosuppressive properties of aqueous humor.
Insulin‐like growth factor‐I inhibits transforming growth factor‐β (TGF‐β) signaling by blocking activation of Smad3 (S3), via a phosphatidylinositol 3‐kinase (PI3K)/Akt‐dependent pathway. Here we ...provide the first report that the kinase activity of Akt is necessary for its ability to suppress many TGF‐β responses, including S3 activation and induction of apoptosis. Wild‐type and myristoylated Akts (AktWT and AktMyr) suppress TGF‐β‐induced phospho‐activation of S3 but not Smad2 (S2), whereas kinase‐dead Akt1 (Akt1K179M) or dominant‐negative PI3K enhances TGF‐β‐induced phospho‐activation of both S2 and S3. Using siRNA, rapamycin (Rap), and adenoviral expression for FKBP12‐resistant and constitutively active TGF‐β type I receptor (ALK5), we demonstrate that mammalian target of Rap (mTOR) mediates Akt1 suppression of phospho‐activation of S3. These and further data on Akt1‐S3 binding do not support a recently proposed model that Akt blocks S3 activation through physical interaction and sequestration of S3 from TGF‐β receptors. We propose a novel model whereby Akt suppresses activation of S3 in an Akt kinase‐dependent manner through mTOR, a likely route for loss of tumor suppression by TGF‐β in cancers.
In a model of pulmonary inflammation and fibrosis induced by the antineoplastic antibiotic, bleomycin, we previously demonstrated that TGF-beta was markedly elevated within 7 d of bleomycin ...administration. At the time of maximal TGF-beta production, TGF-beta 1 was localized by immunohistochemistry to be present almost exclusively in alveolar macrophages. In this study, we have demonstrated that alveolar macrophages stimulated by bleomycin-induced injury secrete large quantities of biologically active TGF-beta 1 when explanted into tissue culture. However, alveolar macrophages from normal saline-treated rats secrete small quantities of biologically inactive TGF-beta. In contrast, splenic macrophages secrete large quantities of inactive TGF-beta and are unaffected by the intratracheal bleomycin treatment. High doses of the corticosteroid methylprednisolone given intramuscularly before and concomitantly with bleomycin administration prevented the influx of alveolar macrophages into the lungs, diminishing both the number of macrophages present in the alveoli and the total lung content of TGF-beta. However, the rate of secretion of TGF-beta by alveolar macrophages recovered from the alveoli was unchanged after corticosteroid treatment. When activated alveolar macrophages were cultured in the presence of several concentrations of dexamethasone that completely suppressed IL-1 secretion, little effect on TGF-beta secretion was observed. The findings in this study demonstrate that during bleomycin-induced injury, alveolar macrophages not only secrete large quantities of active TGF-beta 1, but are a predominant source of the enhanced TGF-beta response seen in this model. Furthermore, the alveolar macrophage secretion of TGF-beta is not inhibited by the presence of high concentrations of corticosteroids.
The role of basal epithelial cells in prostatic function, development and carcinogenesis is unknown. The ability of basal prostatic epithelial cells to acquire a luminal phenotype was explored in ...vitro using the NRP-152 rat dorsal-lateral prostate epithelial cell line as a model system. NRP-152, which was spontaneously immortalized and clonally derived, is an androgen-responsive and nontumorigenic cell line that has a basal cell phenotype under normal growth conditions. However, when placed in mitogen-deficient media, these cells undergo a dramatic morphological change to a luminal phenotype. Under these growth-restrictive conditions, immunocytochemical analysis shows that NRP-152 cells acquire the luminal markers Z0-1 (a tight-junction associated protein), occludin (integral tight-junction protein), and cytokeratin 18, and lose the basal markers cytokeratins 5 and 14. Total protein and mRNA levels of cytokeratins 8, 18, c-CAM 105 (the calcium-independent cell adhesion molecule) and Z0-1, as detected by western and/or northern blot analyses, respectively, are induced, while cytokeratin 5 and 15 are lost, and occludin is unchanged. Concomitant with this differentiation, expression of transforming growth factor-beta2 (TGF-beta2), TGF-beta3, and TGF-beta receptor type II (TbetaRII) is induced, while those of TGF-beta1 and TbetaRI remain essentially unchanged. Mitogens, such as insulin-like growth factor-I and dexamethasone inhibit luminal differentiation, while exogenous TGF-beta induces such differentiation. These data together with TGF-beta neutralization experiments using pan-specific antibody implicate an important role for autocrine TGF-beta in the induction of the luminal differentiation.
Approximately 1 out of every 10 eyes undergoing surgery for retinal detachment develops excessive intraocular fibrosis that can lead to traction retinal detachment and ultimate blindness. This ...disease process has been termed proliferative vitreoretinopathy (PVR). The ability to monitor and grade this fibrotic response accurately within the eye as well as the ability to aspirate vitreous cavity fluid bathing the fibrotic tissue makes this an ideal setting in which to investigate the development of fibrosis. Although laboratory studies have recently shown that transforming growth factor-beta (TGF-beta) can enhance fibrosis, little clinical evidence is yet available correlating the level of this or other growth factors with the degree of fibrosis in a clinical setting. We have found that vitreous aspirates from eyes with intraocular fibrosis associated with PVR have more than three times the amount of TGF-beta (1,200 +/- 300 pM SEM) found in eyes with uncomplicated retinal detachments without intraocular fibrosis (360 +/- 91 pM SEM). Using an in vitro assay, 84-100% of the TGF-beta activity could be blocked with specific antibodies against TGF-beta 2, whereas only 10-21% could be blocked by specific antibodies against TGF-beta 1. TGF-beta 1 was used in an animal model of traction retinal detachment. Since beta 1 and beta 2 have essentially identical biologic effects and only human beta 1 was available in quantities required, beta 1 was chosen for these in vivo studies. The injection of TGF-beta1 plus fibronectin (FN) but not TGF-beta1 alone into the vitreous cavity of rabbits resulted in the increased formation of intraocular fibrosis and traction retinal detachments as compared to control eyes. In previous studies, intravitreal FN levels were also found to be elevated in eyes with intraocular fibrosis.
It has been proposed that intestinal intraepithelial lymphocytes (I-IEL) perform immune surveillance of the epithelial layer (1) and regulate mucosal humoral responses to exogenous Ag (2). To better ...understand the functional potential of this unique population, purified murine I-IEL were analyzed phenotypically and functionally. Initial studies determined that I-IEL could be distinguished based on several phenotypic characteristics including: TCR (TCR-alpha beta vs TCR-gamma delta); Thy-1, CD45R/B220, CD5, and CD8 (CD8 alpha alpha vs CD8 alpha beta) expression. Using anti-TCR mAb, individual I-IEL subsets were activated and examined functionally. Both TCR-alpha beta and TCR-gamma delta I-IEL were found to synthesize an array of lymphokines that included IL-2, IL-3, and IL-6 but not IL-4 or IL-5. Additionally, a number of lymphokines were detected that directly influence epithelial function (IFN-gamma, TNF-alpha, and TGF-beta 1). However, the majority of the I-IEL function was localized within the Thy-1+, CD45R/B220- I-IEL subset. In addition those TCR-alpha beta I-IEL expressing the CD8 alpha beta heterodimer were more easily activated. Thus, a subset of I-IEL have the capacity to respond to TCR-mediated stimuli. The functional activities of these cells may influence both local immune cell populations as well as epithelial differentiation.
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