IPG (inositolphosphate glycan) as a cellular signal for TGF-beta 1 modulation of chondrocyte cell cycle

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Structural and spatial determinants regulating TC21 activation by RasGRF family nucleotide exchange factors

RasGRF family guanine nucleotide exchange factors (GEFs) promote guanosine diphosphate (GDP)/guanosine triphosphate (GTP) exchange on several Ras GTPases, including H-Ras and TC21. Although the mechanisms controlling RasGRF function as an H-Ras exchange factor are relatively well characterized, little is known about how TC21 activation is regulated. Here, we have studied the structural and spatial requirements involved in RasGRF 1/2 exchange activity on TC21. We show that RasGRF GEFs can activate TC21 in all of its sublocalizations except at the Golgi complex. We also demonstrate that TC21 susceptibility to activation by RasGRF GEFs depends on its posttranslational modifications: farnesylated TC21 can be activated by both RasGRF1 and RasGRF2, whereas geranylgeranylated TC21 is unresponsive to RasGRF2. Importantly, we show that RasGRF GEFs ability to catalyze exchange on farnesylated TC21 resides in its pleckstrin homology 1 domain, by a mechanism independent of localization and of its ability to associate to membranes. Finally, our data indicate that Cdc42-GDP can inhibit TC21 activation by RasGRF GEFs, demonstrating that Cdc42 negatively affects the functions of RasGRF GEFs irrespective of the GTPase being targeted.

The biological activity of structurally defined inositol glycans

BACKGROUND: The inositol glycans (IGs) are glycolipid-derived carbohydrates produced by insulin-sensitive cells in response to insulin treatment. IGs exhibit an array of insulin-like activities including stimulation of lipogenesis, glucose transport and glycogen synthesis, suggesting that they may be involved in insulin signal transduction. However, because the natural IGs are structurally heterogeneous and difficult to purify to homogeneity, an understanding of the relationship between structure and biological activity has relied principally on synthetic IGs of defined structure. DISCUSSION: This article briefly describes what is known about the role of IGs in signal transduction and reviews the specific biological activities of the structurally defined IGs synthesized and tested to date. CONCLUSION: A pharmacophore for IG activity begins to emerge from the reviewed data and the structural elements necessary for activity are summarized.

Mutational analysis of the GPI-anchor addition sequence from the circumsporozoite protein of Plasmodium

The plasma membrane of Plasmodium sporozoites is uniformly covered by the glycosylphosphatidylinositol (GPI)-anchored circumsporozoite (CS) protein. Sporozoites form in the mosquito midgut through a budding process that occurs within a multinucleate oocyst underneath the basal lamina of the gut. Earlier genetic studies established that normal sporozoite development requires CS. Mutant parasites lacking CS [CS (-)] do not form sporozoites. Ultrastructural analysis of the oocysts from these parasites revealed that there is an early block in the cytokinesis that occurs within the multinucleate oocysts to generate individual sporozoites. Parasites that are hypomorphic for CS expression gave rise to sporozoites with abnormal morphology. These results proved that CS plays a direct role in the maturation of oocysts and in the normal budding of sporozoites. In this article, we examined if the membrane localization of CS via a GPI-anchor, is crucial for its function during sporozoite formation. We generated three mutants in Plasmodium berghei CS, CS-DeltaGPI, CS-TM1 and CS-TM2. In CS-DeltaGPI, we deleted the signal sequence required for the addition of a GPI-anchor to CS. The resulting protein was found only in the cytoplasm of the oocyst. In CS-TM1 and CS-TM2, the GPI-anchor addition sequence of CS was substituted by the transmembrane domain and truncated (to different degrees) cytoplasmic tail of Plasmodium thrombospondin-related anonymous protein (TRAP). The resulting CS protein was detected on the plasma membrane of the oocysts. The amount of CS in the mutants was similar to that of wild type. The sporozoite budding and development were abrogated in both CS-DeltaGPI and CS-TM mutants. The ultrastructure of the mutant oocysts was indistinguishable from that of the CS (-) parasites. Our results suggest that the GPI-anchor of the CS protein is required for sporogenesis.

Transforming growth factor-beta receptors localize to caveolae and regulate endothelial nitric oxide synthase in normal human endothelial cells

Caveolae (sphingolipid- and cholesterol-rich, 100 nm flask-shaped invaginations of the cell membrane) serve as a nexus of cell signalling. In the present study caveolin-rich lipid raft domains were extracted from HUVEC (human umbilical-vein endothelial cells) using both density gradient and immunoprecipitation techniques, and demonstrated localization of the TGF-beta (transforming growth factor-beta) receptors TbetaRI and TbetaRII to the Cav-1 (caveolin-1)-enriched raft fractions of these normal, human endothelial cells. Immunoprecipitation demonstrated an association between TbetaRI and TbetaRII, as well as an association of the TbetaRs receptors with Cav-1 and eNOS (endothelial nitric oxide synthase), suggesting a mutual co-localization to caveolae; after treatment of HUVEC with 5 ng/ml TGF-beta1 for 15 min, however, co-precipitation of eNOS with TbetaRI, TbetaRII and Cav-1 was diminished. The loss of immunoprecipitable eNOS from Cav-1-enriched fractions was accompanied by a decrease both in phosphorylation of eNOS and in enzymatic activity (conversion of arginine into citrulline). No change in the localization of eNOS to morphologically distinct caveolae could be detected by electron microscopy after treatment of HUVEC with TGF-beta1 for 20 min. The results of these investigations provide evidence that TbetaRI interacts with eNOS in the caveolae of normal, human endothelial cells and has a regulatory function on basal eNOS enzymatic activity.

Removal of phospholipid contaminants through precipitation of glycosylphosphatidylinositols

Parasitic glycosylphosphatidylinositols (GPIs) are thought to be involved in induced cell signaling that leads to proinflammatory responses. Increasing interest in elucidation of the mechanisms involved in signaling pathways drives the finding of rapid and reliable methods to purify GPIs. GPIs are usually extracted using mixtures of chloroform/methanol/water, followed by a phase partition between water and water-saturated n-butanol. GPIs recovered in the butanol phase are separated by thin-layer chromatography, scraped, eluted from the silica, and used for studying the structure-function relationship. The presence of phospholipid contaminants or other hydrophobic components in the samples cannot be excluded. Furthermore, the standard procedures to purify GPIs harbor several drawbacks, including the need to handle large amounts of culture, poor yields, time-consuming, and interfering contaminants. Here we report on the development of a simple and reliable method to isolate and purify both free and bound GPIs from one cell pellet. We exploited the low solubility of GPIs in water-saturated n-butanol to remove the phospholipid contaminants completely. After delipidation, GPI proteins were solubilized from the pellet using a mixture of organic solvent containing ethanol and water.

YKL-40 (cartilage gp-39) induces proliferative events in cultured chondrocytes and synoviocytes and increases glycosaminoglycan synthesis in chondrocytes

YKL-40 (cartilage gp-39), is a mammalian glycoprotein related in sequence to chitinases. Its function is unknown, but it is thought to be involved in tissue remodeling. Immunocytochemical staining of YKL-40 in guinea pig chondrocytes (GPC), rabbit chondrocytes (RC), and rabbit synoviocytes (RS) was higher in dividing cells than in confluent cells, suggesting a participation of YKL-40 in cell cycle events. As assessed by the MTT assay, YKL-40 at 1.9-7.6 nM had dose-dependent mitogenic activity toward the three cell types. At 7.6 nM, YKL-40 increased the number of cells of 42% in GPC, 75% in RC, and 86% in RS after 72 h. YKL-40 also stimulated total proteoglycan synthesis by chondrocytes in a dose-dependent manner as assessed by Na[35SO4] incorporation and cetylpyridinium chloride precipitation. At 9.4 nM, YKL-40 increased proteoglycan synthesis of 42% in GPC and 58% in RC after 24 h. The growth factor properties of YKL-40 may explain the increased tissue remodeling associated with high levels of YKL-40 in joint diseases, and possibly, in malignant pathologies such as breast cancer or colorectal cancer.

Glycosylphosphatidylinositol (GPI) hydrolysis by transforming growth factor-beta1 (TGF-beta1) as a potential early step in the inhibition of epithelial cell proliferation

Glycosylphosphatidylinositol (GPI) was previously identified in rabbit articular chondrocytes as being a precursor of inositolphosphate glycan (IPG), released upon (Transforming Growth Factor-beta) (TGF-beta) exposure, and capable of mimicking the proliferative effects of the growth factor. Here, using mink lung epithelial cells (CCL 64), which are known to be growth-inhibited by TGF-beta, we studied the potential role of GPI-derived molecules in the antiproliferative effect of TGF-beta1. We first identified an endogenous pool of GPI material and three different anionic forms of IPG in epithelial cells pre-labeled with [3H]glucosamine. Shortly (8 min) after TGF-beta1 addition, the cells responded by a rapid and transient hydrolysis of GPI, accompanied by the release of the most anionic form of IPG. This TGF-beta-released IPG, after partial purification, was shown to decrease the proliferation of CCL 64 cells. Moreover, anti-IPG antibodies reduced the effects of TGF-beta and blocked the effects of partially purified IPG. These data strongly suggest that GPI hydrolysis may be an early step of the TGF-beta signalling pathway involved in growth inhibition of epithelial cells.

Inositol phosphoglycans and signal transduction systems in pregnancy in preeclampsia and diabetes: evidence for a significant regulatory role in preeclampsia at placental and systemic levels

Measurements have been made of the urinary content of inositol phosphoglycans IPG P-type and IPG A-type, putative insulin second messengers, in preeclampsia, in type I insulin-treated diabetic pregnant women and their matched control subjects, and nonpregnant women of child-bearing age. The content of IPG P-type and IPG A-type was also measured in the placenta from preeclamptic patients and from normal pregnancies. Pregnancy was associated with an increase, approximately twofold, in urinary output of IPG-P-type relative to nonpregnant controls (P<0.01). The 24-h output of IPG P-type in urine in preeclamptic women was significantly higher (2- to 3-fold) than in pregnant control subjects matched for age, parity, and stage of gestation (P<0.02). In contrast, insulin-dependent diabetic pregnant women did not show any significant change in urinary output of IPG P-type or IPG A-type relative to pregnant control subjects. Evidence for a possible relationship and correlation between the urinary excretion of IPG P-type and markers of preeclampsia, including proteinuria (r = 0.720, P<0.01), plasma aspartate transaminase (r = 0.658, P<0.05), and platelet counts (r = 0.613, P<0.05) is presented. A high yield of IPG P-type was extracted from human placenta, in preeclampsia some 3-fold higher (P = 0.03) than the normal value, whereas no IPG A-type (with lipogenic-stimulating activity) was found. Low concentrations of placental IPG A-type were detected relative to IPG P-type using assay systems dependent upon the effect of this mediator on cAMP-dependent protein kinase or on a proliferation assay using thymidine incorporation into DNA of EGFR T17 fibroblasts. It is postulated that the high urinary excretion IPG P-type in preeclampsia reflects high placental levels and relates to the accumulation of glycogen in the placenta. The paracrine effects of placental IPG P-type (stimulation off other endocrine glands and/or endothelial cells) could contribute to the pathogenesis of the maternal syndrome. A possible theoretical link between elevated placental IPG P-type and apoptosis is proposed.

Specificity in signal transduction among glycosylphosphatidylinositols of Plasmodium falciparum, Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp

Glycosylphosphatidylinositols (GPIs) and related glycoconjugates of parasite origin have been shown to regulate both the innate and acquired immune systems of the host. This is achieved through the activation of novel GPI-dependent signalling pathways in macrophages, lymphocytes and other cell types. Parasite GPIs impart at least two distinct signals to host cells through the structurally distinct inositolphosphoglycan (IPG) and fatty acid domains. Binding of IPG to as yet uncharacterized cell surface receptor(s) leads to activation of src-family protein tyrosine kinases: depending upon structure, GPI-derived fatty acids can either activate or antagonize protein kinase C, and may enter the sphingomyelinase pathway. The degree of fatty acid saturation may also contribute to signalling activity. Thus, variation in structure of parasite GPIs imparts different properties of signal transduction upon this class of glycolipid. The divergent activities of GPIs from various protozoal taxa reflect global aspects of the host/parasite relationship, suggesting that GPI signalling is a central determinant of disease in malaria, leishmaniasis and both American and African trypanosomiases.

Purification and analysis of the neutral glycan moiety of glycosyl phosphatidylinositol from porcine thyroid cells

Metabolic labelling of inositolphosphate glycan with radioactive precursors is not sufficient to characterize and assess the involvement of the glycosyl phosphatidylinositol/inositolphosphate glycan (GPI/IPG) system in porcine thyroid cell signal transduction machinery. A protocol is described for the isolation and purification of free GPI using differential polarity of lipids and sequential thin layer chromatography. The purification until homogeneity of GPI constitutes a required step for gas chromatographic analysis. Next, successive chemical treatments allowed us to remove the neutral glycan moiety of thyroidal GPI, and its composition was obtained by gas chromatography. The proposed structure is consistent with data available for GPI anchor, but differs from compositional analysis data reported for insulin-sensitive GPI. Our results support the existence in porcine thyroid cells of the GPI/IPG system, which can take part in TSH-dependent signal transduction processes.

Glycosyl phosphatidylinositol (GPI)/inositolphosphate glycan (IPG): an intracellular signalling system involved in the control of thyroid cell proliferation

In porcine thyrocytes, TSH alone does not induce cell growth. Recently, it has been demonstrated that acute stimulation by TSH of porcine thyrocytes leads to release an inositolphosphate glycan (IPG) described as a putative second messenger for various growth factors in different cell types. IPG isolated from porcine thyrocytes induces proliferation of fibroblasts EGFR T17 and porcine thyrocytes. In porcine thyrocytes we have confirmed that cell growth requires the presence of both TSH and insulin. This effect is reproduced by 8-bromo cyclic AMP suggesting a mediation by intracellular cyclic AMP. Cooperative effects between 8-bromo cyclic AMP and IPG have also been evidenced and are in favour of a crosstalk between distinct signalling pathways.

Purification of guinea pig YKL40 and modulation of its secretion by cultured articular chondrocytes

The aim of this study was to purify, characterize, and study the regulation at the chondrocyte level of the guinea pig (gp) homologue of human (R) YKL40, a putative marker of arthritic disorders. Studying YKL40 in guinea pigs is of particular interest, as age-related osteoarthritis develops in this species spontaneously. Both N-terminal sequencing and total amino acid composition of gpYKL40 purified from the secretion medium of cultured articular chondrocytes indicate a high degree of identity with hYKL40. gpYKL40 was found to contain complex N-linked carbohydrate, as demonstrated by N-glycosidase F and endoglycosidase F digestion. Isoelectric focusing demonstrated the presence of a major band at pI 6.7. The secretion of gpYKL40 by confluent articular chondrocytes in the extracellular medium was studied by immunoblotting. gpYKL40 was released by chondrocytes continuously over a 7 day period and did not appear to be degraded by proteinases, as its signal intensity in cell-free medium at 37 degrees C did not decrease with time. Thus, gpYKL40 displays high stability and accumulates in extracellular medium without reaching a steady-state level. Among the main factors known to regulate cartilage metabolism, IL-1beta, TNF-alpha, bFGF, or 1,25(OH)2D3 did not alter the basal level of gpYKL40, and retinoic acid had a slight inhibitory effect; TGF-beta and IGF-I and -II dose-dependently and inversely modulated this basal level. TGF-beta at 5 ng/ml decreased extracellular gpYKL40 2.9-fold, whereas IGF-I and IGF-II at 50 ng/ml increased extracellular gpYKL40 3.6- and 3.4-fold, respectively. The present biochemical and biological findings give new insights for studying the function of YKL40 in cartilage.

The role of glycosyl-phosphatidylinositol in signal transduction

Glycosyl-phosphatidylinositol (GPI) lipids have a structural role as protein anchors to the cell surface. In addition, they are implicated in hormone, growth factor and cytokine signal transduction. Their phosphodiesteric hydrolysis mediated by an activated phospholipase results in the generation of water soluble oligosaccharide species termed the inositol phosphoglycan (IPG). This product has been demonstrated to possess biological properties when added exogenously to cells mimicking the biological effects of a variety of extracellular ligands. This may be accomplished since IPG is generic for a family of closely related species which are released in a tissue-specific manner and additionally have cell-specific targets. Micro-organic synthesis has recently been able to shed new light on this topic by the introduction of defined oligosaccharide analogues of IPG for the assessment of their biological activity. These have complemented the findings observed with purified IPG from biological sources thus strengthening the belief that the GPI/IPG signalling system represents a truly novel aspect of transmembrane signalling.

Regulation of host cell function by glycosylphosphatidylinositols of the parasitic protozoa

Antigenic variation, antigenic drift, molecular mimicry, intracellular localization and sequestration in privileged sites are important mechanisms of immune evasion by infectious organisms. Added to this however is the phenomenon by which pathogens deliberately regulate host cell function by the production of glycolipids with agonistic or antagonistic signal transduction capacity. Such pro-active glycolipids are often pathogenicity factors, but they also serve as immunomodulators and immunosuppressants, and these activities may serve as mechanisms of immune evasion. Here we review glycosylphosphatidylinositols and related structures, a novel class of glycolipid common to eukaryotic parasites and their hosts, which recent studies suggest may play a role in immune evasion and immunosuppression by regulating host cell function via the activation or suppression of endogenous host signalling pathways.

An inositolphosphate glycan released by TGF-beta mimics the proliferative but not the transcriptional effects of the factor and requires functional receptors

Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional polypeptide that regulates a number of cellular processes including cell growth and deposition of extracellular matrix protein. Despite the fact that the signal transduction by TGF-beta has been intensively studied, the molecular mechanisms of that pathway are not clear. We have studied the possibility that an inositolphosphate glycan (IPG) is involved in transmission of the TGF-beta 1 signal. We show that TGF-beta 1 induces IPG release in both rabbit articular chondrocytes (RAC), which are growth stimulated by the factor and Mv1Lu cell line, which is growth inhibited. This release requires functional TGF-beta heteromeric receptors in these two cell types. We also demonstrate that IPG mimics TGF-beta 1-induced growth stimulation in mesenchymal cells (+100%) and growth inhibition in epithelial cells (-80%). Moreover TGF-beta receptor I (T beta R-I) is not required for inhibition of proliferation induced by IPG since derivated mutants of the Mv1Lu cell line lacking T beta R-I intracellular domain (R-1B) are significantly inhibited (-65%). Additionally, we show that IPG does not take part in the signalling pathway that leads to activation of matrix gene transcription. These results suggest that TGF-beta effects on growth regulation and extracellular matrix synthesis implicate two different signalling pathways, IPG being only involved in growth regulation.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.

Transforming growth factor beta 1 induces mitogenesis in fetal rat brown adipocytes

The presence of transforming growth factor beta 1 (TGF-beta 1) for 24 or 48 h stimulated DNA synthesis, the percentage of cells in the S + G2/M phases of the cell cycle, and cell number, as compared to quiescent cells. The mitogenic capacity of TGF-beta 1 (1 pM) was similar to that shown by 10% fetal calf serum (FCS). TGF-beta 1 for 48 h increased by 5-fold the percentage of cells containing (3H)thymidine-labeled nuclei as compared to quiescent cels. In addition, single fetal brown adipocytes, showing their typical multilocular fat droplets phenotype, become positive for (3H)thymidine-labeled nuclei in response to TGF-beta 1. Moreover, TGF-beta 1 induced the mRNA expression of a complete set of proliferation-related genes, such as c-fos (30 min), c-myc and beta-actin (2 h), and H-ras, cdc2 kinase, and glucose 6-phosphate dehydrogenase (G6PD) at 4 and 8 h, as compared to quiescent cells. Concurrently, TGF-beta 1 for 12 h increased the protein content of proliferating cellular nuclear antigen (PCNA) by 6-fold and p21-ras by 2-fold. Although our results demonstrate that TGF-beta 1 induces the expression of very early genes related to cell proliferation, TGF-beta 1 could be acting either as a mitogen or as a survival factor in induce proliferation to fetal brown adipocytes.

Role of glycosyl-phosphatidylinositol hydrolysis as a mitogenic signal for epidermal growth factor

We have investigated the role of the hydrolysis of glycosyl-phosphatidylinositol (GPI) as one of the signalling pathways elicited after interaction of epidermal growth factor (EGF) with its specific plasma membrane receptor (EGFR). Endogenous GPI was characterized in both NIH 3T3 mouse fibroblast cells and in EGFR-transfected NIH 3T3 cells (designated EGFR T17). GPI molecules isolated from both cell lines were identical and they incorporated radioactivity from both sugar and fatty acid substrates. Incubation of EGFR T17 cells with EGF, produced a rapid and transient hydrolysis of GPI. Maximum hydrolysis occurred after a 1-min incubation with 50 nM EGF. No such effects of EGF were observed in the parental cell line. Both inositol phosphoglycan (IPG)- and EGF-induced cell proliferation was inhibited in the presence of an IPG-antibody to different extents. The relationship between GPI hydrolysis and the activity of the EGFR was studied using the tyrosine kinase inhibitors tyrphostin (RG50864) and genistein. These agents were able to significantly inhibit EGF-mediated cell proliferation, EGF-dependent hydrolysis of GPI and EGF-regulated autophosphorylation of the EGFR. It is concluded that GPI hydrolysis is one of the earliest intracellular events generated in response to EGF.

Erythropoietin stimulates glycosylphosphatidylinositol hydrolysis in rat erythroid progenitor cells and inositolphosphate glycan modulates their proliferation

The involvement of a glycosylphosphatidylinositol/inositolphosphate glycan (GPI/IPG) system in the erythropoietin (Epo) signal transduction was investigated. Endogenous GPI was evidenced in extracts of normal Epo-responsive cells after incorporation of [3H]glucosamine, [3H]inositol and [32P]orthophosphate. Incubation of these cells with Epo produced a rapid and transient hydrolysis of GPI with parallel release of IPG. IPG production was Epo dose dependent and the maximal effect was obtained with the same concentration of Epo which gave the maximal mitogenic effect, i.e. 1 U/ml. The number and size of erythroid colonies (CFU-E) were increased by the addition of purified rat erythroid IPG to the culture medium, but not to the same extent as with a maximal Epo treatment. Exogenous IPG effect was dose dependent. In the presence of suboptimal Epo concentrations, IPG has been found to potentiate Epo-induced CFU-E growth. These results support the hypothesis that a GPI/IPG based signal transduction system may be involved in Epo-induced cell proliferation.

Effects of transforming growth factor beta 1 on the adenylyl cyclase-cAMP pathway in prostate cancer

We reported previously that MATLyLu rat prostate cancer cells engineered to overproduce transforming growth factor beta 1 (TGF beta 1) produce larger, more metastatic tumors in vivo. We recognized that this ability of TGF beta 1 to act as a positive modulator of prostate tumor behavior might be due to effects of TGF beta 1 on the host and/or on the tumor cells. In this study we demonstrated that the cells themselves respond to endogenously produced TGF beta 1, and that the adenylyl cyclase (AC)-cAMP pathway is affected. TGF beta 1-overproducing cells had lower membrane AC activity, lower intracellular cAMP content, and a lower Gs alpha protein level than did control cells. Prostate cancer cells were growth inhibited by 8-bromo-cAMP or forskolin, agents that elevate intracellular cAMP. Thus, TGF beta 1 overproduction affects the phenotype of the tumor cells, deliberate activation of endogenously produced latent TGF beta 1 is not required (indicating that the cells themselves are capable of activating latent TGF beta 1), and TGF beta 1 overproduction lowers the cellular concentration of the growth inhibitor cAMP. Therefore, TGF beta 1 overproduction could affect tumor behavior in vivo in part via a direct effect on the tumor cells.

Caveolae: where incoming and outgoing messengers meet

Plasmalemmal caveolae were first identified as an endocytic compartment in endothelial cells, where they appear to move molecules across the cell by transcytosis. More recently, they have been found to be sites where small molecules are concentrated and internalized by a process called potocytosis. A growing body of biochemical and morphological evidence indicates that a variety of molecules known to function directly or indirectly in signal transduction are enriched in caveolae. This raises the possibility that a third function for caveolae is to process hormonal and mechanical signals for the cell. Insights gained from studying potocytosis suggest several different ways that this membrane specialization might function to integrate incoming and outgoing cellular messages.