LRIG1 is a triple threat: ERBB negative regulator, intestinal stem cell marker and tumour suppressor.

In baseball parlance, a triple threat is a person who can run, hit and throw with aplomb. Leucine-rich repeats and immunoglobulin-like domains 1 (LRIG1) is a cell surface protein that antagonises ERBB receptor signalling by downregulating receptor levels. Over 10 years ago, Hedman et al postulated that LRIG1 might be a tumour suppressor. Recently, Powell et al provided in vivo evidence substantiating that claim by demonstrating that Lrig1 loss in mice leads to spontaneously arising, highly penetrant intestinal adenomas. Interestingly, Lrig1 also marks stem cells in the gut, suggesting a potential role for Lrig1 in maintaining intestinal epithelial homeostasis. In this review, we will discuss the ability of LRIG1 to act as a triple threat: pan-ERBB negative regulator, intestinal stem cell marker and tumour suppressor. We will summarise studies of LRIG1 expression in human cancers and discuss possible related roles for LRIG2 and LRIG3.

Antioxidants enhance the cytotoxicity of chemotherapeutic agents in colorectal cancer: a p53-independent induction of p21WAF1/CIP1 via C/EBPbeta.

Colorectal cancer (CRC) is the second leading cause of cancer deaths in the United States. Five-fluorouracil (5FU) remains the single most effective treatment for advanced disease, despite a response rate of only 20%. Herein, we show that the antioxidants pyrrolidinedithiocarbamate and vitamin E induce apoptosis in CRC cells. This effect is mediated by induction of p21WAF1/CIP1, a powerful inhibitor of the cell cycle, through a mechanism involving C/EBPbeta (a member of the CCAAT/enhancer binding protein family of transcription factors), independent of p53. Antioxidants significantly enhance CRC tumor growth inhibition by cytotoxic chemotherapy in vitro (5FU and doxorubicin) and in vivo (5FU). Thus, chemotherapeutic agents administered in the presence of antioxidants may provide a novel therapy for colorectal cancer.

Apical enrichment of human EGF precursor in Madin-Darby canine kidney cells involves preferential basolateral ectodomain cleavage sensitive to a metalloprotease inhibitor.

EGF precursor (proEGF) is a member of the family of membrane-anchored EGF-like growth factors that bind with high affinity to the epidermal growth factor receptor (EGFR). In contrast to human transforming growth factor-alpha precursor (proTGFalpha), which is sorted basolaterally in Madin-Darby canine kidney (MDCK) cells (Dempsey, P., and R. Coffey, 1994. J. Biol. Chem. 269:16878-16889), we now demonstrate that human proEGF overexpressed in MDCK cells is found predominantly at the apical membrane domain under steady-state conditions. Nascent proEGF (185 kD) is not sorted but is delivered equally to the apical and basolateral membranes, where it is proteolytically cleaved within its ectodomain to release a soluble 170-kD EGF form into the medium. Unlike the fate of TGFalpha in MDCK cells, the soluble 170-kD EGF species accumulates in the medium, does not interact with the EGFR, and is not processed to the mature 6-kD peptide. We show that the rate of ectodomain cleavage of 185-kD proEGF is fourfold greater at the basolateral surface than at the apical surface and is sensitive to a metalloprotease inhibitor, batimastat. Batimastat dramatically inhibited the release of soluble 170-kD EGF into the apical and basal medium by 7 and 60%, respectively, and caused a concordant increase in the expression of 185-kD proEGF at the apical and basolateral cell surfaces of 150 and 280%, respectively. We propose that preferential ectodomain cleavage at the basolateral surface contributes to apical domain localization of 185-kD proEGF in MDCK cells, and this provides a novel mechanism to achieve a polarized distribution of cell surface membrane proteins under steady-state conditions. In addition, differences in disposition of EGF and TGFalpha in polarized epithelial cells offer a new conceptual framework to consider the actions of these polypeptide growth factors.

Extracellular matrix regulates whey acidic protein gene expression by suppression of TGF-alpha in mouse mammary epithelial cells: studies in culture and in transgenic mice.

Whey acidic protein (WAP) is an abundant rodent milk protein. Its expression in mouse mammary epithelial cell cultures was previously found to require the formation of an extracellular matrix (ECM)-induced three-dimensional alveolar structure. In the absence of such structures, cells were shown to secrete diffusible factors leading to suppression of WAP expression. We demonstrate here that (a) TGF-alpha production and secretion by mammary cells is downregulated by the basement membrane-dependent alveolar structure, and (b) compared with beta-casein, WAP expression is preferentially inhibited both in culture and in transgenic mice when TGF-alpha is added or overexpressed. Thus, (c) the enhanced TGF-alpha production when cells are not in three-dimensional structures largely accounts for the WAP-inhibitory activity found in the conditioned medium. Since this activity can be abolished by incubating the conditioned medium with a function blocking antibody to TGF-alpha. The data suggest that ECM upregulates WAP by downregulating TGF-alpha production. We also propose that changes in TGF-alpha activity during mouse gestation and lactation could contribute to the pattern of temporal expression of WAP in the gland. These results provide a clear example of cooperation among lactogenic hormones, ECM, and locally acting growth factors in regulation of tissue-specific gene expression.

Overexpression of transforming growth factor alpha in psoriatic epidermis.

Transforming growth factor alpha (TGF-alpha) is produced by and required for the growth of epithelial cells and is angiogenic in vivo. Since epidermal hyperplasia and angiogenesis are hallmarks of psoriasis, TGF-alpha gene expression was analyzed in epidermal biopsies of normal and psoriatic skin. TGF-alpha messenger RNA and protein are much more abundant in lesional psoriatic epidermis than in normal-appearing skin of psoriatic patients or in normal epidermis. In contrast, messenger RNA levels of transforming growth factor beta 1 (TGF-beta 1), which inhibits epithelial cell growth, are not significantly different in normal, uninvolved, and lesional psoriatic epidermis. Thus, psoriatic epidermal hyperplasia may involve increased expression of a keratinocyte mitogen (TGF-alpha) rather than deficient expression of a growth inhibitor (TGF-beta 1).

Oncogenic K-RAS subverts the antiapoptotic role of N-RAS and alters modulation of the N-RAS:gelsolin complex.

Activating mutations in members of the RAS family of genes are among the most common genetic events in human tumorigenesis. Once thought to be functionally interchangeable, it is increasingly recognized that the classical members of this protein family (H-RAS, N-RAS and K-RAS4B) exhibit unique and shared functions that are highly context-dependent. Herein, we demonstrate that the presence of an oncogenic KRAS allele results in elevated levels of GTP-bound N-RAS (N-RAS.GTP) in two human colorectal cancer cell lines, HCT 116 and DLD-1, compared to their isogenic counterparts in which the mutant KRAS allele has been disrupted by homologous recombination. N-RAS subserves an antiapoptotic role in cells expressing wild-type K-RAS; this function is compromised, however, by the presence of mutant K-RAS, and these cells display increased sensitivity to apoptotic stimuli. We additionally identify a physical interaction between N-RAS and gelsolin, a factor that has been shown to promote survival and show that the N-RAS:gelsolin complex is modulated differently in wild-type and mutant K-RAS environments following apoptotic challenge. These findings represent the first biochemical evidence of a functional relationship between endogenous RAS proteins and identify a dynamic physical interaction between endogenous N-RAS and gelsolin that correlates with survival.

Localization of transforming growth factor alpha and its receptor in gastric mucosal cells. Implications for a regulatory role in acid secretion and mucosal renewal.

Transforming growth factor alpha (TGF alpha) shares with epidermal growth factor (EGF) structural homology (35%), a common cell-surface membrane receptor (TGF alpha/EGF receptor), and a nearly identical spectrum of biological activity, including inhibition of gastric acid secretion. Herein, we report expression of TGF alpha mRNA in normal gastric mucosa of the adult guinea pig, rat, and dog. TGF alpha mRNA was also detected in matched surgically resected gastric mucosa and adjacent gastric carcinoma from 10 patients, and in gastric mucosa adjacent to a benign ulcer from an additional patient. TGF alpha protein was quantitated by radioimmunoassay and was present in tumor and adjacent mucosa. TGF alpha/EGF receptor mRNA was also detected in gastric mucosa from all species studied. Localization of TGF alpha and TGF alpha/EGF receptor mRNA expression was examined in samples of unfractionated guinea pig gastric mucosa and from chief cell-enriched and parietal cell-enriched fractions. All samples exhibited TGF alpha and TGF alpha/EGF receptor expression. The TGF alpha signal was greatest in the parietal cell fraction (5.8-fold increase), but was also enhanced in the chief cell fraction (1.9-fold increase) relative to the unfractionated gastric mucosa. Like TGF alpha expression, TGF alpha/EGF receptor mRNA expression was most intense in the parietal cell-enriched fraction (7.8-fold increase), but was also increased in the chief cell-enriched fraction (2.7-fold increase) relative to the unfractionated guinea pig gastric mucosa. We conclude that TGF alpha and TGF alpha/EGF receptor genes are expressed in normal adult mammalian gastric mucosa. These findings, when interpreted in light of described actions of TGF alpha and EGF, provide evidence that local production of TGF alpha could play an important role in the regulation of acid secretion and mucosal renewal in the stomach.

Induction of heparin-binding epidermal growth factor-like growth factor mRNA in rat kidney after acute injury.

Previous studies have suggested that EGF or other members of the EGF family of mitogenic proteins are involved in proliferation of renal tubular epithelial cells occurring during recovery from injury to the kidney. The present studies examined whether expression of mRNA for the recently identified heparin-binding EGF-like growth factor (HB-EGF) is regulated in response to renal injury induced by either ischemia/reperfusion or mercuric chloride. Increased expression of HB-EGF mRNA was demonstrated in the post-ischemic kidney within 45 min of unilateral ischemia/reperfusion in the rat. Induction of HB-EGF mRNA occurred only when ischemia was followed by reperfusion, and was not eliminated by removal of blood cells from the post-ischemic kidney by saline perfusion. In situ hybridization with 35S-labeled antisense riboprobes of HB-EGF indicated that compared with control, there was increased HB-EGF mRNA expression in the 6 h post-ischemic kidney in the inner cortex and outer medulla in a patchy distribution, with the greatest expression in the inner stripe of the outer medulla. Expression occurred primarily in tubular epithelial cells. Recombinant human HB-EGF stimulated [3H]-thymidine incorporation in both primary cultures of rabbit proximal tubule cells and NRK 52E normal rat kidney epithelial cells, with potency similar to that of EGF. Induction of HB-EGF mRNA was observed in tubules freshly isolated from rat renal cortex or outer medulla when the tubules were subjected to reoxygenation after incubation in anoxic conditions. The nephrotoxin, mercuric chloride, also caused induction of HB-EGF mRNA both in vivo and in isolated rat cortical tubules. The anoxia/reoxygenation-induced expression of HB-EGF mRNA in isolated tubules was inhibited by the free radical scavengers, di- and tetra-methylthiourea, indicating involvement of reactive oxygen species. These findings indicate that HB-EGF mRNA is inducible in the kidney in vivo by acute tubular injury and suggest that HB-EGF may act as an autocrine/paracrine growth factor involved in proliferation of tubular epithelial cells and repair of the kidney.

Hepatic processing of transforming growth factor beta in the rat. Uptake, metabolism, and biliary excretion.

Transforming growth factor beta (TGF beta), a recently discovered polypeptide, modulates growth of normal and neoplastic cells. Since little is known concerning in vivo disposition of TGF beta, we performed studies to examine the hepatic processing of biologically active 125I-TGF beta in the rat. After intravenous injection, 125I-TGF beta disappeared from the plasma with an initial t1/2 of 2.2 min; partial hepatectomy delayed the plasma disappearance of 125I-TGF beta by 80%. 60 min after intrafemoral injection, 63% of the recovered label was present in liver and/or bile; by 90 min, most of the label removed by the liver (83%) had been slowly excreted into bile. Nearly all the label in bile (96%) was soluble in trichloracetic acid and not immunoprecipitable by specific antiserum. Colchicine and vinblastine inhibited cumulative biliary excretion of label by 28 and 37%, respectively; chloroquine and leupeptin each increased the amount of label in bile that was precipitable by trichloracetic acid and that coeluted with authentic 125I-TGF beta on molecular sieve chromatography. There was efficient first-pass hepatic extraction of 125I-TGF beta (36%) in the isolated perfused rat liver, which was inhibited by unlabeled TGF beta (but not by epidermal growth factor, EGF) and by lectins in a dose-dependent manner; prolonged fasting also decreased clearance (26%). After fractionation of liver by differential or isopycnic centrifugation, radiolabel codistributed with marker enzymes for lysosomes. The results indicate rapid, extensive, inhibitable, and organ-selective extraction of TGF beta by the liver. After extraction, TGF beta undergoes efficient transhepatic transport, extensive intracellular metabolism, and slow but complete biliary excretion of its metabolites. Liver fractionation studies and pharmacologic manipulations suggest that these processes are associated with organelles that include microtubules and lysosomes. The data suggest that the liver is a major target tissue or site of metabolism for biologically active TGF beta.

Transforming growth factor alpha protection against drug-induced injury to the rat gastric mucosa in vivo.

This study was designed to determine whether transforming growth factor alpha (TGF alpha) protects rat gastric mucosa against ethanol- and aspirin-induced injury. Systemic administration of TGF alpha dose-dependently decreased 100% ethanol-induced gastric mucosal injury; a dose of 50 micrograms/kg delivered intraperitoneally 15 min before ethanol decreased macroscopic mucosal injury by > 90%. At the microscopic level, TGF alpha prevented deep gastric necrotic lesions and reduced disruption of surface epithelium. Pretreatment with orogastric TGF alpha (200 micrograms/kg) only partially (40%) decreased macroscopic ethanol damage. Intraperitoneal administration of TGF alpha at a dose of 10 micrograms/kg, which does not significantly inhibit gastric acid secretion, decreased aspirin-induced macroscopic damage by > 80%. TGF alpha protection does not seem to be mediated by prostaglandin, glutathione, or ornithine decarboxylase-related events, as evidenced by lack of influence of the inhibition of their production. Pretreatment with the sulfhydryl blocking agent N-ethylmaleimide partially abolished (40%) the protective effect of TGF alpha. In addition, systemic administration of TGF alpha resulted in a two-fold increase in tyrosine phosphorylation of phospholipase C-gamma 1 and in a time- and dose-dependent increase in levels of immunoreactive insoluble gastric mucin; these events occurred in a time frame consistent with their participation in the protective effect of TGF alpha.

Inhibition of human colon cancer cell growth by selective inhibition of cyclooxygenase-2.

A considerable amount of evidence collected from several different experimental systems indicates that cyclooxygenase-2 (COX-2) may play a role in colorectal tumorigenesis. Large epidemiologic studies have shown a 40-50% reduction in mortality from colorectal cancer in persons taking aspirin or other nonsteroidal antiinflammatory drugs on a regular basis. One property shared by all of these drugs is their ability to inhibit COX, a key enzyme in the conversion of arachidonic acid to prostaglandins. Two isoforms of COX have been characterized, COX-1 and COX-2. COX-2 is expressed at high levels in intestinal tumors in humans and rodents. In this study, we selected two transformed human colon cancer cell lines for studies on the role of COX-2 in intestinal tumorigenesis. We evaluated HCA-7 cells which express high levels of COX-2 protein constitutively and HCT-116 cells which lack COX-2 protein. Treatment of nude mice implanted with HCA-7 cells with a selective COX-2 inhibitor (SC-58125), reduced tumor formation by 85-90%. SC-58125 also inhibited colony formation of cultured HCA-7 cells. Conversely, SC-58125 had no effect on HCT-116 implants in nude mice or colony formation in culture. Here we provide evidence that there may be a direct link between inhibition of intestinal cancer growth and selective inhibition of the COX-2 pathway.

Helicobacter pylori upregulates expression of epidermal growth factor-related peptides, but inhibits their proliferative effect in MKN 28 gastric mucosal cells.

Acute exposure to Helicobacter pylori causes cell damage and impairs the processes of cell migration and proliferation in cultured gastric mucosal cells in vitro. EGF-related growth factors play a major role in protecting gastric mucosa against injury, and are involved in the process of gastric mucosal healing. We therefore studied the acute effect of H. pylori on expression of EGF-related growth factors and the proliferative response to these factors in gastric mucosal cells (MKN 28) derived from gastric adenocarcinoma. Exposure of MKN 28 cells to H. pylori suspensions or broth culture filtrates upregulated mRNA expression of amphiregulin (AR) and heparin-binding EGF-like growth factor (HB-EGF), but not TGFalpha. This effect was specifically related to H. pylori since it was not observed with E. coli, and was independent of VacA, CagA, PicA, PicB, or ammonia. Moreover, H. pylori broth culture filtrates stimulated extracellular release of AR and HB-EGF protein by MKN 28 cells. AR and HB-EGF dose-dependently and significantly stimulated proliferation of MKN 28 cells in the absence of H. pylori filtrate, but had no effect in the presence of H. pylori broth culture filtrates. Inhibition of AR- or HB-EGF- induced stimulation of cell growth was not mediated by downregulation of the EGF receptor since EGF receptor protein levels, EGF binding affinity, number of specific binding sites for EGF, or HB-EGF- or AR-dependent tyrosine phosphorylation of the EGF receptor were not significantly altered by incubation with H. pylori broth culture filtrates. Increased expression of AR and HB-EGF were mediated by an H. pylori factor > 12 kD in size, whereas antiproliferative effects were mediated by both VacA and a factor < 12 kD in size. We conclude that H. pylori increases mucosal generation of EGF-related peptides, but in this acute experimental model, this event is not able to counteract the inhibitory effect of H. pylori on cell growth. The inhibitory effect of H. pylori on the reparative events mediated by EGF-related growth factors might play a role in the pathogenesis of H. pylori-induced gastroduodenal injury.

Targeted disruption of the Kvlqt1 gene causes deafness and gastric hyperplasia in mice.

The KvLQT1 gene encodes a voltage-gated potassium channel. Mutations in KvLQT1 underlie the dominantly transmitted Ward-Romano long QT syndrome, which causes cardiac arrhythmia, and the recessively transmitted Jervell and Lange-Nielsen syndrome, which causes both cardiac arrhythmia and congenital deafness. KvLQT1 is also disrupted by balanced germline chromosomal rearrangements in patients with Beckwith-Wiedemann syndrome (BWS), which causes prenatal overgrowth and cancer. Because of the diverse human disorders and organ systems affected by this gene, we developed an animal model by inactivating the murine Kvlqt1. No electrocardiographic abnormalities were observed. However, homozygous mice exhibited complete deafness, as well as circular movement and repetitive falling, suggesting imbalance. Histochemical study revealed severe anatomic disruption of the cochlear and vestibular end organs, suggesting that Kvlqt1 is essential for normal development of the inner ear. Surprisingly, homozygous mice also displayed threefold enlargement by weight of the stomach resulting from mucous neck cell hyperplasia. Finally, there were no features of BWS, suggesting that Kvlqt1 is not responsible for BWS.

Selective inhibition of growth-related gene expression in murine keratinocytes by transforming growth factor beta.

Transforming growth factor beta (TGF beta) is a potent inhibitor of epithelial cell proliferation. A nontumorigenic epidermal growth factor (EGF)-dependent epithelial cell line, BALB/MK, is reversibly growth arrested by TGF beta. TGF beta will also abrogate EGF-stimulated mitogenesis of quiescent BALB/MK cells. Increased levels of calcium (greater than 1.0 mM) will induce differentiation in BALB/MK cells; in contrast, TGF beta-mediated growth inhibition does not result in induction of terminal differentiation. In the present study, the effects of TGF beta and calcium on growth factor-inducible gene expression were examined. TGF beta markedly decreased c-myc and KC gene expression in rapidly growing BALB/MK cells and reduced the EGF induction of c-myc and KC in a quiescent population of cells. TGF beta exerted its control over c-myc expression at a posttranscriptional level, and this inhibitory effect was dependent on protein synthesis. TGF beta had no effect on c-fos gene expression, whereas 1.5 mM calcium attenuated EGF-induced c-fos expression in quiescent cells. Expression of beta-actin, however, was slightly increased in both rapidly growing and EGF-restimulated quiescent BALB/MK cells treated with TGF beta. Thus, in this system, TGF beta selectively reduced expression of certain genes associated with cell proliferation (c-myc and KC), and at least part of the TGF beta effect was at a posttranscriptional level.

Synergistic interaction of the Neu proto-oncogene product and transforming growth factor alpha in the mammary epithelium of transgenic mice.

Transgenic mice expressing either the neu proto-oncogene or transforming growth factor (TGF-alpha) in the mammary epithelium develop spontaneous focal mammary tumors that occur after a long latency. Since the epidermal growth factor receptor (EGFR) and Neu are capable of forming heterodimers that are responsive to EGFR ligands such as TGF-alpha, we examined whether coexpression of TGF-alpha and Neu in mammary epithelium could cooperate to accelerate the onset of mammary tumors. To test this hypothesis, we interbred separate transgenic strains harboring either a mouse mammary tumor virus/TGF-alpha or a mouse mammary tumor virus/neu transgene to generate bitransgenic mice that coexpress TGF-alpha and neu in the mammary epithelium. Female mice coexpressing TGF-alpha and neu developed multifocal mammary tumors which arose after a significantly shorter latency period than either parental strain alone. The development of these mammary tumors was correlated with the tyrosine phosphorylation of Neu and the recruitment of c-Src to the Neu complex. Immunoprecipitation and immunoblot analyses with EGFR- and Neu-specific antisera, however, failed to detect physical complexes of these two receptors. Taken together, these observations suggest that Neu and TGF-alpha cooperate in mammary tumorigenesis through a mechanism involving Neu and EGFR transactivation.

Identification of rCop-1, a new member of the CCN protein family, as a negative regulator for cell transformation.

By using a model system for cell transformation mediated by the cooperation of the activated H-ras oncogene and the inactivated p53 tumor suppressor gene, rCop-1 was identified by mRNA differential display as a gene whose expression became lost after cell transformation. Homology analysis indicates that rCop-1 belongs to an emerging cysteine-rich growth regulator family called CCN, which includes connective-tissue growth factor, CYR61, CEF10 (v-src inducible), and the product of the nov proto-oncogene. Unlike the other members of the CCN gene family, rCop-1 is not an immediate-early gene, it lacks the conserved C-terminal domain which was shown to confer both growth-stimulating and heparin-binding activities, and its expression is lost in cells transformed by a variety of mechanisms. Ectopic expression of rCop-1 by retroviral gene transfers led to cell death in a transformation-specific manner. These results suggest that rCop-1 represents a new class of CCN family proteins that have functions opposing those of the previously identified members.

Complex regulation of transforming growth factor beta 1, beta 2, and beta 3 mRNA expression in mouse fibroblasts and keratinocytes by transforming growth factors beta 1 and beta 2.

Regulation of transforming growth factor beta 1 (TGF beta 1), TGF beta 2, and TGF beta 3 mRNAs in murine fibroblasts and keratinocytes by TGF beta 1 and TGF beta 2 was studied. In quiescent AKR-2B fibroblasts, in which TGF beta induces delayed stimulation of DNA synthesis, TGF beta 1 autoregulation of TGF beta 1 expression was observed as early as 1 h, with maximal induction (25-fold) after 6 to 12 h. Increased expression of TGF beta 1 mRNA was accompanied by increased TGF beta protein production into conditioned medium of AKR-2B cells. Neither TGF beta 2 nor TGF beta 3 mRNA, however, was significantly induced, but both were apparently down regulated at later times by TGF beta 1. Protein synthesis was not required for autoinduction of TGF beta 1 mRNA in AKR-2B cells. Nuclear run-on analyses and dactinomycin experiments indicated that autoregulation of TGF beta 1 expression is complex, involving both increased transcription and message stabilization. In contrast to TGF beta 1, TGF beta 2 treatment of quiescent AKR-2B cells increased expression of TGF beta 1, TGF beta 2, and TGF beta 3 mRNAs, but with different kinetics. Autoinduction of TGF beta 2 mRNA occurred rapidly with maximal induction at 1 to 3 h, enhanced TGF beta 3 mRNA levels were observed after 3 h, and increased expression of TGF beta 1 occurred later, with maximal mRNA levels obtained after 12 to 24 h. Nuclear run-on analyses indicated that TGF beta 2 regulation of TGF beta 2 and TGF beta 3 mRNA levels is transcriptional, while TGF beta 2 induction of TGF beta 1 expression most likely involves both transcriptional and posttranscriptional controls. In BALB/MK mouse keratinocytes, minimal autoinduction of TGF beta 1 occurred at only the 12- and 24-h time points and protein synthesis was required for this autoinduction. The results of this study provide an example in which TGF beta 1 and TGF beta 2 elicit different responses and demonstrate that expression of TGF beta 1, and TGF beta 3 are regulated differently. The physiological relevance of TGF beta 1 autoinduction in the context of wound healing is discussed.

Differential trafficking of transforming growth factor-beta receptors and ligand in polarized epithelial cells.

Epithelial cells in vivo form tight cell-cell associations that spatially separate distinct apical and basolateral domains. These domains provide discrete cellular processes essential for proper tissue and organ development. Using confocal imaging and selective plasma membrane domain activation, the type I and type II transforming growth factor-beta (TGFbeta) receptors were found to be localized specifically at the basolateral surfaces of polarized Madin-Darby canine kidney (MDCK) cells. Receptors concentrated predominantly at the lateral sites of cell-cell contact, adjacent to the gap junctional complex. Cytoplasmic domain truncations for each receptor resulted in the loss of specific lateral domain targeting and dispersion to both the apical and basal domains. Whereas receptors concentrate basolaterally in regions of direct cell-cell contact in nonpolarized MDCK cell monolayers, receptor staining was absent from areas of noncell contact. In contrast to the defined basolateral polarity observed for the TGFbeta receptor complex, TGFbeta ligand secretion was found to be from the apical surfaces. Confocal imaging of MDCK cells with an antibody to TGFbeta1 confirmed a predominant apical localization, with a stark absence at the basal membrane. These findings indicate that cell adhesion regulates the localization of TGFbeta receptors in polarized epithelial cultures and that the response to TGFbeta is dependent upon the spatial distribution and secretion of TGFbeta receptors and ligand, respectively.

Production of transforming growth factors by human colon cancer lines.

Three human colon cancer lines (SW 480, SW 620, WIDR) were characterized as to their production of molecules with transforming growth factor (TGF)-like activity. Production of both TGF alpha-like and TGF beta-like activity was quantitated, as were cellular receptors for these molecules, and growth response in soft agar to exogenous epidermal growth factor (EGF) (as a substitute for TGF alpha) and TGF beta. Serum-free medium conditioned by these cells showed differing amounts of TGF alpha-like and TGF beta-like competing activity in EGF and TGF beta radioreceptor assays. Likewise the cells showed differing abilities to bind 125I-labeled EGF and TGF beta. SW 620 cells produced relatively large quantities of TGF alpha-like activity and had no detectable EGF receptors; specific TGF beta binding was observed. SW 480 cells produced the most TGF beta-like activity and had no measurable TGF beta membrane receptors, but EGF receptors were detectable. WIDR cells had both EGF and TGF beta membrane receptors and produced relatively low levels of EGF and TGF beta receptor-competing activity. All three of the cell lines grew spontaneously in soft agar (in medium containing 10% serum). In contrast to other carcinoma cell lines, exogenous EGF and TGF beta had no significant effect on soft agar growth of the colon carcinoma cells. The production of both TGF alpha-like and TGF beta-like polypeptides by colon carcinoma cell lines has been shown, yet involvement of these factors in autostimulatory activity could not be demonstrated. The possibility that these endogenous factors could be involved in paracrine stimulation of stromal cells remains to be explored.

Role of transforming growth factor beta 1 in induction of colon carcinoma differentiation by hexamethylene bisacetamide.

The differentiation agent hexamethylene bisacetamide (HMBA) increased expression of transforming growth factor beta 1 (TGF beta 1) mRNA in HT29 colon carcinoma cells. The increase was evident after 24 h and was maintained at levels 4-5-fold the control levels for at least 5-13 days. No increase in expression of TGF beta 2 or TGF alpha mRNA was observed. Both TGF beta 1 and HMBA induced loss of expression of a cell surface malignancy marker on HT29 cells, and both decreased cell growth in serum-free medium. Exogenously applied TGF beta 1 mimicked the growth-arresting effect of HMBA on three surgically resected moderately differentiated colon carcinomas in serum-free primary culture. Both TGF beta 1 and HMBA increased the tumor growth fraction in a second group of three more aggressive colon carcinomas, while neither agent had any measurable growth-modulating activity on two other colon carcinomas. The induction of TGF beta 1 mRNA by HMBA along with the parallel biological effects of HMBA and exogenously applied TGF beta 1 on resected carcinomas and on HT29 cells suggest that the effects of HMBA on colon carcinoma cells may be mediated in part by induction of TGF beta 1.