Inhibition of CACO-2 cell proliferation by (n-3) fatty acids: possible mediation by increased secretion of insulin-like growth factor binding protein-6

Recent insights into the actions of IGFBP-6

IGFBP-6 is an O-linked glycoprotein that preferentially binds IGF-II over IGF-I. It is a relatively selective inhibitor of IGF-II actions including proliferation, survival and differentiation of a wide range of cells. IGFBP-6 has recently been shown to have a number of IGF-independent actions, including promotion of apoptosis in some cells and inhibition of angiogenesis. IGFBP-6 also induces migration of tumour cells including rhabdomyosarcomas by an IGF-independent mechanism. This chemotactic effect is mediated by MAP kinases. IGFBP-6 binds to prohibitin-2 on the cell surface and the latter is required for IGFBP-6-induced migration by a mechanism that is independent of MAP kinases. IGFBP-6 may enter the nucleus and modulate cell survival and differentiation. IGFBP-6 expression is decreased in a number of cancer cells and it has been postulated to act as a tumour suppressor. IGFBP-6 expression is increased in a smaller number of cancers, which may reflect a compensatory mechanism to control IGF-II actions or IGF-independent actions. The relative balance of IGF-dependent and IGF-independent actions of IGFBP-6 in vivo together with the related question regarding the roles of IGFBP-6 binding to IGF and non-IGF ligands are keys to understanding the physiological role of this protein.

The multifaceted effects of omega-3 polyunsaturated Fatty acids on the hallmarks of cancer

Omega-3 polyunsaturated fatty acids, in particular eicosapentaenoic acid, and docosahexaenoic acid have been shown to have multiple beneficial antitumour actions that affect the essential alterations that dictate malignant growth. In this review we explore the putative mechanisms of action of omega-3 polyunsaturated fatty acid in cancer protection in relation to self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, apoptosis, limitless replicative potential, sustained angiogenesis, and tissue invasion, and how these will hopefully translate from bench to bedside.

Insulin-like growth factor-binding protein-6 and cancer

The IGF (insulin-like growth factor) system is essential for physiological growth and it is also implicated in a number of diseases including cancer. IGF activity is modulated by a family of high-affinity IGF-binding proteins, and IGFBP-6 is distinctive because of its marked binding preference for IGF-II over IGF-I. A principal role for IGFBP-6 is inhibition of IGF-II actions, but recent studies have indicated that IGFBP-6 also has IGF-independent effects, including inhibition of angiogenesis and promotion of cancer cell migration. The present review briefly summarizes the IGF system in physiology and disease before focusing on recent studies on the regulation and actions of IGFBP-6, and its potential roles in cancer cells. Given the widespread interest in IGF inhibition in cancer therapeutics, increasing our understanding of the mechanisms underlying the actions of the IGF ligands, receptors and binding proteins, including IGFBP-6, will enhance our ability to develop optimal treatments that can be targeted to the most appropriate patients.

Conjugated linoleic acid synthesis-related protein proteasome subunit α 5 (PSMA5) is increased by vaccenic acid treatment in goat mammary tissue

This study was conducted to identify proteins associated with the endogenous synthesis of conjugated linoleic acid (CLA) from trans-vaccenic acid (TVA; trans-11 C18:1, a precursor for CLA endogenous synthesis) in mammary tissues. Six lactating goats were divided into 2 groups. One group was given an intravenous bolus injection of TVA (150mg) twice daily over 4 d; the other group received saline injections. Treatment with TVA increased the concentration of cis-9,trans-11 CLA and TVA in goat milk. Additionally, TVA treatment increased the expression of stearoyl-CoA desaturase (SCD) in mammary tissue. Using 2-dimensional gel electrophoresis and electrospray ionization quadrupole time-of-flight mass spectrometry, 3 proteins affected by infusions of TVA were identified. Proteasome (prosome, macropain) subunit α type 5 (PSMA5) was upregulated, whereas peroxiredoxin-1 and translationally controlled tumor protein 1 were downregulated in TVA-treated animals compared with the vehicle-injected controls. Only the effect of TVA on PSMA5 could be confirmed by Western blot analysis. To further explore the regulation of PSMA5 in mammary epithelial cells when TVA is converted into CLA, we used a differentiated bovine mammary epithelial cell line treated with TVA for 6h. Changes in cis-9,trans-11 CLA concentrations and mRNA expression patterns of both SCD and PSMA5 were monitored. The concentration of cis-9,trans-11 CLA increased after TVA treatment. The mRNA expression level of PSMA5 was significantly elevated to 6h, but SCD mRNA expression only increased in 2h after TVA treatment. These results indicate that PSMA5 is highly expressed in goat mammary tissue and bovine mammary epithelial cells when TVA is converted into CLA. Our data suggest that PSMA5 protein is associated with CLA biosynthesis in mammary tissue.

Cholesterol ester and free fatty acids are modulated by policosanol in CaCo-2 intestinal cells

OBJECTIVE

To investigate policosanol absorption by brush border membrane (BBM), metabolism in CaCo-2 enterocytes, and transport of policosanol metabolites across the basolateral membrane (BLM). It was hypothesized that policosanol is partially oxidized into fatty acids and then is incorporated into other lipids.

METHODS

Policosanol was emulsified with phosphatidylcholine in the culture medium. The viability of cells was assessed via an MTT (3-[4,5]dimethylthiazol-2-yl-2,5-diphenyltetrazolim) assay. Control cells received only the same amount of "vehicle" (phosphatidylcholine) without policosanol. CaCo-2 cell monolayer and medium were collected; lipid was extracted and analyzed by thin-layer chromatography (TLC) and gas liquid chromatography (GLC).

RESULTS

Eighty-six percent of policosanol added to the cell culture medium was absorbed after 48 hours' incubation. The amount of cholesterol ester fatty acid was significantly increased both in the cells and in the basolateral medium, and was reduced in the apical medium. Policosanol increased the quantity of free fatty acids in the basolateral medium and reduced the free fatty acid content of CaCo-2 cells. Further evaluation of lipid profiles indicated that policosanol modulated the fatty acid profile of cholesterol ester in the basolateral medium.

CONCLUSION

It was concluded that policosanol or policosanol metabolites may modulate lipid metabolism and/or transport following absorption by the BBM, partial oxidation by the intestinal epithelium, and transport of policosanol metabolites across the BLM.

trans-10,cis-12 Conjugated Linoleic Acid Inhibits the G1–S Cell Cycle Progression in DU145 Human Prostate Carcinoma Cells

Conjugated linoleic acid (CLA) is a group of positional and geometric isomers of linoleic acid, and has evidenced anti-cancer activities in experimental animal cancer models and in vitro studies. The two predominant isomers of CLA are cis-9,trans-11 CLA (c9t11) and trans-10,cis-12 CLA (t10c12). The present study was performed to study the effect of the individual CLA isomers on DU145 cell growth. The cells were incubated in serum-free medium with different concentrations of the fatty acids. Treatment of cells with t10c12 (at 2.5-10 micromol/L) resulted in a dose-dependent reduction in the numbers of viable cells, whereas c9t11 CLA at a concentration of 5 micromol/L slightly increased viable cell numbers at 3 days (P < .05). DNA flow cytometric analysis revealed that the treatment of DU145 cells with t10c12 for 24 hours induced a small but significant increase in the number of cells in the G1 phase, accompanied by a complementary decrease in cells in the S phase. c9t, however, had no effect on cell cycle progression. To determine the molecular mechanisms underlying t10c12-induced G1 arrest, the levels of cell cycle regulatory proteins were estimated by western blot analyses. t10c12 induced a marked increase in p21(CIP1/WAF1) protein levels in a dose-dependent manner. p27(KIP1) was not affected by t10c12. t10c12 moderately decreased cyclin A and cyclin D1 protein levels (P > .05). However, t10c12 did not affect the expression of cyclin-dependent kinase (CDK) 2, CDK4, or cyclin E. t10c12 increased p21(CIP1/WAF1) bound to CDK2 and attenuated CDK2 activity. These results indicate that t10c12-induced p21(CIP1/WAF1) binds to CDK, and inhibits the activity of this enzyme, which results in the observed decrease in the G1-S progression in DU145 cells.

Trans-10,cis-12, not cis-9,trans-11, conjugated linoleic acid inhibits G1-S progression in HT-29 human colon cancer cells

Commercial preparations of conjugated linoleic acid (CLA) contain both positional and geometric isomers of octadecadienoic acid, with cis-9,trans-11 CLA (c9t11) and trans-10,cis-12 CLA (t10c12) as the principal isomers. We showed previously that CLA reduced the incidence of colon tumors in rats treated with 1,2-dimethylhydrazine. In addition, our previous in vitro studies showed that t10c12 inhibited the growth of HT-29 and Caco-2 human colon cancer cells, whereas c9t11 had no effect on cell growth. In the present study, to examine the effects of the CLA isomers on cell cycle and cell cycle regulatory proteins, we treated HT-29 cells with various concentrations (0-4 micromol/L) of the individual CLA isomers. A DNA flow cytometric analysis revealed that t10c12 induced a G1 arrest, whereas c9t11 had no effect on the cell cycle. Western blot analysis of total cell lysates revealed no alteration in the protein expression of cyclin A, cyclin D, cyclin E, cyclin-dependent kinase (CDK) 2, or CDK4 due to t10c12 treatment. However, t10c12 substantially increased the protein expression and mRNA accumulation of the CDK inhibitor p21(CIP1/WAF1). The t10c12 isomer increased the association of p21(CIP1/WAF1) with CDK2 and proliferating cell nuclear antigen, but decreased the levels of phosphorylated retinoblastoma protein (Rb), with an increase in the levels of hypophosphorylated Rb protein. An in vitro kinase assay using histone H1 as a substrate showed that the activities of CDK2 were significantly decreased by t10c12. These results indicate that t10c12 exerts its growth inhibitory effects in colon cancer cells through the induction of G1 cell cycle arrest. The induction of p21(CIP1/WAF1) may be one of the mechanisms by which t10c12 inhibits cell cycle progression in HT-29 cells.

Application of a continuous bioreactor cascade to study the effect of linoleic acid on hybridoma cell physiology

The aim of the present study is to demonstrate the use of controlled bioreactors for toxicological studies. As a model system the effect of linoleic acid on hybridoma cells is studied in two well-controlled continuously operated bioreactors placed in series. In the first reactor the effect on rapid proliferating cells can be studied, while in the second reactor a special steady state is created, which allows studying the effect on apoptotic cells. Experiments are done at 0, 25, and 50 microM linoleic acid. At the end of the experiment with 50 microM linoleic acid, the concentration of linoleic acid is increased stepwise to determine the cytotoxic level. For rapid proliferating cells exposed to 25 and 50 microM stimulation of growth was observed. At 50 microM there was at the same time an increase in cell death through apoptosis. For stressed apoptotic cells linoleic acid caused partial growth inhibition at 25 and 50 microM and arrest of cell proliferation in the G(2)/M phase at 50 microM. For both, rapid proliferating cells and stressed apoptotic cells, complete growth inhibition occurred at 85 microM, with cells being arrested in the G(2)/M phase and dying mainly through necrosis. Cells in the bioreactor system appeared to be more sensitive towards linoleic acid than cells grown in multi-well plates. (IC(50) = 300 microM; IC(100) = 400 microM). Altogether the results of the present study reveal that the biostat experiments allow detailed analysis of the effect of a bioactive ingredient on cell physiology and behavior.

Inhibition of colon cancer cell proliferation by the dietary compound conjugated linoleic acid is mediated by the CDK inhibitor p21CIP1/WAF1

Our previous studies indicated that dietary conjugated linoleic acid (CLA) inhibits colon tumor cell proliferation in vitro and in vivo. To identify mechanisms by which CLA regulates growth arrest, the HT-29 human colon carcinoma cell line was treated with various physiological concentrations of CLA and analyzed by flow cytometry. We detected a dose-dependent increase in the percentage of cells arrested in G1 after CLA treatment that was accompanied by induction of the cyclin dependent kinase (CDK) inhibitor p21CIP1/WAF. CLA addition also led to increased p21 expression in HCT116 and SW480 cells, indicating that p21 induction is a general consequence of CLA treatment in colon cancer cells. Since both HT-29 and SW480 cells have mutant p53, our data indicate that p53 is not essential for induction of p21. In addition to an increase in p21 levels, HT-29 cell growth arrest was also accompanied by moderate decreases in Cyclin A, D1, E, and proliferating cell nuclear antigen (PCNA) levels. Following CLA treatment, p21 associated with and inhibited CDK4 and CDK2, and this correlated with reduced phosphorylation of retinoblastoma proteins. Increased association of p21 with PCNA was also detected. Dietary CLA inhibits cell cycle progression by inducing p21, which negatively regulates the growth promoting activities of CDK/cyclins and PCNA. These studies indicate that physiological concentrations of CLA inhibit growth of colon cancer cells with either wild-type or mutant p53, and may have therapeutic benefits in vivo.

Trans-10,cis-12, not cis-9,trans-11, conjugated linoleic acid decreases ErbB3 expression in HT-29 human colon cancer cells

AIM: To examine whether trans-10,cis-12 CLA (t10c12) or cis-9,trans-11 CLA (c9t11) inhibits heregulin (HRG)-β-stimulated cell growth and HRG-β-ErbB3 signaling in HT-29 cells.

METHODS: We cultured HT-29 cells in the absence or presence of the CLA isomers and/or the ErbB3 ligand HRG-β. MTT assay, [³H]thymidine incorporation, Annexin V staining, RT-PCR, Western blotting, immunoprecipitation, and in vitro kinase assay were performed.

RESULTS: HRG-β increased cell growth, but did not prevent t10c12-induced growth inhibition. T10c12 inhibited DNA synthesis and induced apoptosis of HT-29 cells, whereas c9t11 had no effect. T10c12 decreased the levels of ErbB1, ErbB2, and ErbB3 proteins and transcripts in a dose-dependent manner, whereas c9t11 had no effect. Immunoprecipitation/Western blot studies revealed that t10c12 inhibited HRG-β-stimulated phosphorylation of ErbB3, recruitment of the p85 subunit of phosphoinositide 3-kinase (PI3K) to ErbB3, ErbB3-associated PI3K activities, and phosphorylation of Akt. However, c9t11 had no effect on phospho Akt levels. Neither t10c12 nor c9t11 had any effect on HRG-β-induced phosphorylation of ERK-1/2.

CONCLUSION: These results indicate that the inhibition of HT-29 cell growth by t10c12 may be induced via its modulation of ErbB3 signaling leading to inhibition of Akt activation.

IGFBP-6 five years on; not so 'forgotten'?

Insulin-like growth factor binding protein (IGFBP)-6 is unique among IGFBPs for its IGF-II binding specificity. IGFBP-6 inhibits growth of a number of IGF-II-dependent cancers, including rhabdomyosarcoma, neuroblastoma and colon cancer. Although the major action of IGFBP-6 appears to be inhibition of IGF-II actions, a number of studies suggest that it may also have IGF-independent actions. Gene array studies show regulation of IGFBP-6 in many circumstances that are consistent with antiproliferative actions. However, other studies show the opposite, so that IGFBP-6 may be acting as a counter-regulator in these situations or it may have other as yet undetermined actions. Both the N-terminal and C-terminal domains of IGFBP-6 contribute to high affinity IGF binding, and the C-terminal domain appears to confer its IGF-II specificity. The three-dimensional structure of the C-domain of IGFBP-6 contains a thyroglobulin type 1 fold, and the IGF-II binding site is located in the proximal half of this domain adjacent to the glycosaminoglycan binding site. Future studies are needed to further delineate the putative IGF-independent actions of IGFBP-6 and to build on the structural information to enhance our understanding of this IGFBP. This is particularly significant since IGFBP-6 provides an attractive basis for therapy of IGF-II-dependent tumors.

The role of the insulin-like growth factor system in colorectal cancer: review of current knowledge

BACKGROUND

The insulin-like growth factor system, which includes insulin-like growth factors (IGF-I and IGF-II), IGF receptors (IGF-IR and IGF-IIR) and IGF binding proteins (IGFBPs), plays an important role in epithelial growth, anti-apoptosis and mitogenesis. There is a growing body of evidence showing that IGFs control growth and proliferation of several types of cancer. This review introduces the latest information on the biology of the IGF system and its pathophysiological role in the development of colorectal cancer.

DISCUSSION

The growth promoting effects of IGF-I and IGF-II on cancer cells are mediated through the IGF-IR, which is a tyrosine kinase and cancer cells with a strong tendency to metastasise have a higher expression of the IGF-IR. Most of the IGFs in circulation are bound to the IGFBPs, which regulate the bioavailability of the IGFs. All IGFBPs inhibit IGF action by high affinity binding, while some of them also potentiate the effects of IGFs. Colon cancer cells produce specific proteases that degrade the IGFBP so that the IGF will be free to act on the cancer cell in an autocrine manner. Therefore, the IGFBPs play a crucial role in the development of the cancer.

CONCLUSION

The current knowledge about the link between IGFs and colon cancer is mainly based on in vitro investigations. Further in vivo study is needed to understand the exact role of the IGF system, especially its binding proteins, so that they can be manipulated for the prevention and treatment of colorectal cancer.

n-3 Polyunsaturated fatty acids and colon cancer prevention

The incidence of colon cancer in industrialised countries has increased since the early 1970s. It is estimated that more than one-third of cases are associated with factors related to a Western diet. Both the type and amount of dietary fats consumed have been implicated in colon cancer aetiology. Recent studies have demonstrated that n-3 polyunsaturated fatty acids (PUFAs), commonly found in fish oil (FO), could prevent colon cancer development. Evidences show that n-3 PUFAs act at different stages of cancer development and through several mechanisms including the modulation of arachidonic acid-derived prostaglandin synthesis, and Ras protein and protein kinase C expression and activity. As a result, n-3 PUFAs limit tumour cell proliferation, increase apoptotic potential along the crypt axis, promote cell differentiation and possibly limit angiogenesis. The modulatory actions of n-3 PUFAs on the immune system and their anti-inflammatory effects might also play a role in reducing colon carcinogenesis. There remains, nevertheless, some ambiguity over the safety of n-3 PUFAs with respect to secondary tumour formation. However, it appears that n-3 PUFAs may be of use in colon cancer prevention.

trans-10, cis-12 conjugated linoleic acid reduces insulin-like growth factor-II secretion in HT-29 human colon cancer cells

We previously demonstrated that a mixture of conjugated linoleic acid (CLA) isomers decreases colon cancer incidence in rats treated with 1,2-dimethylhydrazine. Our in vitro studies have also shown that CLA inhibits the growth of HT-29 cells, a human colon cancer cell line. When we compared the individual potencies of the two main isomers found in the mixture of CLA isomers (e.g., cis-9, trans-11 [c9t11] and trans-10, cis-12 [t10c12]), t10c12 CLA decreased viable cell numbers in a dose-dependent manner. By contrast, c9t11 CLA had no effect. Therefore, the present study examined whether the decreased cell growth is related to changes in secretion of insulin-like growth factor (IGF)-II and/or IGF-binding proteins (IGFBPs) that have been shown to regulate HT-29 cell proliferation. Cells were incubated in serum-free medium with various concentrations of the individual CLA isomers, and immunoblot analysis of 24-hour, serum-free, conditioned media using a monoclonal anti-IGF-II antibody was performed. HT-29 cells secreted both mature 7,500 apparent molecular weight (M(r)) and higher-M(r) forms of IGF-II. t10c12 CLA decreased the levels of the higher-M(r) and the mature form of IGF-II in a dose-dependent manner, whereas c9t11 CLA had no effect. Ligand blot analysis of conditioned medium using (125)I-IGF-II revealed that the production of IGFBP-2 and IGFBP-4 was also decreased by t10c12 CLA, whereas c9t11 CLA had no effect. Exogenous IGF-II abrogated the growth inhibition induced by t10c12 CLA. These results indicate that inhibition of HT-29 cell growth by t10c12 CLA may be mediated by decreasing IGF-II secretion in these cells.

Conjugated linoleic acid downregulates insulin-like growth factor-I receptor levels in HT-29 human colon cancer cells

Conjugated linoleic acid (CLA) has chemoprotective properties in a variety of experimental cancer models. We have previously observed that dietary CLA inhibits colon tumorigenesis induced by 1,2-dimethylhydrazine in rats. In addition, our in vitro studies have shown that CLA inhibits DNA synthesis and induces apoptosis in HT-29 cells, the human colon adenocarcinoma cell line. The insulin-like growth factor (IGF) system regulates the growth of HT-29 cells by an autocrine mechanism. The present study examined whether the growth inhibitory effect of CLA is related to changes in the IGF system in HT-29 cells. To determine whether CLA inhibits IGF-II production, HT-29 cells were incubated in serum-free medium in the presence of various concentrations of CLA. CLA decreased protein levels of both mature and pro IGF-II and IGF-II transcripts. Whereas exogenous IGF-I and IGF-II produced an increase in cell number, neither IGF-I nor IGF-II counteracted the negative growth regulatory effect of CLA. Reverse transcriptase-polymerase chain reaction and Western blot analysis of total cell lysates revealed that CLA decreased IGF-I receptor (IGF-IR) transcript and protein levels in a dose-dependent manner. Immunoprecipitation/Western blot studies revealed that CLA inhibited IGF-I-induced phosphorylation of IGF-IR and insulin-receptor substrate (IRS)-1, recruitment of the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3K) to IGF-IR, IGF-IR-associated PI3K activity, and phosphorylated Akt and extracellular signal-regulated kinase (ERK)-1/2 levels. In conclusion, the inhibition of cell proliferation and induction of apoptosis by CLA in HT-29 cells may be mediated in part by its ability to decrease IGF-II synthesis and to downregulate IGF-IR signaling and the PI3K/Akt and ERK-1/2 pathways.

Trans-10,cis-12-conjugated linoleic acid inhibits Caco-2 colon cancer cell growth

A commercially available mixture of conjugated linoleic acid (CLA) isomers decreases colon cancer cell growth. We compared the individual potencies of the two main isomers in this mixture [cis-9,trans-11 (c9t11) and trans-10,cis-12 (t10c12)] and assessed whether decreased cell growth is related to changes in secretion of insulin-like growth factor II (IGF-II) and/or IGF-binding proteins (IGFBPs), which regulate Caco-2 cell proliferation. Cells were incubated in serum-free medium with different concentrations of the individual CLA isomers. t10c12 CLA dose dependently decreased viable cell number (55 +/- 3% reduction 96 h after adding 5 microM t10c12 CLA). t10c12 CLA induced apoptosis and decreased DNA synthesis, whereas c9t11 CLA had no effect. Immunoblot analysis of 24-h serum-free conditioned medium using a monoclonal anti-IGF-II antibody revealed that Caco-2 cells secreted both a mature 7,500 molecular weight (M(r)) IGF-II and higher M(r) forms of IGF-II. The levels of the higher M(r) and the mature form of IGF-II were decreased 50 +/- 3% and 22 +/- 2%, respectively, by 5 microM t10c12 CLA. c9t11 CLA had no effect. Ligand blot analysis of conditioned medium using 125I-labeled IGF-II revealed that t10c12 CLA slightly decreased IGFBP-2 production; c9t11 CLA had no effect. Exogenous IGF-II reversed t10c12 CLA-induced growth inhibition and apoptosis. These results indicate that CLA-inhibited Caco-2 cell growth is caused by t10c12 CLA and may be mediated by decreasing IGF-II secretion in Caco-2 cells.

Dietary n-6 and n-3 polyunsaturated fatty acids and colorectal carcinogenesis: results from cultured colon cells, animal models and human studies

During the past few decades, many studies have been conducted to evaluate the effects of n-6 and n-3 polyunsaturated fatty acids (PUFAs) on colorectal carcinogenesis. This report provides a brief overview of the recent studies that have been performed in cultured colon cells, animal models as well as of the population-based and short-term biomarker studies with humans. No differential effect between n-6 and n-3 PUFAs has been observed in vitro. Results from animal models indicate that n-6 PUFAs have a tumor enhancing effect, predominantly during the post-initiation phase. n-3 PUFAs may protect against colorectal carcinogenesis during both the initiation and post-initiation phase. Population-based human studies show little or no associations between n-6 or n-3 PUFA intake and colorectal cancer. Short-term biomarker studies in humans suggest though that fish oil (FO) supplementation with high amounts of n-3 PUFAs may protect against colorectal carcinogenesis and that n-6 PUFA supplementation may increase the risk.

Inhibition of Caco-2 cell proliferation by all-trans retinoic acid: role of insulin-like growth factor binding protein-6

The present study examined the effects of all-trans retinoic acid (tRA) on proliferation and expression of the IGF system in Caco-2 human colon adenocarcinoma cells. tRA inhibited Caco-2 cell proliferation in a dose-dependent manner, with a 40 +/- 2% decrease in cell number observed 48 h after the addition of 1 microM tRA. Ligand blot analysis of IGFBPs in conditioned media revealed that Caco-2 cells produced three IGFBPs of M(r): 34,000 (IGFBP-2), 24,000 (IGFBP-4), and 32,000 (IGFBP-6). The concentrations of IGFBP-2 and IGFBP-4 decreased by 48 +/- 6 and 70 +/- 13%, respectively, whereas that of IGFBP-6 increased by 698 +/- 20% with 1 microM tRA. tRA decreased mRNA levels of IGFBP-2 and IGFBP-4 by 20 +/- 3 and 50 +/- 8%, respectively, whereas tRA increased IGFBP-6 mRNA by 660 +/- 20%. tRA did not alter levels of IGF-II mRNA or peptide. To examine if endogenous IGFBP-6 inhibits cell proliferation, Caco-2 cells were transfected with an IGFBP-6 cDNA expression construct or pcDNA3 vector only and stable clones were selected. Clones overexpressing IGFBP-6 grew more slowly than vector controls and achieved final densities 30-55% lower than those of vector controls. Accumulation of IGFBP-6 mRNA and concentrations of IGFBP-6 peptide in conditioned media were increased by 200-250 and 220-250%, respectively, in the IGFBP-6 clones compared with controls. Increased expression of IGFBP-6, which has a high binding affinity for IGF-II, following tRA treatment suggests that the decreased proliferation caused by tRA may result, at least in part, from IGFBP-6-mediated disruption of the IGF-II autocrine loop in these colon cancer cells.

Synthetic low-calcaemic vitamin D(3) analogues inhibit secretion of insulin-like growth factor II and stimulate production of insulin-like growth factor-binding protein-6 in conjunction with growth suppression of HT-29 colon cancer cells

The aims of the present study were to compare the ability of various synthetic analogues of 1 alpha,25-dihydroxyvitamin D(3) [1 alpha,25-(OH)(2)D(3)] to inhibit proliferation of HT-29 cells, a human colon adenocarcinoma cell line. HT-29 cells were incubated for 144 h with various concentrations (0-100 nM) of 1 alpha,25-(OH)(2)D(3), or the analogues EB1089, CB1093 or 1 beta,25-(OH)(2)D(3). All these analogues except 1 beta,25-(OH)(2)D(3) inhibited cell proliferation, but relative potencies and efficacies of EB1089 and CB1093 were much greater than that of the native vitamin. Cells grew in serum-free medium, reaching a plateau density at day 10 of culture, and addition of 10 nM 1 alpha,25-(OH)(2)D(3) or 1 beta,25-(OH)(2)D(3) did not alter the long-term growth characteristics of HT-29 cells. However, cells treated with 10 nM EB1089 or CB1093 grew at a rate slower than control and reached final densities that were 53+/-1 and 36+/-2% lower than control, respectively. Immunoblot analysis of serum-free conditioned medium using a monoclonal anti-insulin-like growth factor-(IGF)-II antibody showed that both 10 nM EB1089 and CB1093 markedly inhibited secretion of both mature 7500 M(r) and higher M(r) forms of IGF-II. Ligand blot and immunoblot analyses of conditioned media revealed the presence of IGFBPs of M(r) 24,000 (IGFBP-4), 30,000 (glycosylated IGFBP-4), 35,000 (IGFBP-2) and 32,000-34,000 (IGFBP-6). The level of IGFBP-2 was decreased by 42+/-8 and 49+/-7% by 10 nM EB 1089 and CB1093, respectively, compared to controls. IGFBP-6 was increased approximately twofold by EB1089 and CB1093, and exogenously added IGFBP-6 inhibited HT-29 cell proliferation. These results suggest that inhibition of HT-29 cell proliferation by EB1089 and CB1093 may be attributed, at least in part, to the decreased secretion of IGF-II. The increase in IGFBP-6 concentration coupled with its high affinity for IGF-II may also contribute to decreased cellular proliferation by an indirect mechanism involving sequestration of endogenously produced IGF-II.