Regulation of expression and activity of four PKC isozymes in confluent and mechanically stimulated UMR-108 osteoblastic cells

The transcript (mRNA), protein levels, enzyme activity, and cellular localization of four protein kinase C (PKC) isozymes identified in rat osteogenic sarcoma cells (UMR-108) were studied at confluent density and during mechanical stress (cyclic stretch). Western blot analysis indicated that growth to confluent density significantly increased the protein levels of cPKC-alpha (11.6-fold), nPKC-delta (5.3-fold), and nPKC-epsilon (22.0-fold) but not aPKC-zeta. Northern blot analysis indicated a significant (2.3-fold) increase in the 10 kb transcript of cPKC-alpha, a slight (1.3-fold) increase in that of nPKC-epsilon but no detectable change in that of the remaining isozymes. Enzyme activity assays of the individually immunoprecipitated isozymes yielded detectable kinase activity only for PKC-alpha, PKC-delta, and PKC-epsilon and only in confluent cells, corroborating the selective increase of these isozymes at confluent density. The UMR-108 cells showed a dramatic orientation response to mechanical stress with cell reshaping and alignment of the cell long axis perpendicular to the axis of force, remodeling of the actin cytoskeleton, and the appearance of multiple peripheral sites which stained for actin, vinculin, and PKC in separate experiments. Longer term mechanical stress beyond 24 h, however, resulted in no significant change in the mRNA level, protein level, or enzyme activity of any of the four PKC isozymes investigated. The results indicate that there are isozyme-selective increases in the protein levels of PKC isozymes of osteoblastic UMR-108 cells upon growth to confluence which may be regulated at the transcriptional or the post-transcriptional level. The results from UMR-108 cells support the earlier proposal (Carvalho RS, Scott JE, Suga DM, Yen EH. 1994. J Bone Miner Res 9(7):999-1011) that PKC could be involved in the early phase of mechanotransduction in osteoblasts through the activation of focal adhesion assembly/disassembly and the remodeling of the actin cytoskeleton.

Recent advances in the use of vitamin A (retinoids) in the prevention and treatment of cancer

Vitamin A, its physiologic metabolites, and synthetic derivatives (retinoids) have been shown to have protective effects against the development of certain types of cancer. In addition, pharmacologic amounts of retinoids have been used with some success in the treatment of a few human tumors. The chemoprevention effect of retinoids is most likely exerted at the tumor-promotion phase of carcinogenesis. Retinoids block tumor promotion by inhibiting proliferation, inducing apoptosis, inducing differentiation, or a combination of these actions. Clinically, isotretinoin (13-cis-retinoic acid) significantly decreases the incidence of second primary tumors in patients with head-and-neck cancer and reduces appearance of non-melanoma skin cancer in patients with xeroderma pigmentosum. Retinoic acid has proved to be an effective treatment for promyelocytic leukemia. However, retinoid resistance limits its use as a single agent. Clinical trials are in progress to determine the efficacy of retinoids in treating other types of cancer such as neuroblastoma and breast carcinoma. The development of receptor-selective retinoids and selective inhibitors of retinoid metabolism may lead to further use of retinoids in both chemoprevention and treatment of cancer.

Effect of receptor-selective retinoids on growth and differentiation pathways in mouse melanoma cells

Treatment of B16 mouse melanoma cells with all-trans-retinoic acid (ATRA) results in inhibition of cell proliferation and induction of differentiation. Accompanying these events is an induction of retinoic acid receptor beta (RARbeta) expression, an increase in protein kinase Calpha (PKCalpha) expression, and enhanced activator protein-1 (AP-1) transcriptional activity. These cells express nuclear RARalpha and RARgamma and nuclear retinoid X receptors (RXR) alpha and beta constitutively. We tested the ability of receptor-selective retinoids to induce the biochemical changes found in ATRA-treated melanoma cells and also tested their effectiveness in decreasing anchorage-dependent and -independent growth. The RXR-selective ligand (2E,4E)-6-(5,6,7,8-tetrahydro-3,5,5,8, 8-pentamethyl-2-naphthalenyl)-3,7-dimethyl-2,4,6-octatrienoic acid (SR11246) was most effective at inhibiting anchorage-dependent growth, whereas the RARgamma-selective ligand 6-[(5,6,7, 8-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)(hydroxyimino)methyl]-2-naphthalen ecarbo xylic acid (SR11254) was most potent at inhibiting anchorage-independent growth. In contrast, 4-(5,6,7,8-tetrahydro-5,5, 8,8-tetramethyl-2-naphthalenecarboxamido)-benzoic acid (Am580), an RARalpha-selective ligand, was the most effective receptor-selective agonist for inducing RARbeta mRNA and increasing the amount of PKCalpha protein. All of the retinoids induced a concentration-dependent increase in AP-1 transcriptional activity, with little difference in effectiveness among the receptor-selective retinoids. A synergistic increase in the amount of PKCalpha was found when an RAR-selective agonist was combined with an RXR-selective agonist. One possible explanation for this result is that an RXR-RAR heterodimer in which both receptors are liganded is required for maximum expression of this critical component of the ATRA-induced differentiation pathway. Our data suggest that synthetic retinoids can activate different growth and differentiation pathways preferentially in B16 melanoma cells, due, most likely, to their ability to activate a different subset of receptors.

Cloning and characterization of the murine PKC alpha promoter: identification of a retinoic acid response element

Protein kinase C (PKC) is a family which consists of multiple isoforms whose distinct physiological roles within the cell are unknown. We have previously demonstrated that levels of PKC alpha mRNA, protein, and enzyme activity in B16 melanoma cells can be modulated by retinoic acid. We investigated this regulation by cloning and characterizing the promoter region of the murine PKC alpha gene. A 13 kb mouse genomic fragment containing the 5' flanking region, first exon, and first intron was isolated and sequenced. Two transcription initiation sites were identified at 919 and 925 bp upstream from the translation start site. The promoter region contained a TATA-like box at -93 bp upstream of the transcription start site, but no CAAT box. Promoter activity differed between cell lines and correlated with the levels of PKC alpha expressed in these cell lines. Reporter gene assays showed that the region between -179 and -452 bp likely contains a silencer element(s). The promoter activity of a -179 bp fragment in B16 cells was stimulated twofold by retinoic acid. Within this region (-93 to -65 bp) there is a retinoic acid response element. An oligonucleotide spanning this region specifically bound exogenous RAR-RXR heterodimers and endogenous RAR from B16 nuclear extracts. These results suggest that retinoic acid increases PKC alpha gene expression in B16 cells, at least in part, through direct transcriptional stimulation of its promoter.

Characterization of density-dependent regulation of the tyrosinase gene promoter: role of protein kinase C

The rate-limiting step in melanogenesis is catalyzed by tyrosinase, a multifunctional enzyme encoded by the albino locus. We have previously reported that depletion of protein kinase C by long-term treatment of B16 mouse melanoma cells with phorbol dibutyrate (PDBu) prevented cell density-dependent melanogenesis. This was accompanied by a lack of induction of tyrosinase protein and mRNA. We report here the effect of PDBu on the functional activity of the mouse tyrosinase promoter by reporter gene assay and its effect on the binding of nuclear proteins from B16 cells to the "M-box" region of the mouse tyrosinase promoter. Short-term PDBu treatment of B16 cells transfected with a mouse tyrosinase promoter-luciferase construct resulted in increased reporter gene activity, while long-term PDBu treatment inhibited reporter gene activity. Using an oligonucleotide containing the M-box and its flanking residues in electrophoretic mobility shift assays, we found a density-dependent change in the pattern of DNA-protein complexes. One complex was found to be negatively regulated by long-term PDBu treatment. Competition experiments with various mutated oligonucleotides demonstrated that both the M-box and flanking residues are important for nuclear protein binding. The complex whose formation was inhibited by long-term PDBu treatment was shown to contain the basic helix-loop-helix leucine zipper protein microphthalmia-associated transcription factor (MITF). These results suggest that chronic PDBu treatment might inhibit tyrosinase expression (and subsequent melanogenesis) by affecting the amount or function of MITF.

Effects of protein kinase C alpha overexpression on A7r5 smooth muscle cell proliferation and differentiation

Smooth muscle cell differentiation and proliferation are increasingly seen to be intimately tied to the etiology of atherosclerosis and hypertension. To determine the role of PKC alpha in the regulation of smooth muscle cell differentiation and proliferation, the rat embryonic smooth muscle cell line A7r5 was transfected with an expression vector containing the full-length PKC alpha cDNA. Neomycin-resistant clones which exhibited increased PKC alpha levels compared to wild-type cells were selected. The A7r5 cells overexpressing PKC alpha had altered morphology and decreased growth rates compared to wild-type cells and cells transfected only with the neomycin resistance gene. Electrophoretic mobility shift assays showed that nuclear extracts from overexpressing clones gave a different pattern of protein-DNA binding to an AP-1 consensus oligonucleotide compared to wild-type cells. In contrast to the growth characteristics of these clones, their levels of cell differentiation marker proteins such as vinculin and desmin were not affected by PKC alpha overexpression. Moreover, the smooth muscle-specific differentiation marker alpha-actin was markedly reduced, while beta-actin levels were found to remain unchanged. Northern blot analysis confirmed that alpha-actin downregulation occurred at the RNA level. Western blot analysis revealed that A7r5 cells have five different PKC isoforms and that these isoform protein levels were not changed by PKC alpha overexpression. These findings suggest that PKC alpha regulates growth and differentiation of A7r5 smooth muscle cells and that these changes might result from altered expression/function of AP-1 transcription factors.

Binding of type II nuclear receptors and estrogen receptor to full and half-site estrogen response elements in vitro

The mechanism by which retinoids, thyroid hormone (T3) and estrogens modulate the growth of breast cancer cells is unclear. Since nuclear type II nuclear receptors, including retinoic acid receptor (RAR), retinoid X receptor (RXR) and thyroid hormone receptor (TR), bind direct repeats (DR) of the estrogen response elements (ERE) half-site (5'-AGGTCA-3'), we examined the ability of estrogen receptor (ER) versus type II nuclear receptors, i.e. RARalpha, beta and gamma, RXRbeta, TRalpha and TRbeta, to bind various EREs in vitro . ER bound a consensus ERE, containing a perfectly palindromic 17 bp inverted repeat (IR), as a homodimer. In contrast, ER did not bind to a single ERE half-site. Likewise, ER did not bind two tandem (38 bp apart) half-sites, but low ER binding was detected to three tandem copies of the same half-site. RARalpha,beta or gamma bound both ERE and half-site constructs as a homodimer. RXRbeta did not bind full or half-site EREs, nor did RXRbeta enhance RARalpha binding to a full ERE. However, RARalpha and RXRbeta bound a half-site ERE cooperatively forming a dimeric complex. The RARalpha-RXRbeta heterodimer bound the Xenopus vitellogenin B1 estrogen responsive unit, with two non-consensus EREs, with higher affinity than one or two copies of the full or half-site ERE. Both TRalpha and TRbeta bound the full and the half-site ERE as monomers and homodimers and cooperatively as heterodimers with RXRbeta. We suggest that the cellular concentrations of nuclear receptors and their ligands, and the nature of the ERE or half-site sequence and those of its flanking sequences determine the occupation of EREs in estrogen-regulated genes in vivo .

Characterization of retinoic acid-induced AP-1 activity in B16 mouse melanoma cells

Retinoic acid (RA) induces differentiation of B16 mouse melanoma cells, which is accompanied by an increase in protein kinase Calpha (PKCalpha) as well as a selective enrichment of nuclear PKCalpha. We report here that RA also increases AP-1 activity in these cells. Transient transfection of B16 cells with luciferase reporter gene constructs indicated that RA induced a concentration-dependent increase in AP-1 activity. Acute treatment (2 h) of B16 cells with phorbol dibutyrate (PDB) increased AP-1 activity by 10-fold. RA treatment did not change the expression of Jun family members; however, it decreased the expression of c-Fos. In contrast acute PDB treatment induced c-Fos expression, while having little effect on c-Jun. Five DNA-protein complexes were formed with nuclear extracts from B16 cells and an oligonucleotide containing an AP-1 consensus sequence. Several complexes were decreased in cells treated with RA. Conversely, certain complexes were increased in cells acutely treated with PDB. The slowest migrating complexes were shown to contain Fos family members. Down-regulation of PKC inhibited both the acute PDB-induced and the RA-induced increase in AP-1 activity. The selective PKC enzyme inhibitor, bisindolylmaleimide, reduced PDB-stimulated AP-1 activity, but enhanced RA-induced AP-1 activity. These results together with our previous studies suggest the intriguing possibility that PKC protein, but not enzyme activity, may be required for RA-induced AP-1 activity.

Regulation of melanogenesis in B16 mouse melanoma cells by protein kinase C

Melanogenesis is regulated by a variety of environmental and hormonal factors. In this study, we showed that protein kinase C (PKC) plays a major role in regulating melanogenesis in B16 mouse melanoma cells. Chronic treatment of B16 cells with phorbol dibutyrate resulted in a concentration-dependent loss of density-dependent induction of tyrosinase activity, which correlated positively with a concentration-dependent loss of PKC enzyme activity. In contrast, B16 clones overexpressing PKC alpha had increased tyrosinase activity. Different phorbol derivatives inhibited tyrosinase activity and depleted cellular PKC alpha in a manner that reflected their reported tumor-promoting activity. Western blotting analysis showed that phorbol dibutyrate decreased the amount of the brown locus gene product (TRP-1) by 50% and lowered the amount of the albino locus gene product (tyrosinase) to undetectable levels. None of the phorbol derivatives affected the level of the slaty locus protein (TRP-2). The decrease in tyrosinase and TRP-1 protein levels was found to be due to a decrease in the mRNA encoded by these genes. In addition to inhibiting the density-dependent increase in tyrosinase activity, phorbol dibutyrate inhibited some, but not all, of the 8-bromocyclic AMP-induced increase in tyrosinase activity. This was accompanied by a decrease in the amount of tyrosinase protein induced by 8-bromocyclic AMP. Although 8-bromocyclic AMP did not change the level of TRP-1, it did reverse the decrease in the amount of this protein induced by phorbol dibutyrate. The amount of TRP-2 was not altered by any of these agents. These data suggest that PKC regulates melanogenesis primarily by controlling the constitutive expression of tyrosinase and, to a lesser extent, TRP-1.

Inhibition of retinoic acid receptor function and retinoic acid-regulated gene expression in mouse melanoma cells by calreticulin. A potential pathway for cyclic AMP regulation of retinoid action

Calcium is a second messenger that controls a wide variety of cellular functions. Because of its multiple actions, there is a stringent requirement for calcium homeostasis, and this is achieved in part by a system of transport and storage proteins such as calreticulin located in the endoplasmic reticulum. Calreticulin is also found in the nucleus, suggesting that it may have a role in transcriptional regulation. It has been reported that calreticulin can inhibit steroid-regulated gene transcription by preventing receptor binding to DNA. Here we report that overexpression of the calreticulin gene in B16 mouse melanoma cells resulted in a decrease in retinoic acid (RA)-stimulated reporter gene expression. Gel shift analysis showed that purified calreticulin inhibited the binding of endogenous RAR to a beta-RA response element oligonucleotide, only if added prior to the addition of the oligonucleotide. Co-immunoprecipitation studies suggest a physical interaction between RAR and calreticulin. Transfection of the calreticulin gene into B16 cells inhibited the RA induction of protein kinase Calpha, a marker of RA-induced differentiation. We also found that cyclic AMP increased the expression of calreticulin. Cyclic AMP may act to antagonize RA action by both decreasing RAR expression (Y. Xiao, D. Desai, T. Quick, and R. M. Niles, J. Cell Physiol., in press) and stimulating calreticulin levels.

Control of retinoic acid receptor expression in mouse melanoma cells by cyclic AMP

Retinoic acid receptor (RAR) alpha and gamma mRNAs were constitutively expressed in B16 melanoma cells with or without retinoic acid (RA) treatment. RAR beta mRNA, however, was significantly expressed only after exposure to RA. Induction of RAR beta by RA occurred within 1 h and was not inhibited by cycloheximide (i.e., did not require new protein synthesis). All three RAR mRNA levels were dramatically decreased with 8-bromo-cyclic AMP treatment and could not be rescued by addition of RA. Analysis of RAR gamma revealed that this decrease occurred within 1 h of exposure to 8-bromo-cyclic AMP and was not blocked by simultaneous treatment with cycloheximide. The stability of RAR gamma mRNA was not altered by cyclic AMP treatment. Nuclear extracts from 8-bromo-cyclic AMP-treated cells showed a large decrease in protein binding to a retinoic acid response element (RARE) oligonucleotide compared to control cells. This correlated with a marked reduction of RA-stimulated RARE-reporter gene activity in transfected cells which were treated with cyclic AMP. Pretreatment of B16 cells with cyclic AMP prior to RA addition dramatically reduced induction of PKC alpha, an early marker of RA-induced cell differentiation. Thus, cyclic AMP can antagonize the action of RA most likely via its ability to inhibit RAR expression.

Phorbol 12-myristate 13-acetate enhances nm23 gene expression in murine melanocytes but not in syngeneic B16-BL6 melanoma variants

The nm23 gene has been described as a potential metastasis suppressor gene in certain rodent and human tumors. We previously demonstrated that tyrosine and phenylalanine restriction suppresses metastatic heterogeneity of B16-BL6 murine melanoma and selects for tumor variants with decreased metastatic potential. In this study, we investigated nm23 expression in the highly metastatic B16-BL6 (ND) melanoma, its nutritionally derived poorly metastatic (LT) variant, and the syngeneic non-tumorigenic Mel-ab melanocytes. No differences in nm23 expression were observed between ND and LT cells, and nm23 expression varied between different isolates. Previously, we showed that metastatic potential of 1-ND cells decreases and is not altered in 1-LT cells after prolonged in vitro cell passage; however, nm23 expression is equivalently increased by 2-fold. In 2-ND and 2-LT cells, expression of nm23 is not different at higher in vitro cell passage. Expression of nm23 decreased about 2-fold when phorbol 12-myristate 13-acetate (PMA) was removed from Mel-ab cells, which induces these cells to become quiescent. Although membrane-associated protein kinase C (PKC) activity decreased after prolonged PMA treatment in all cells, neither nm23 expression nor proliferation of ND and LT cells was affected by PMA. These data indicate that nm23 expression is related to proliferative activity rather than to the suppression of metastatic potential.

The relationship between susceptibility to retinoic acid treatment and protein kinase C alpha expression in murine melanoma cell lines

Retinoic acid (RA)-induced differentiation of B16 mouse melanoma cells is accompanied by a large increase in the amount of PKCalpha protein. Overexpression of PKCalpha in these cells results in a more differentiated phenotype. To determine if these findings had general applicability to murine melanomas, we investigated the relationship between sensitivity to RA and induction of PKCalpha in three different murine melanoma cell lines. RA inhibited the anchorage-dependent growth of all three cell lines, with JB/MS being the most sensitive, S91 intermediate, and RPMI the least affected. RA also inhibited soft agar colony formation in JB/MS, but had little effect on RPMI. All cell lines expressed PKCalpha, but not beta or gamma. RA induced a large concentration-dependent increase in PKCalpha protein in JB/MS (6- to 10-fold), a smaller increase in S91 (2- to 3-fold), and very little induction of PKCalpha in RPMI. Previously we had observed that the amount of PKCalpha increased with the density of B16 cells in culture. We found that this density-dependent increase in PKCalpha occurred in three out of four melanoma cell lines examined. These results suggest that PKCalpha plays an important role in RA-induced murine melanoma cell differentiation.

Retinoic acid specifically increases nuclear PKC alpha and stimulates AP-1 transcriptional activity in B16 mouse melanoma cells

B16 melanoma cells differentiate upon treatment with retinoic acid (RA). This differentiation process is accompanied by an increase of protein kinase C alpha (PKC alpha) mRNA and protein levels. Overexpression of PKC alpha in these cells results in a more differentiated phenotype, suggesting the importance of this protein in the control of differentiation by RA. The purpose of the study reported here was to determine the subcellular distribution of the RA-induced PKC alpha, whether the RA-induced increase in PKC alpha protein levels was accompanied by an increase in in situ enzyme activity, and whether RA altered AP-1 transcriptional activity. We found that RA treatment increased PKC alpha protein levels in all subcellular compartments examined, but it also induced a selective enrichment in nuclear-associated PKC alpha levels. Treating cells with an active phorbol ester induced translocation of PKC alpha to membrane fractions, but had no effect on nuclear PKC alpha levels. RA also increased PKC enzymatic activity in intact cells as determined by phosphorylation of the PKC-specific endogenous substrate MARCKS. However, while RA induced a five- to eightfold increase in total cellular PKC alpha protein levels, it only increased MARCKS phosphorylation by twofold. In light of the increase in in situ PKC enzyme activity and the enrichment of nuclear PKC alpha, we determined whether AP-1 activity might be increased in RA-treated cells. Use of luciferase reporter gene constructs with or without AP-1 elements transfected into B16 cells indicated that RA induced a four- to fivefold increase in AP-1 transcriptional activity. These results suggest a hypothesis whereby RA-induced nuclear PKC alpha might lead to increased AP-1 activity and show that RA-induced growth inhibition and differentiation are not always accompanied by an inhibition of AP-1 activity as has been proposed by other investigators.

Expression and regulation of retinoid X receptors in B16 melanoma cells

Recently, a new subfamily of nuclear retinoid receptors that is distinct from that of RARs has been identified and named Retinoid X receptors (RXRs). These receptors specifically bind 9-cis-retinoic acid (9cisRA), but not all-trans-retinoic acid (ATRA). We determined which RXR subtypes were expressed in B16 mouse melanoma cells and then studied the effect of ATRA, 8-bromo-cyclic AMP (8BrcA), and phorbol dibutyrate (PDB) on RXR mRNA levels. ATRA induces differentiation in these cells while 8BrcA and PDB antagonize the RA-induced differentiation of B16 melanoma cells. Northern analysis demonstrated the expression of RXR alpha and RXR beta mRNA in B16 cells, but RXR gamma was not detectable. Further analysis using RT-PCR also failed to detect RXR gamma in these cells. Long-term RA treatment decreased the expression of RXR alpha, but not RXR beta mRNAs. PDB did not alter the expression of either RXR mRNAs, however, 8BrcA treatment resulted in a time dependent decrease in the amount of RXR beta, but not RXR alpha mRNA. Inhibition of protein synthesis by cycloheximide resulted in a large increase in RXR alpha and RXR beta mRNA levels. This effect of cycloheximide was time and concentration dependent with maximal stimulation of RXR alpha and RXR beta mRNAs occurring at 4 h of treatment. Inhibition of transcription with actinomycin D completely abolished the cycloheximide-induced increase of RXR beta. In contrast to its effect on other genes, such as immediate response genes, cycloheximide treatment did not increase the half-life of RXR beta mRNA. Nuclear run-on assays showed that cycloheximide treatment of intact B16 melanoma cells stimulated the transcription rate of RXR beta, but not RXR alpha. These results suggest the presence of an unstable transcription factor that negatively regulates the expression of RXR beta in B16 melanoma cells. In addition, since RXR beta is the predominant isotype in B16 cells, 8BrcA may, at least partially, inhibit RA-induced differentiation through down-regulation of this RXR.

Correlation of retinoid binding affinity to retinoic acid receptor alpha with retinoid inhibition of growth of estrogen receptor-positive MCF-7 mammary carcinoma cells

Both anchorage-dependent growth and anchorage-independent growth of the estrogen receptor-positive mammary carcinoma cell line MCF-7 are inhibited by all-trans-retinoic acid. This cell line has nuclear retinoic acid receptors (RARs) alpha and gamma. The natural retinoids all-trans-retinoic acid and 9-cis-retinoic acid and a series of 12 conformationally restricted retinoids, which showed a range of binding selectivities for these receptors and had either agonist or antagonist activity for gene transcriptional activation by the RARs, were evaluated for their abilities to inhibit anchorage-dependent (adherent) and anchorage-independent (clonal) growth of MCF-7 cells. Correlation analyses were performed to relate growth inhibition by these retinoids with their binding affinity to RAR alpha or RAR gamma. Inhibition of anchorage-dependent growth in culture after 7 days of retinoid treatment correlated with binding to RAR alpha (n = 14; P < or = 0.001) and not to RAR gamma (n = 14; P > 0.1). Both the RAR alpha-selective retinoid agonists and the two RAR antagonists that were evaluated inhibited adherent cell growth. The RAR gamma-selective agonists had very low growth inhibitory activity (< 10%) at concentrations as high as 12.5 microM. These results suggest that RAR alpha is the retinoid receptor involved in the inhibition of adherent cell growth by retinoids and that transcriptional activation by this receptor on a RAR response element does not appear to be required for this process to occur. For this series of retinoids, inhibition of anchorage-independent growth after 21 days of retinoid treatment only correlated (n = 12; P < or = 0.005) with binding affinity to RAR alpha for the retinoid agonists, although the RAR gamma-selective retinoids displayed weak activity. The RAR antagonists were very poor inhibitors of growth. These results suggest that activation of gene transcription by RAR alpha appears to be required for inhibition of anchorage-independent growth by retinoids in this estrogen receptor-positive mammary carcinoma cell line.

Use of vitamins A and D in chemoprevention and therapy of cancer: control of nuclear receptor expression and function. Vitamins, cancer and receptors

Vitamin A is metabolized to several biologically active compounds, the best known of which is retinoic acid. This compound has been shown to inhibit the growth of a variety of tumor cells and to induce a more differentiated phenotype in several tumor types. Vitamin D is metabolized to the active compound 1,25-dihydroxyvitamin D3. This vitamin is well-known for its role in maintaining calcium homeostasis in the body. Recently it has been shown that vitamin D3 can also inhibit tumor cell replication and stimulate differentiation of selected tumor types. Retinoic acid is being used clinically to treat promyelocytic leukemia, head and neck tumors as well as cervical dysplasia. Use of vitamin D3 clinically has been restricted by its affect on calcium metabolism. Recently, however, new analogs of vitamin D3 have been developed which have much less calcium mobilizing activity, yet still retain their tumor inhibitory properties. The action of both of these vitamins is mediated by nuclear receptors which have the same structure as steroid receptors. There are three nuclear retinoic acid receptors (RAR alpha, beta, and gamma), but only one vitamin D3 nuclear receptor. These receptors are expressed in very small amounts. Since the ligand should be in vast excess of receptor (ie not limiting), we explored the possibility that response to vitamin A might be mediated by control of RAR expression. Using B16 mouse melanoma cells as a model system, we found that RAR alpha and gamma mRNAs were constitutively expressed. RAR beta mRNA was induced by treatment of the cells with RA. Induction of RAR beta mRNA occurred within 1h and was not inhibited by cycloheximide. The mRNA for all three RARs was dramatically decreased with 8-bromo-cyclic AMP treatment and could not be rescued by addition of RA. Analysis of RAR gamma revealed that this decrease occurred within 1h of exposure to 8-bromo-cyclic AMP and was not blocked by simultaneous treatment with cycloheximide. Nuclear extracts from cyclic AMP-treated cells showed a large decrease in protein binding to a retinoic acid response element (RARE) oligonucleotide compared to control cells. This correlated with a marked reduction of RA-stimulated RARE-reporter gene activity in transfected cells which were treated with cyclic AMP. Pre-treatment of B16 cells with cyclic AMP prior to RA addition dramatically reduced induction of PKC alpha, an early marker of RA-induced cell differentiation. Thus, cyclic AMP can antagonize the physiological actions of RA via its ability to inhibit RAR expression.

Characterization of conventional protein kinase C (PKC) isotype expression during F9 teratocarcinoma differentiation. Overexpression of PKC alpha alters the expression of some differentiation-dependent genes

F9 teratocarcinoma is a useful model for studying early embryogenesis since these cells can differentiate into primitive or parietal endoderm under the influence of retinoic acid or retinoic acid and cyclic AMP, respectively. We have found that three isoforms of protein kinase C (PKC alpha, -beta, and -gamma) were expressed in undifferentiated stem cells. When the cells were treated with retinoic acid either alone or in the presence of cAMP for 120 h, PKC alpha mRNA and protein levels increased, whereas those of PKC beta and PKC gamma became undetectable. These changes began within 24 h of drug treatment and were complete by 48-72 h. In order to determine the functional significance of the induction of PKC alpha during F9 differentiation, we established two stable transfectants that overexpressed PKC alpha protein between 4- and 5-fold compared to wild type cells. Characterization of these cell lines revealed an altered pattern of expression of some of the markers of F9 differentiation. The clone that had the highest amount of PKC alpha protein constitutively expressed mRNA for type IV collagen and c-Jun, which are not normally expressed until 24-48 h of treatment with differentiation agents. In the other overexpressing clone, these markers were induced much faster than in wild type cells. The growth rate of both overexpressing clones was less than wild type cells, while the expression of the PKC beta protein in these clones was similar to the levels found in differentiated F9 cells. However, other markers of differentiation, including the cellular morphology and levels of pST6-135 and c-myc RNA, responded to agents identically in both wild type and PKC-alpha-overexpressing clones. Therefore, overexpression of PKC alpha is not sufficient to induce full differentiation of F9 cells. However, our data suggest that certain pathways that lead to the expression of differentiation-dependent genes are regulated by PKC alpha protein levels.

Expression of TGF-beta during in vitro differentiation of hamster tracheal epithelial cells

The control of growth and differentiation of tracheal epithelial cells is poorly understood. Retinoic acid seems to be essential for the growth and secretory cell differentiation of hamster tracheal epithelial (HTE) cells in culture. In this study, we tested the hypothesis that one way by which retinoic acid (RA) stimulates growth is by decreasing transforming growth factor beta (TGF beta) expression or activity or both. HTE cells were very sensitive to TGF beta-induced growth inhibition. TGF beta 1 was more potent than TGF beta 2 with 50% inhibition of growth achieved at a concentration less than 0.1 ng/ml. A single TGF beta 1 transcript of 2.4 kb was expressed in HTE cells, and the amount increased by fourfold as cell proliferation decreased and differentiation increased. No TGF beta 2 mRNA could be detected in proliferating undifferentiated HTE cells, but two distinct mRNAs (5.1 and 3.5 kb) were observed to be induced in a transient fashion in RA-treated cells which correlated with the onset of differentiation. The amount of biologically active TGF beta in conditioned media from HTE cells at different stages of growth and differentiation in primary culture was determined by the mink lung epithelial cell growth inhibition assay and the use of neutralizing antibodies. These assays indicated a large increase in the total amount of TGF beta at the time the cells slowed their growth and started to differentiate. The activity was due primarily to TGF beta 1. Interestingly, cells treated with RA had a major component of "preactivated" (non-latent) TGF beta 1 compared to control cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of human retinoic acid receptor alpha 1 expressed in recombinant baculovirus-infected Sf9 insect cells

Full-length human retinoic acid receptor alpha 1 (hRAR alpha 1) was expressed in Sf9 insect cells using the baculovirus expression vector system (BEVS). Western blot analysis using a specific anti-RAR peptide antiserum detected two major protein bands with apparent mol wts of approximately 54 and approximately 51 kDa in extracts from insect cells infected with recombinant hRAR alpha 1 Autographica californica (AcNPV) baculovirus. Analysis of recombinant extracts from Sf9 cells labeled in vivo with [32P]orthophosphate suggested that the recombinant protein was phosphorylated. A component in the recombinant nuclear extracts specifically bound [3H]all-trans-retinoic acid (RA) and sedimented in sucrose density gradient centrifugation as a single, symmetric peak with a sedimentation coefficient of approximately 3.6S, corresponding to a protein of approx 50 kDa. Scatchard analyses determined that [3H]RA was bound in recombinant extracts by a single class of binding sites with an apparent dissociation constant of approximately 0.3 nM and nuclear and cytoplasmic extracts contained approximately 1200 and approximately 200 pmoles, respectively, of unoccupied receptor per mg protein. In competitive ligand binding assays, relative binding affinities of 9-cis- and 13-cis-RA for hRAR alpha 1 in nuclear extracts were about threefold and sixfold lower than all-trans-RA, whereas all-trans-retinol, -retinaldehyde, and -retinyl acetate demonstrated relatively weak binding. In gel mobility shift assays, the electrophoretic migration of a [32P]-labeled oligonucleotide containing the retinoic acid response element of the RAR beta gene was retarded in the presence of recombinant nuclear and cytoplasmic extracts. The apparent complex formation between recombinant hRAR alpha 1 and beta RARE was greatly enhanced by the addition of nuclear extract from wild-type AcNPV-infected Sf9 cells, possibly because of heterodimer formation between recombinant hRAR alpha 1 and a metazoan RXR homolog. Thus, recombinant hRAR alpha 1 expressed at high levels in Sf9 insect cells exhibited biochemical properties of the native protein, including nuclear translocation, specific high affinity ligand and RARE binding, and possible heterodimer formation.

Interactions between retinoic acid and protein kinase C in induction of melanoma differentiation

Retinoic acid treatment of B16 mouse melanoma cells induces a differentiated phenotype. This is accompanied by a decrease in monolayer growth rate, loss of the ability to form colonies in soft agarose, increased production of melanin and other melanocyte-specific markers. In addition, retinoic acid treatment of these cells decreases their tumorigenicity when injected subcutaneously into mice. Our laboratory has found that an early biochemical change after the addition of retinoic acid is a large increase in PKC. PKC is an enzyme whose activity is activated by diacylglycerol and calcium and has been shown to be an important mediator of substances that stimulate growth or differentiation. Since PKC is a multi-gene family, it was important for us to determine which isotype(s) was expressed in B16 cells and which type was induced by retinoic acid. We found that only PKC-alpha is expressed in these cells, and this is the form that is induced by retinoic acid. The retinoic acid-induced increased in PKC-alpha is found at both the RNA and protein level. The mechanism of induction is not yet clear since there is only a small increase in the transcription rate and no change in the stability of the mRNA for PKC-alpha in treated cells. In addition, the induction of PKC by retinoic acid can be blocked by inhibitors of protein synthesis, suggesting that the induction requires the synthesis of new protein(s). In order to determine the role of increased PKC-alpha in the retinoic acid-induced differentiation, we transfected full-length PKC-alpha cDNA in mammalian expression vectors into B16 cells. Two clones that stably overexpressed PKC-alpha to different levels were isolated. The phenotype of these clones resembled WT cells treated with retinoic acid, i.e. they had longer doubling times, decreased ability to form colonies in soft agar, increased melanin production, and decreased tumorigenicity in mice. Recent data suggest a role for the RAR-beta in mediating the effect of retinoic acid on PKC induction. B16 cells express a very low amount of RAR-beta mRNA. The level is increased drastically by retinoic acid treatment without any requirement for protein synthesis. When B16 cells were transfected with and overexpressed RAR-beta, they also expressed more PKC-alpha mRNA and protein, and the induction of PKC by retinoic acid was not blocked by protein synthesis inhibitors. In summary, these finding suggest a key role for PKC-alpha in the pathway by which retinoic acid induces B16 mouse melanoma differentiation.

Quantitative ultrastructural analysis of the relationship between cell growth, shape change, and mucosecretory differentiation in cultured hamster tracheal epithelial cells exposed to retinoic acid

Hamster tracheal epithelial cells in extended (32 degrees C) primary culture with and without supplemental retinoic acid (RA) were studied during the proliferative (5 days) and differentiation phases (11 days) by correlative transmission electron microscopy (EM) and light microscopic (LM) autoradiography to quantify the relationship between cell proliferation, shape change, and mucin granule expression. In retinyl acetate-containing control medium, cell numerical density was higher and [3H]thymidine labeling index (LI) lower at day 11 compared with day 5. The addition of 10(-7) M RA to the medium caused an increase in cell numerical density at both times. LI was increased by RA at 5 days and decreased at 11 days. Measurements of cell shape in ultrathin sections adjacent to LM autoradiographs made in the vertical plane demonstrated an RA-induced change from flat to cuboidal at 5 days and a more columnar phenotype at 11 days. Cells containing mucin granules were of two main types based on their ultrastructure. One type, seen at 5 and 11 days, contained diminutive mucin granules and had an LI of 50% at 11 days. Its LI and frequency (26%) were unaltered by RA. The other type, less frequent (15%) and present only at 11 days, was more columnar and contained mucous granules similar to those found in vivo. RA doubled the frequency of this cell type but did not affect its LI (11%). Cells of this type with more than five mucin granules in EM profile did not incorporate thymidine. The data indicate that RA accelerates and enhances cell shape change toward a more cuboidal phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased expression of protein kinase C alpha plays a key role in retinoic acid-induced melanoma differentiation

Differentiation of B16 mouse melanoma cells induced by retinoic acid (RA) is preceded by a large increase in protein kinase C alpha (PKC alpha) mRNA and protein. To determine the role of PKC alpha in the differentiation program, we stably transfected B16-F1 cells with a plasmid containing the full length PKC alpha cDNA driven by an SV40 promoter. Two out of thirty-two colonies screened were determined to overexpress PKC by 2-4-fold according to Western blot analysis and PKC enzyme activity. When compared to control cells (wild-type cells and cells transfected only with the neomycin resistance gene), PKC alpha overexpressing clones displayed longer doubling times, diminished anchorage-independent growth, and increased melanin production. RA treatment of control cells mimicked these phenotypic characteristics. When injected subcutaneously into syngeneic mice, PKC alpha overexpressing clones produced smaller tumors and had longer latencies than control cells. These findings, combined with the fact that phorbol esters down-regulate PKC and antagonize RA action suggest that PKC alpha plays a key role in the RA-induced melanoma differentiation.

Regulation of protein kinase C gene expression by retinoic acid in B16 mouse melanoma cells

We have previously shown that the retinoic acid (RA)-induced growth arrest and differentiation of B16 mouse melanoma cells is accompanied by a large increase in the amount and activity of protein kinase C (PKC). Since PKC is a multigene family, we investigated which isoforms were expressed in control and RA-treated B16 melanoma cells, and characterized the manner by which RA regulates PKC gene expression. We found that RA treatment of B16 cells resulted in an increase in PKC alpha mRNA beginning at 4-8 h and reached a maximum of 10- to 12-fold over control levels by 48 h. There was also a small amount of PKC gamma mRNA, present only in 48-h RA-treated cells, but no PKC beta mRNA was detected. The effect of RA on PKC alpha mRNA induction was not direct since the induction was abolished when cycloheximide was included in the incubation medium. Nuclear run-on experiments showed that the RA-induced increase in PKC alpha steady-state mRNA was not entirely due to an increase in transcriptional activity, as the increase in PKC alpha transcription was only 2- to 3-fold over control, which is not enough to account for the 10- to 15-fold increase in steady state levels. There was also no change in PKC alpha mRNA stability in RA-treated B16 cells compared to untreated cells. The 10.9-kb PKC alpha message in both control and RA-treated cells was less stable than the 3.8-kb PKC alpha message. Therefore, we propose that the major level of control of PKC alpha mRNA levels by RA is post-transcriptional, either RNA processing or transport out of the nucleus.

B16 mouse melanoma cells selected for resistance to cyclic AMP-mediated growth inhibition are cross-resistant to retinoic acid-induced growth inhibition

B16 mouse melanoma cells are grown inhibited by cyclic AMP or by retinoic acid (RA). However, the combination of these two agents results in less growth inhibition than either agent alone. In order to investigate this interaction, cells were selected for resistance to 8-bromo-cyclic AMP-induced growth inhibition. Two clones (3 and 7) which demonstrated significant resistance were isolated. When these two clones were treated with retinoic acid (RA) it was observed that they also exhibited different degrees of resistance to this growth inhibitor. This cross-resistance did not appear to be due to a lack of uptake or retention of the respective inhibitors, since the mutants took up and retained more 3H-cAMP and 3H-RA than wild type cells, suggesting that the dual resistance was not due to an amplification of P-glycoprotein. The mutation confering cAMP-resistance did not appear to involve cyclic AMP-dependent protein kinase, since both catalytic activity and the amount of cAMP protein binding was similar in wild type and mutants. Thus, the mutation must be beyond the interaction of cAMP with cAMP-dependent protein kinase. We have previously reported that RA induces protein kinase C in B16 melanoma cells (Niles and Loewy: Cancer Res. 49:4483-4487, 1989). Therefore, we measured the ability of RA to induce protein kinase C in the cyclic AMP-resistant mutants. We found an inverse correlation between RA-induced protein kinase C activity and growth inhibition in these mutants. The data reported here suggest that cyclic AMP regulates some step in the RA signal transduction pathway.

Effects of growth medium and cyclic AMP analogues on the cAMP-induced differentiation of F9 teratocarcinoma cells

F9 teratocarcinoma cells differentiate into parietal endodermlike cells when treated with retinoic acid (RA) and cyclic AMP (cAMP). We have previously found that F9 cells can be induced to differentiate by treatment with cAMP in the absence of RA. In the course of determining why other investigators had failed to observe cAMP-induced differentiation, we found that the growth medium played an important role in determining the response of F9 cells to differentiating agents. When F9 cells were grown in minimal essential medium (MEM) and treated with either 8-bromo-cAMP (8BrcA) + 1-methyl, 3-isobutylxanthine (MIX), or dibutyryl cAMP (DBcA) + theophylline (T), they differentiated to parietal endodermlike cells without any requirement for exogenous RA. However, when F9 cells were grown in Dulbecco's modified Eagle's medium (DME), DBcA/T failed to induce differentiation alone and required exogenous RA to induce formation of parietal endoderm-like cells. 8BrcA/MIX alone was still able to induce some differentiation, although the extent was not as great as those cells grown in MEM. These results could not be explained by the different growth-promoting properties of the two culture media because there was no difference in the doubling time of F9 cells grown in either medium. Likewise, RA and cAMP both inhibited growth to the same extent in either medium. Inasmuch as almost all published reports on F9 cell differentiation have used DME as a growth medium, and RA with or without DBcA/T as the differentiating agents, these studies would not have had the appropriate conditions to detect the cAMP-induced differentiation of F9 cells.

Cyclic adenosine monophosphate-mediated induction of F9 teratocarcinoma differentiation in the absence of retinoic acid

F9 teratocarcinoma stem cells differentiate into parietal endoderm-like cells when given retinoic acid (RA) and dibutyryl cyclic adenosine monophosphate (DB-cAMP). It is generally accepted that the stem cells are resistant to the action of cAMP alone and need to be primed by RA in order to respond to cAMP. In this report, we demonstrate that F9 stem cells differentiate into parietal endoderm-like cells in the absence of exogenous RA when treated with cholera toxin and 1-methyl,3-isobutyl xanthine (CT/MIX) or 8-bromo-cAMP/MIX (8B2-cAMP/MIX). Cells treated with CT/MIX or 8B2-cAMP/MIX were morphologically similar to parietal endoderm-like cells, produced high amounts of plasminogen activator, and synthesized both type IV collagen and laminin mRNA. Conversely, markers made in abundance by stem cells such as stage-specific embryonic antigen (SSEA-1) and an mRNA species of 6.8 kb (pST6-135) were markedly reduced in CT/MIX-treated cells. To prove that cAMP alone could induce differentiation Lipidex-1000, a hydrophobic gel, was used to remove 80-90% of the endogenous serum retinoids. F9 cells grown in this retinoid-depleted serum and treated with 8B2-cAMP/MIX differentiated to parietal endoderm-like cells as shown by both dramatic changes in morphology and induction of type IV collagen mRNA. Our results indicate that the differentiation of F9 to parietal endoderm-like cells can be induced by increased intracellular cAMP and is not strictly dependent on the addition of RA.

The effect of retinoic acid on growth and proto-oncogene expression in hamster tracheal epithelial cells

Retinoic acid (RA) has been shown to be required for the maintenance of epithelial differentiation. Vitamin A deficiency in hamsters induces the tracheal epithelial cells to undergo squamous metaplasia. Reversing the vitamin deficiency restores the tracheal epithelial cells to their normal morphology and function. Using a hamster tracheal epithelial (HTE) cell culture system which undergoes differentiation to predominantly secretory cells in vitro, we found that RA can convert flat, squamous-like cells to compact, cuboidal-like cells, and that it stimulated cell proliferation. The mitogenic response to RA was maximal at 10(-7) M and required at least 48 h of treatment to observe the effect. RNA levels of growth-related genes during the growth and differentiation phases of primary HTE cultures were examined by Northern analysis. RA maintained a high level of c-myc RNA expression in preconfluent cultures, whereas untreated cells had low amounts of c-myc RNA. Expression of RNA for the replication-dependent histone 3.2 followed a similar pattern, i.e., its level was high in retinoid-treated versus control preconfluent cultures. In confluent (fully differentiated) HTE cell cultures, both retinoid-treated and control cells had low RNA levels of c-myc and histone 3.2. c-fos RNA levels were undetectable in either control or treated cells at any stage during primary culture. The RNA level of c-Ha-ras was very low in both control and treated cultures and did not vary with the state of growth or differentiation, except that when RA-treated cultures reached confluence, no c-Ha-ras RNA was detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium butyrate suppresses the transforming activity of an activated N-ras oncogene in human colon carcinoma cells

The transforming activity of DNA from a newly established undifferentiated human colon carcinoma cell line (MIP-101) was tested in the NIH-3T3 transfection assay. Southern blot analysis of the transfectant DNA revealed the presence of a human N-ras oncogene. Treatment of MIP-101 cells with the maturational agent sodium butyrate induced a more normal phenotype, including diminished growth rate, elimination of anchorage independent growth, and decreased tumorigenicity (R. Niles, S. Wilhelm, P. Thomas, and N. Zamcheck (1988) J. Cancer Invest. 6, 39). Here we report that there is a significant reduction in the transforming efficiency of the DNA from butyrate-treated MIP-101 cells. A nonspecific reduction in total DNA uptake as an explanation for these findings was eliminated by showing that there was similar uptake and expression of the thymidine kinase gene from the DNA of butyrate-treated and control MIP cells. Butyrate treatment had no detectable effect on the overall structure, methylation, and level of expression of the human N-ras gene from MIP-101 cells. An NIH-3T3 transformant ability after treatment with sodium butyrate. Although butyrate suppressed several transformed properties similar to MIP-101 cells, DNA from control and treated cultures had an identical level of transforming activity. The results suggest that the environment of the MIP cells may contain additional elements not present in the NIH-3T3 transformants which are required to observe the effect of butyrate on reduction of transforming activity.

Induction of protein kinase C in mouse melanoma cells by retinoic acid

Retinoic acid inhibits the proliferation of B16 mouse melanoma cells. It also eliminates the ability of these cells to grow in soft agar. These biological actions of retinoic acid have been shown to be accompanied by an increase in the amount of cyclic AMP-dependent protein kinase and an induction of a new isozyme form (RII beta). In this report we demonstrated that retinoic acid-treated B16 melanoma cells had large increases in protein kinase C activity. This increased enzyme activity was accompanied by increases in both the number of phorbol dibutyrate binding sites and the amount of immunoreactive protein kinase C. Other treatments (melanocyte-stimulating hormone, serum deprivation) which inhibited the growth of these cells did not increase protein kinase C activity. When B16 melanoma cells were treated for a prolonged time (72 h) with phorbol dibutyrate, protein kinase C activity was barely detectable. Under these conditions, melanin production was inhibited and cell growth was accelerated. When retinoic acid was added together with phorbol dibutyrate, it prevented the growth stimulatory effect of the phorbol ester and increased protein kinase C activity. However, the absolute activity of the enzyme was still below that found in control cells and very much lower than in cells treated with retinoic acid alone. Taken together with our previous findings, we propose that the increase in protein kinase C might be part of a differentiation program induced by retinoic acid.

Human neutrophil elastase causes glycoconjugate release from the epithelial cell surface of hamster trachea in organ culture

It is known that human neutrophil elastase (HNE) treatment of hamster tracheal explants causes the release of glycoconjugates, most of which appear to have the characteristics of mucus glycoproteins. This study was designed to determine the origin of HNE-induced glycoconjugate release from 1-day-old cultures of adult hamster trachea. After confirming that HNE treatment released glycoconjugates from cultures labeled with tritiated glucosamine, light microscopic autoradiograms and electron micrographs were prepared. Untreated cultures and cultures treated with inactivated HNE served as controls. HNE treatment caused a 40 to 50% decrease in the silver grain count on the external apical surfaces of secretory cells (p less than 0.05) and ciliated cells (p less than 0.01). Silver grain counts in secretory and ciliated cell cytoplasm, submucosa, and nontissue background were not significantly different from controls. The percentage of nongranulated secretory cells and the number of secretory granules in granulated secretory cells were similar in the HNE-treated and untreated controls. There was no evidence of constitutive release of radiolabeled glycoproteins, or of discharge of secretory granules from the secretory cells. We conclude that HNE releases mucins and other glycoconjugates from the external surfaces of both secretory and ciliated cells in tracheal organ culture.

Membrane differentiation markers of airway epithelial secretory cells

We describe here a system for culturing epithelial cells isolated from hamster trachea, which results in a highly enriched population of mucus-secreting cells. The culture system has enabled us to study the process of secretory cell differentiation in vitro. We found that epithelial secretory cells, in vivo and after 5 days in vitro, selectively bind the lectin Helix pomatia agglutinin (HPA) to apical and, to a lesser extent, basolateral surfaces as well as to mucin granules and intracellular secretory organelles. SDS-PAGE gels of detergent extracts of secretory cells cultured for 5 days reveal three HPA-binding glycoproteins with MW of 120 KD, 220 KD, and greater than 400 KD. The high-MW glycoprotein appears identical to mucin, since it is found in secretions from intact trachea and in spent media from 5-day cultures. It does not appear in spent media from 3-day cultures when cells contain few mucous granules and secrete little mucin. The 220 KD HPA-binding glycoprotein is also present in 5-day but not in 3-day cultures. In contrast, the 120 KD glycoprotein is present at both times. HPA-gp120 is a hydrophobic integral membrane protein, whereas HPA-gp220 and mucin are hydrophilic and are membrane associated. These studies define three membrane glycoproteins, one of which is specific for the tracheal epithelial secretory cell regardless of its mucous content, whereas the other two glycoproteins correlate with mucin secretion. They also demonstrate that, in the fully differentiated state, mucin is bound in a non-covalent fashion to the apical plasma membrane of the tracheal epithelial secretory cell.

The effect of sodium butyrate and retinoic acid on growth and CEA production in a series of human colorectal tumor cell lines representing different states of differentiation

The effect of sodium butyrate and retinoic acid added singly or in combination on substrate-dependent growth, colonization efficiency in soft agar, and carcino-embryonic antigen (CEA) production in three human colorectal carcinoma cell lines differing in their degree of differentiation was studied. All three colon cancer cell lines regardless of their state of differentiation had their growth markedly slowed by sodium butyrate, and to a lesser extent by retinoic acid. When both agents were added together, a small synergistic inhibition of growth was noted in all the cell lines. Butyrate eliminated colony formation in soft agar in all three cell lines, however, retinoic acid only reduced colony formation in the well differentiated cell line DLD-2. Sodium butyrate was able to induce CEA production in the undifferentiated cell (MIP-101) and the moderately differentiated cells (clone D) which were previously negative for this marker. It also enhanced the baseline production of CEA in the well differentiated cells (DLD-2). Retinoic acid did not induce CEA production in clone D or MIP-101 cells, but did enhance the production of CEA in DLD-2 cells. When both retinoic acid and sodium butyrate were added together, CEA production was either additive (DLD-2) or was inhibited (clone D and MIP-101). One explanation of these results is that only well differentiated cells have functional cellular retinoic acid-binding protein (cRABP), and that certain actions of retinoic acid (inhibition of anchorage-dependent growth) are independent of the presence of cRABP.

Human neutrophil elastase releases cell surface mucins from primary cultures of hamster tracheal epithelial cells

Primary hamster tracheal epithelial cells growing on a collagen gel matrix produce high molecular weight mucins indistinguishable from mucins produced in vivo. Using a modified version of these confluent cultures, we have demonstrated here that (i) release of mucins can be stimulated by human neutrophil elastase (HNE; EC 3.4.21.37); (ii) HNE can degrade mucins, and both mucin release and degradation by HNE require an active catalytic site; and (iii) there are at least two pools of mucins in these cells: one is a rapidly turning-over spontaneously releasable constitutive pool, the other is a slowly turning-over HNE-releasable pool. We provide evidence that the HNE-releasable mucins are membrane bound and associated with the secretory cell apical surface.

Isolation and characterization of an undifferentiated human colon carcinoma cell line (MIP-101)

An undifferentiated human colon carcinoma cell line was established from tumor tissue obtained from metastasis to the liver of colonic adenocarcinoma in a patient with fulminant Dukes D colorectal carcinoma. Histological analysis of the tumor biopsy from the liver confirmed the hospital pathology report of poorly differentiated colonic adenocarcinoma. Explants of this tumor tissue xenografted into a nude mouse were used to establish an epithelioid-like cell culture line, MIP-101. The cell line formed tumors in nude mice that histologically appeared undifferentiated and did not stain for carcinoembryonic antigen (CEA). No CEA was present either by radioimmunoassay (RIA) of the culture supernatant or by immunoperoxidase staining of the tumors or monolayers. MIP-101 appears to be one of the most undifferentiated human colon carcinoma cells lines available. It should prove useful in the search for markers of undifferentiated colonic cancer and in studies of colonic cancer differentiation.

Serine proteases stimulate mucous glycoprotein release from hamster tracheal ring organ culture

The effects of several neurohumoral agents and serine proteases on glycoconjugate release from hamster tracheal organ cultures were assessed. The beta-adrenergic agonist isoproterenol inhibited glycoconjugate release, and its effect was abolished by the specific beta-blocking agent propranolol. A cholinergic agonist, pilocarpine, marginally increased glycoconjugate release, and its effect was abolished by the antagonist atropine. Human neutrophil elastase and porcine pancreatic trypsin consistently increased glycoconjugate release by 1.8 to 2.8-fold. When the proteases were inactivated, they were no longer effective in stimulating glycoconjugate release. Histologic and electron microscopic analysis of the protease-treated organ cultures revealed no discernible toxic reaction. In addition, organ cultures prelabeled with chromium 51 did not release an increased amount of radioactivity when treated with the proteases. Biochemical analysis of the glycoconjugates released into the culture medium showed them to be of high molecular weight (90% eluted in the void volume of a Sepharose 6B column) and to be resistant to digestion with hyaluronidase and heparinase, properties consistent with mucous glycoproteins. The mechanism of protease-induced glycoconjugate release is unknown. We speculate that stimulation of airway secretory cells by serine proteases of neutrophilic or other inflammatory cell origin may play a role in the increased airway secretion that is characteristic of acute tracheobronchitis.

The effect of retinoic acid on protein phosphorylation in mouse melanoma cells

Vitamin A inhibits growth and increases the activity of cAMP-dependent protein kinase in B16 mouse melanoma cells. In this report we show that retinoic acid (RA) treatment of intact cells alters their subsequent in vitro protein phosphorylation, but we could not demonstrate any changes in in vivo protein phosphorylation. A 48-h treatment with RA results in a concentration-dependent decrease of protein phosphorylation of a 95K molecular weight (MW) protein in both supernatant and particulate fractions. The phosphorylation of this protein does not appear to be regulated by cAMP. Proteins at 92K and 82K MW in the supernatant fraction are increased in phosphorylation. The former (but not the latter) is regulated by cAMP. In the particulate fraction a variety of proteins 12K-68K MW are increased in phosphorylation, as the cells are treated with increasing amounts of RA. The phosphorylation of most of these proteins is regulated by cAMP. Another inhibitor of B16 cell growth, melanocyte-stimulating hormone (MSH) also alters protein phosphorylation. At short incubation periods (1 h), this hormone stimulates phosphorylation of a number of proteins (17-40K MW), while in longer incubation periods (48 h) phosphorylation is inhibited. All of these phosphorylations appear to be regulated by cAMP. We attempted to repeat these observations using intact-cell phosphorylation with 32PO4. In two experiments we saw small changes in the phosphorylation of proteins. In most experiments, however, we could find no change in the phosphoproteins. Further experiments have led us to question the in vivo phosphorylation, since treatment of the cells with MSH, cholera toxin, or db-cAMP also did not affect intact-cell protein phosphorylation. We have previously documented that under these latter conditions cAMP levels are greatly elevated and cAMP-dependent protein kinase is activated. The in vitro phosphorylation results suggests that in RA-treated cells, kinase activities and/or protein substrate levels are changing. However, the physiological significance of the particular MW phosphoproteins changes we have described must await resolution of the in vivo phosphorylation data.

The effect of retinoic acid on cyclic-AMP-binding proteins in mouse melanoma cells

We have previously reported [(1980) J. Biol. Chem. 255, 5999-6002] that retinoic acid inhibited growth and increased cyclic-AMP-dependent protein kinase activity in mouse melanoma cells. A variant melanoma line having depressed levels of cyclic-AMP-dependent protein kinase was not growth-inhibited by retinoic acid. In this report we describe the effect of retinoic acid on cyclic AMP binding proteins in B16 mouse melanoma cells. Using the technique of photoaffinity labeling, we found three major proteins of Mr 49 000, 52 000, and 55 000 which were specifically labeled with 8-N3-[32P]AMP in both control and treated cells. Based upon their molecular weight, relative affinity for 8-N3-[32P]AMP and comigration with standards, we have designated the 49 000-Mr and 55 000-Mr species as RI and RII respectively. The position of the intermediate band (Mr 52 000) was not affected by pre-incubation with ATP or alkaline phosphatase, and two-dimensional gel analysis indicated that it had the same pI as RI. Retinoic acid increased the 8-N3-[32P]AMP labeling of RI within 24 h, reaching a maximal six fold increase by 48 h. These increases were limited to the 40 000 X g supernatant fraction and occurred prior to any growth inhibition. By using increasing concentrations of 8-N3-cAMP we were able to construct a saturation curve for RI binding. Calculation of apparent Kd values from these curves showed nearly identical affinities for RI binding of 8-N3-cAMP from control and retinoic-acid-treated cells. Therefore we conclude that retinoic acid is increasing the amount of RI rather than altering its properties. Corroboration of these results was obtained by DEAE-cellulose chromatography. Peak I (corresponding to type I protein kinase) from retinoid-treated cells was increased about six fold in binding activity.

In vitro characteristics of the lipid-filled interstitial cell associated with postnatal lung growth: evidence for fibroblast heterogeneity

This study explores the in vitro modulation of the lipid-filled phenotype of the lipid interstitial cell (LIC) isolated from the developing rat lung. Isolated LIC lose their cytoplasmic lipid droplets when cultured in fetal bovine serum (FBS) but retain their potential for lipid storage, since they rapidly reaccumulate lipid when subcultured in neonatal rat serum (NRS) and to a lesser extent in adult rat serum (ARS). The return of LIC to a lipid-filled state may not represent cell differentiation, since it occurs in the presence of bromodeoxyuridine. NRS contains twice the free fatty acids (FFA) of FBS and ARS, and doubling the FFA concentration of FBS and ARS increases LIC storage lipids. Serum triglyceride (TG) is 10 times higher in ARS and 23 times higher in NRS than in FBS. Since LIC lipoprotein lipase (LPL) activity is in the range of 3T3-L1 adipocytes (0.56 vs. 1.72 units/mg DNA), the LIC has the potential of incorporating serum lipoprotein-triglyceride. The LPL activity of LIC is 9-12 times that of fetal and adult rat lung fibroblasts and 50 times that of human lung, trachea, or skin fibroblasts; LIC are probably a source of endothelial LPL in the developing lung. The response of LIC and ARLF cyclic-AMP to hormones known to influence lipid synthesis or degradation showed that: only LIC responded to glucagon; prostaglandin E1 was a more potent stimulus to LIC; isoproterenol was a more potent stimulus to ARLF; and neither cell responded to ACTH. The unique nature of LIC tends to support further the concept of fibroblast heterogeneity within tissues.

Differential sensitivity of normal and chemically transformed epithelial cells to cholera toxin

We have been studying the regulation of growth by cyclic adenosine 3':5'-monophosphate (cyclic AMP) and other factors in untransformed (K16) and chemically transformed (W8) rat liver epithelial cells. Initially, we found that 8-bromocyclic adenosine 3':5'-monophosphate was a more potent inhibitor of cell replication in K16 than in W8 cells. In addition, the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX) caused marked growth inhibition in K16 but not in W8 cells. Through the use of cholera toxin (CT) with or without MIX, we elevated intracellular cyclic AMP levels in a quantifiable fashion. With CT alone or combined with MIX, we observed a dose-dependent morphological change in W8 cells, which consisted of extensive "process" formation. K16 morphology was not altered at any concentration of CT +/- MIX tested. K16 cell growth was only marginally inhibited by CT alone, but markedly inhibited by CT plus MIX. W8 cell growth was moderately inhibited by CT alone or combined with MIX. Analysis of cyclic AMP levels revealed that, at all concentrations of CT +/- MIX and at all time periods tested, W8 cells produced significantly more cyclic AMP than K16 cells. It appears that morphological changes and growth inhibition are not necessarily linked and that MIX may inhibit K16 cell replication by means other than its ability to increase intracellular cyclic AMP levels.

Retinoic acid increases cyclic AMP-dependent protein kinase activity in murine melanoma cells

The influence of all trans-retinoic acid on cyclic AMP metabolism was examined in B16-F1 mouse melanoma cells. Treatment of these cells with retinoic acid resulted in a dose-dependent inhibition of cell growth which was accompanied by a concentration-dependent increase in both basal and cyclic AMP-stimulated protein kinase activity, Intracellular levels of cyclic AMP, however, were not altered by retinoid treatment. A protein kinase-deficient variant of B16-F1 (MR-4) did not exhibit decreased growth or increased protein kinase activity in response to retinoic acid treatment. At least 24 h of incubation was required before increased protein kinase activity could be detected in treated B16-F1 cells. Retinoic acid treatment increased the Vmax of protein kinase, but the Ka for cyclic AMP activation was not altered. These findings suggest that in B16 mouse melanoma cells, cyclic AMP-dependent protein kinase may be a target for the growth inhibitory effects of the retinoid.

Differential growth inhibition in two human carcinoma cell lines by cyclic adenosine 5'-monophosphate analogs

The inhibition of cell replication in two human carcinoma cell lines by various cyclic AMP analogs was explored. In all instances, the addition of the cyclic nucleotide phosphodiesterase inhibitor 1-methyl-3-isoburylxanthine resulted in synergistic growth inhibition by the analogs. A correlation was found between an analog's ability to inhibit growth and its ability to activate protein kinase. A differential effect of the breakdown product 8-bromo-AMP (8-BrAMP) on cell replication in the two cell lines was observed; i.e., one cell type was extremely sensitive to inhibition by 8-BrAMP and the growth inhibition could not be reversed by uridine, whereas the other cell type was less sensitive to 8-BrAMP and the growth inhibition was significantly reversed by uridine.

Regulation of phosphatidylcholine metabolism by cyclic AMP in a model alveolar type 2 cell line

The influence of cyclic AMP on the metabolism of phosphatidylcholine, the major component of pulmonary surfactant was examined in a cell line (A549) with type 2 pneumonocyte characteristics. It was found that cyclic AMP increased both the total amount of phosphatidylcholine and disaturated phosphatidylcholine as well as the incorporation of [3H]choline into these fractions. The effect was specific for cyclic AMP since 5'-AMP, adenosine, and cyclic GMP did not alter phosphatidylcholine or disaturated phosphatidylcholine levels. Cyclic AMP had no effect on phosphatidylcholine and disaturated phosphatidylcholine metabolism in another non-type 2 human epithelial cell line (MA-160). Since the ability of various cyclic AMP analogs to increase phosphatidylcholine and disaturated phosphatidylcholine levels was correlated with their ability to activate protein kinase, it seems likely that a protein phosphorylation mechanism is involved in controlling phosphatidylcholine metabolism.