Antagonism between epidermal growth factor and phorbol ester tumor promoters in human breast cancer cells

Phosphorylation at serine 482 affects stability of NF90 and its functional role in mitosis

OBJECTIVES

NF90 is a multifunctional double-strand RNA binding protein with documented roles in transcription, mRNA stability, translation, RNA processing and transport, and mitosis. It is a phosphoprotein that interacts with, and is a substrate for, several protein kinases. The study described here was initiated to gain better understanding of specific NF90 phosphorylation sites and their relationship to mechanisms by which NF90 performs its various functions.

MATERIALS AND METHODS

Phosphoproteomic studies have identified NF90 serine 482 (S482) as a major phosphorylation site in vivo. Site-specific mutations were introduced at this site and the mutated proteins were expressed in MCF7 cells by transfection. Western blotting was used to examine NF90 expression, stability, and responsiveness to protein kinase activators and inhibitors. Flow cytometry was used to examine effects of NF90 mutation on cell cycle progression.

RESULTS

Non-phosphorylatable mutant S482A was unstable compared to phosphomimetic S482E mutant. NF90-S482A expression was greatly enhanced by inhibiting proteasomal degradation or by activating PKC. Identical treatments had little effect on NF90-S482E. In contrast to WT NF90 or NF90-S482E, cells stably expressing NF90-S482A accumulated in M phase when treated with TPA.

CONCLUSIONS

Phosphorylation at S482 is important for NF90 stability and in regulating its functional role during mitosis. Based on the sequence surrounding S482, mitotic kinase PLK1 is a strong candidate for the enzyme that phosphorylates NF90 at this site.

Conjugated linoleic acid attenuates cyclooxygenase-2 transcriptional activity via an anti-AP-1 mechanism in MCF-7 breast cancer cells

Overexpression of cyclooxygenase-2 (COX-2) is regarded as a causative factor in the onset of tumorigenesis of the breast. In this study, we investigated the effects of conjugated linoleic acid (CLA) on COX-2 transcription in MCF-7 breast cancer cells. Results of transient transfection studies revealed that treatment with a CLA mix or selected isomers (c9, t11-CLA; t10, c12-CLA) at concentrations ranging from 20 to 80 micromol/L, attenuated COX-2 transcription induced by the proinflammatory agent 12-O-tetradecanoylphorbol-13-acetate (TPA). In addition, the CLA mix inhibited TPA-induced activity of the collagenase-1 promoter. Using electrophoretic mobility shift assays, we found that the CLA mix reduced TPA-induced recruitment of nuclear proteins to a cAMP response element (CRE) in the COX-2 promoter and a consensus TPA-responsive element (TRE) in the collagenase-1 promoter. Both CRE and TRE are binding sites for activator protein-1 (AP-1). Binding studies revealed that the t10, c12-CLA isomer was more effective than the CLA mix or c9, t11-CLA in reducing binding of cJun to either the COX-2 CRE or collagenase-1 TRE, whereas linoleic acid increased binding to both elements. Overexpression of the AP-1 member, c-Jun, reversed the inhibitory effects of the CLA mix on COX-2 transcription, and restored binding of nuclear proteins to the CRE and TRE. Collectively, these results suggest that CLA represses AP-1-mediated activation of COX-2 transcription.

Altered expression and localization of PKC eta in human breast tumors

Protein kinase C (PKC) eta is a PKC isoform whose upregulation is associated with differentiation in many epithelial tissues, including the rat mammary gland. The purpose of this study was to examine whether PKC eta is altered, in expression or localization, in human breast cancer. Paraffin sections of 49 in situ breast lesions, 29 invasive breast tumors, and nine normal breast biopsies were examined for PKC eta expression by immunohistochemistry. Adjacent regions of normal epithelium, and in situ lesions that were present adjacent to invasive lesions were also analyzed. In normal epithelium, regardless of the presence of adjacent in situ or invasive lesions, PKC eta was present in the cytoplasm of the luminal epithelium, and increased in areas of normal lobular development, similar to normal rat mammary gland. PKC eta staining intensity was homogeneous in normal lobules, but heterogeneous in in situ and invasive lesions, being focally increased in cells with aberrant nuclear morphology. In situ lesions were similar to adjacent normal epithelium in average staining intensity, regardless of whether invasion was also present. However, the invasive lesions themselves were significantly decreased in staining intensity compared to adjacent in situ lesions. In addition, 75% of invasive breast cancer lesions showed decreased staining relative to adjacent normal epithelium, compared to 37% of in situ lesions. The invasive tumors which possessed high PKC eta staining were associated with positive lymph node status. These results demonstrate that quantitative and qualitative alterations in PKC eta occur in human breast cancers.

Conjugated linoleic acid inhibits proliferation and induces apoptosis of normal rat mammary epithelial cells in primary culture

The trace fatty acid conjugated linoleic acid (CLA) inhibits rat mammary carcinogenesis when fed prior to carcinogen during pubertal mammary gland development or during the promotion phase of carcinogenesis. The following studies were done to investigate possible mechanisms of these effects. Using a physiological model for growth and differentiation of normal rat mammary epithelial cell organoids (MEO) in primary culture, we found that CLA, but not linoleic acid (LA), inhibited growth of MEO and that this growth inhibition was mediated both by a reduction in DNA synthesis and a stimulation of apoptosis. The effects of CLA did not appear to be mediated by changes in epithelial protein kinase C (PKC) since neither total activity nor expression nor localization of PKC isoenzymes alpha, beta II, delta, epsilon, eta, or zeta were altered in the epithelium of CLA-fed rats. In contrast, PKCs delta, epsilon, and eta were specifically upregulated and associated with a lipid-like, but acetone-insoluble, fibrillar material found exclusively in adipocytes from CLA-fed rats. Taken together, these observations demonstrate that CLA can act directly to inhibit growth and induce apoptosis of normal MEO and may thus prevent breast cancer by its ability to reduce mammary epithelial density and to inhibit the outgrowth of initiated MEO. Moreover, the changes in mammary adipocyte PKC expression and lipid composition suggest that the adipose stroma may play an important in vivo role in mediating the ability of CLA to inhibit mammary carcinogenesis.

Alterations in the expression and localization of protein kinase C isoforms during mammary gland differentiation

Protein kinase C (PKC) is involved in signaling that modulates the proliferation and differentiation of many cell types, including mammary epithelial cells. In addition, changes in PKC expression or activity have been observed during mammary carcinogenesis. In order to examine the involvement of specific PKC isoforms during normal mammary gland development, the expression and localization of PKCs alpha, delta, epsilon and zeta were examined during puberty, pregnancy, lactation, and involution. By immunoblot analysis, expression of PKC alpha, delta, epsilon and zeta proteins was increased in mammary epithelial organoids during the transition from puberty to pregnancy. In mammary gland frozen sections, PKCs alpha, delta, epsilon and zeta were stained in the luminal epithelium and myoepithelium, in varying isoform-and developmental stage-specific locations. PKC alpha was found in a punctate apical localization in the luminal epithelium during pregnancy. During lactation, PKC epsilon was present in the nucleus, and PKC zeta was concentrated in the subapical region of the luminal epithelium. Additionally, marked staining for PKCs alpha, delta, epsilon, and zeta was observed in the myoepithelial cells at the base of ducts and alveoli. This basal ductal and alveolar staining differed in intensity in a developmentally-specific fashion. During most time points (virgin, pregnant, lactating, and early involution), myoepithelial cells of the duct were more intensely stained than those lining the alveoli for PKCs alpha, delta, epsilon and zeta. During late involution (days 9-12), the preferential staining of ducts was lost or reversed, and the myoepithelial cells lining the regressing alveolar structures stained equally (PKCs epsilon and zeta) or more intensely (PKCs alpha and delta), coincident with the thickening of the myoepithelial cells surrounding the regressing alveoli. The increased PKC isoform staining at the base of alveoli during involution suggests that alveolar regression may be influenced by alterations in signaling in the alveolar myoepithelium.

Inhibition of AP-1 and NF-kappaB by manganese-containing superoxide dismutase in human breast cancer cells

One of the primary antioxidant enzymes, manganese-containing superoxide dismutase (MnSOD), has shown the ability to reverse malignant phenotypes in a variety of human tumor cells that are low or absent in MnSOD expression. We have observed that overexpression of human MnSOD in human breast cancer MCF-7 cells inhibits tumor growth both in vitro and in vivo. The signaling pathway underlying the MnSOD induced tumor suppression is unknown. We demonstrate here that transcriptional and DNA binding ability of AP-1 and NF-kappaB, but not SP-1, were inhibited (by 50%) in the MCF-7 cell line overexpressing MnSOD. When transiently expressing, MnSOD inhibited AP-1 but increased NF-kappaB transactivation, which can be abolished by sodium pyruvate, a hydrogen peroxide scavenger. To analyze the target genes responsible for MnSOD-induced tumor suppression, genes related to tumor growth and responsive to AP-1 or NF-kappaB were analyzed. AP-1 responsive collagenase I, stromelysin I, and NF-kappaB responsive IL-1 and IL-6 were down-regulated in the MnSOD stable transfectants compared to the control cell lines. Since TPA induces differentiation in human breast cancer cells and up-regulates MnSOD gene in HeLa cells, MnSOD expression and AP-1 and NF-kappaB activity were measured under TPA treatment. The results showed that TPA induced endogenous MnSOD expression and inhibited both AP-1 and NF-kappaB. Together, these results suggest that tumor suppression by overexpressing MnSOD is related to a modulation of AP-1 and NF-kappaB, which causes a down-regulation of genes responsible for tumor malignant phenotype.

Gap junction Cx26 gene modulation by phorbol esters in benign and malignant human mammary cells

Connexin (Cx) 26, a major gap junction protein expressed in mammary epithelial cells, has been considered to be a tumor suppressor gene candidate. This study investigated the molecular mechanism of transcriptional up-regulation of Cx26 by phorbol ester (TPA) in human immortalized MCF-10 mammary epithelial cells and MDA-MB-231 mammary cancer cells. Such up-regulation was mediated through the protein kinase C pathway and could be blocked by the PKC inhibitor, calphostin C. Based on the results of the nuclear run-on assay, there was a TPA-induced increase in the rate of transcriptional initiation. We identified a TPA-induced DNase I hypersensitivity (DH) region approximately 1 kb 5' upstream of the ATG translation starting site. Sequence analysis revealed that this DH region was located in intron 1 and contained two TRE-like TGAT/ATCA elements, two 5'TTCA3' motifs and a 5'AGGAAG3' PEA3 motif. Both TRE-like elements were capable of binding AP1. TPA inducibility of this DH region was seen by the CAT reporter assay and appeared to be direction-dependent suggesting a functional cooperation between PEA3/TTCA and TRE.

Inverse regulation of oestrogen receptor and epidermal growth factor receptor gene expression in MCF-7 breast cancer cells treated with phorbol ester

In human breast cancer cell lines, an inverse relationship exists between the basal levels of oestrogen receptor (ER) and epidermal growth factor receptor (EGF-R) gene expression. In addition, the tumour-promoting phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) inhibits ER and stimulates EGF-R expression in MCF-7 breast cancer cells. This study aimed to define further the potential mechanisms involved in the modulation of ER and EGF-R gene expression by TPA. ER mRNA levels were reduced after 3 h and declined to 30% of control between 12 and 72 h after exposure to 10 nM TPA. This decrease in mRNA levels was preceded by an apparent fall in ER transcription rate. There was no effect on the stability of ER mRNA following pretreatment for 3-24 h with TPA, supporting the conclusion that the fall in ER mRNA levels was predominantly due to a decrease in ER transcription rate. Levels of EGF-R mRNA increased 10-fold by 12 h due predominantly to an increased transcription rate. The TPA-induced decrease in ER mRNA was unaffected by the simultaneous administration of the protein synthesis inhibitor cycloheximide, whereas the increase in EGF-R mRNA was inhibited by co-incubation with cycloheximide. These data indicate a requirement for continuing protein synthesis for the TPA effect on EGF-R but not on ER mRNA levels. Because the modulation of ER and EGF-R gene expression by TPA is likely to involve the protein kinase C (PKC) signal transduction pathway, the effects of other known activators of PKC were investigated. The non-phorboid tumour promoter mezerein modulated ER (an 80% decrease) and EGF-R (a 20-fold increase) mRNA levels in a similar manner to TPA. In contrast, neither 1,2-dioctanoyl-sn-glycerol (DiC8) nor 1-oleoyl-2-acetyl-sn-glycerol (OAG), both permeant analogues of the endogenous physiological activators of PKC, affected ER and EGF-R mRNA levels. These latter results were not due to a lack of efficacy because a single administration of DiC8 was as effective as TPA in inducing c-fos mRNA at 30 min. However DiC8 was less active in the later induction of c-myc mRNA. These data demonstrate reciprocal regulation of ER and EGF-R gene expression by TPA, involving effects on transcriptional events, which appear to be mediated by sustained activation of PKC.

Early gene responses associated with transforming growth factor-beta 1 growth inhibition and autoinduction in MCF-7 breast adenocarcinoma cells

In the human breast carcinoma cell line (MCF-7), exogenous TGF-beta 1 induces a dose-dependent inhibition of cell proliferation. In a MCF-7 cell subline [MCF-7(-)], which has an undetectable level of type II TGF-beta receptor, exogenous TGF-beta 1 does not inhibit cell proliferation but is still able to induce its own message. In both cell lines, TGF-beta 1 stimulates expression of c-jun, whereas a rapid, transient and marked increase in c-fos mRNA is only observed in the MCF-7 cells sensitive to the growth inhibitory effect of TGF-beta 1. Depletion of protein kinase C abolishes the c-fos but not the c-jun response to TGF-beta 1. Our results suggest that growth inhibition and autoinduction by TGF-beta 1 are mediated by different signalling pathways. In addition, a PKC-dependent increase in c-fos expression seems to be associated with the growth inhibitory effect of TGF-beta 1.

Cell-adhesion-disrupting action of interleukin 6 in human ductal breast carcinoma cells

Recombinant baculovirus-derived interleukin 6 (IL-6) disrupts the attachment of human ductal breast carcinoma subline ZR-75-1-Tx cells to neighbors and the substratum in culture without inhibiting the proliferation of the cells. The nonadherent cells lack pseudopodia and do not translocate directionally. These findings stand in contrast to the earlier observations in the Ro subline of ZR-75-1 cells in which IL-6 induces cell-cell separation without detachment of the cells from the substratum, with the cells displaying pseudopodia, increased motility, and decreased proliferation. The IL-6-induced ZR-75-1-Tx cell detachment and rounding are reversible by incubation of the treated cells in IL-6-free medium for several days. The distinctive changes induced by IL-6 in ZR-75-1-Ro cells are similarly reversible. Either acidic fibroblast growth factor or phorbol 12-myristate 13-acetate can replace serum as a cofactor in IL-6-induced ZR-75-1-Tx cell detachment. Our findings indicate that genetic changes can occur in breast carcinoma cells that through cytokine action markedly affect cell structure, adhesiveness, and motility.

Phorbol 12-myristate 13-acetate stimulates proliferation and ductal morphogenesis and inhibits functional differentiation of normal rat mammary epithelial cells in primary culture

The effect of the tumor promoter phorbol 12-myristate 13-acetate (PMA) on proliferation and differentiation of normal mammary epithelial cells from 50-day-old virgin rats was investigated using a model system that allows for full morphological and functional development of the cells. In this model, mammary epithelial cells are grown within a reconstituted basement membrane in a defined serum-free medium. PMA at a concentration of 10(-6) M effected translocation of protein kinase C from cytosol to membrane. At the same concentration, it stimulated cell proliferation both in the presence and absence of EGF, and this stimulation was observed even when PMA exposure was limited to 15 min at the time of each media change. In contrast to the effect on proliferation, PMA at concentrations of 10(-7) and 10(-6) M inhibited functional differentiation as assessed by casein accumulation. Phorbol 12,13-dibutyrate at 10(-6) M also stimulated proliferation and inhibited casein accumulation and was more effective than PMA in both cases. In contrast, the nonactive tumor promoter 4-alpha PMA had no effect on either proliferation or differentiation. One of the most striking effects of PMA was its ability to stimulate an atypical ductal morphogenesis, as manifested by the formation of intricate web-like colonies, and to inhibit the development of the well-differentiated alveolar-like multilobular colonies. PMA was also shown to completely suppress the growth of the squamous-like colonies that develop when EGF is absent or deficient. These effects of phorbol esters in mammary epithelial cells to stimulate proliferation, inhibit functional differentiation, and stimulate the development of ductal colonies are consistent with the suggestion that the signal transduction pathways evoked by PMA could act to stimulate the growth of initiated cells or render normal cells more sensitive to carcinogen.

Estrogen-inducible pS2 protein is not the key regulatory component in the proliferation of human breast cancer cells (MCF-7)

Exposure of human breast cancer cells (MCF-7) to tumor promoters such as 12-O-tetradecanoyl phorbol 13-acetate (TPA) for 24 h at concentrations of 1-100 nM resulted in marked inhibition of DNA synthesis but a 3-5-fold increase in the amount of pS2 protein in the medium. These results support our previous suggestion that pS2 protein is not involved in the mechanism controlling proliferation of MCF-7 cells. During treatment with TPA, the intracellular content of pS2 protein was constant, suggesting that TPA did not induce secretion of pS2 protein but rather de novo synthesis of the protein. The increase in the pS2 protein content of the medium by TPA was inhibited by simultaneous addition of cycloheximide, but not by that of actinomycin D. Northern-blot hybridization analysis showed that the amount of pS2 mRNA was unchanged by treatment of the cells with TPA. These results indicate that TPA does not induce transcription of the pS2 gene, and suggest that the main effect of TPA results from the induction of translation of pS2 mRNA.

Regulation of prolactin receptor expression by the tumour promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate in human breast cancer cells

In both the normal and malignant human breast, cellular sensitivity to the proliferative and differentiative activities of the lactogenic hormones is conferred by expression of the prolactin receptor (PRLR). The PRLR is regulated by steroid hormones; however, recent findings have suggested that PRLR may also be regulated by protein kinase C. To examine this possibility we have studied the effect of various modulators of PKC activity on PRLR binding activity and gene expression in five PRLR positive human breast cancer cell lines. Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), a tumour promoter and modulator of PKC activity, decreased PRLR binding activity in all cell lines examined. In MCF-7 cells, 10 nM TPA caused a 70% loss of PRLR mRNA after 12 h, paralleled 3 h later by a comparable loss of cell surface PRLR. Mezerein, a non-phorbol ester modulator of PKC activity and 1,2-dioctanoyl-sn-glycerol, a permeant analogue of the endogenous activator of PKC, also reduced PRLR binding activity and gene expression in a time- and concentration-dependent manner. Cycloheximide failed to abrogate the TPA-induced decline in PRLR mRNA levels, indicating that this process was not dependent upon continuing protein synthesis. No change in the stability of PRLR mRNA was observed during 24 h of TPA treatment and TPA reduced the rate of PRLR gene transcription within 3 h of treatment. These results demonstrate that modulators of PKC activity reduce PRLR binding activity and gene expression, implicating this signal transduction pathway in PRLR regulation.

Antiproliferative effect of phorbol esters on MCF-7 human breast adenocarcinoma cells: relationship with enhanced expression of transforming growth-factor-beta 1

In human breast carcinoma MCF-7 cells, phorbol diesters inhibit proliferation and induce cell maturation. We have recently reported that exogenous TGF-beta 1 reverses the resistance of a breast adenocarcinoma MCF-7 subline (MCF-7:RPh-4) to these phorbol ester effects. Here, we investigated the involvement of TGF-beta 1 in the PKC-mediated inhibition of breast-cancer cell proliferation. Parental MCF-7-conditioned medium contained a 20-fold higher transforming activity on NRK-49F fibroblasts than the TPA-resistant subline. TPA increased TGF-beta activity in MCF-7 conditioned medium. MCF-7 cells also expressed more TGF-beta 1 mRNA than the resistant subline. TPA induced a dose-dependent increase in TGF-beta 1 mRNA levels that paralleled the inhibitory effect on MCF-7 proliferation. The lower level of TGF-beta mRNA expression in TPA resistant subline was not modified after addition of TPA, but was significantly increased in the presence of exogenous TGF-beta 1. These data argue in favor of a role of endogenous TGF-beta 1 in the maturation process induced by protein kinase C activation.

Phorbol ester and bryostatin effects on growth and the expression of oestrogen responsive and TGF-beta 1 genes in breast tumour cells

The phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) (10 nM) produce a marked reduction in the growth, measured by thymidine uptake, of MCF-7 cells in full growth medium, but had only a small effect on MDA-MB-231 and T47D cells. Bryostatin alone also inhibited growth but to a lesser extent than seen with TPA. The effect of TPA on MCF-7 cells was partially reversed by bryostatin, added simultaneously or after TPA, suggesting bryostatin does not simply mimic TPA in this system. Even though both are believed to act via effects on protein kinase C, bryostatin appears to act as antagonist to the effect of TPA as well as a partial agonist on its own. When the oestrogen receptor positive MCF-7 and T47D cells were maintained in charcoal stripped serum, the increase in DNA synthesis on stimulation with oestradiol was inhibited with 50 nM TPA in MCF-7 cells but not in T47D cells. The effects of these treatments on the expression of two well characterised oestrogen responsive genes pNR2(pS2) and pNR100 (Cathepsin-D) were examined. Rather than preventing transcription of these oestrogen responsive genes, TPA alone increased pNR2 and pNR100 levels in MCF-7 cells and the combined effect of oestradiol and TPA had a marked synergistic effect in increasing the transcript levels of these genes. In T47D cells pNR2 transcripts were not detected and the increase in pNR100 mRNA levels were not affected by TPA. We conclude that the inhibitory effects of TPA on the growth stimulation of MCF-7 cells by oestradiol was not due to a general inhibition of the expression of oestrogen responsive genes. An alternative possibility examined was that the growth inhibitory effect of TPA on MCF-7 cells might be due to stimulation of TGF-beta 1, acting as an autocrine inhibitory growth factor. Oestradiol treatment of MCF-7 cells reduced the levels of TGF-beta 1 mRNA whereas TPA produced a marked increase. The combined effect of TPA and oestradiol further increased TGF-beta 1 mRNA above the levels seen with TPA alone. Bryostatin had little effect on TGF-beta 1 expression either alone or in combination with oestradiol. These observations are consistent with the hypothesis that the inhibitory effect of TPA on MCF-7 cells may be partly due to autocrine inhibition by TGF-beta 1.

Phorbol ester-induced inhibition of proliferation of Daudi Burkitt's lymphoma cells by impairment of cytokinesis

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) exerts a dose-dependent effect on Daudi cell proliferation. A low concentration has a slight mitogenic effect but higher concentrations inhibit proliferation. The inhibitory effect is associated with increases in cell size, macromolecular content, and incorporation of precursors into RNA and protein. Cell cycle analysis indicates that TPA at 1-10 nM leads to an apparent accumulation of cells in G2/M phase. However, within this population a significant proportion of cells undergo nuclear division but fail to carry out cytokinesis, giving rise to cells with two or more nuclei. Consistent with this, DNA synthesis continues in cells which cease to divide in the presence of TPA. The ability of the phorbol ester to inhibit proliferation can thus be attributed mainly to an inhibition of cytokinesis rather than DNA replication.

The effect of fetal calf serum on growth arrest caused by activators of protein kinase C

The growth of human-derived A549 lung carcinoma cells is inhibited by activators of protein kinase C (PKC) such as 12-O-tetradecanoylphorbol- 13-acetate (TPA). In this study, the effect of serum deprivation on TPA-induced growth retardation has been investigated. Cells cultured with 10% FCS and TPA (10(-8) M) stopped growing for 6 days, whereas inhibition of DNA synthesis caused by TPA in cells which were grown in medium containing the serum substitute ultraser lasted for less than 48 hr. The ability of cells to respond to the growth-inhibitory potential of TPA decreased with decreasing amounts of FCS in the cellular medium. Addition of fetuin or epidermal growth factor (EGF) to incubates with serum-deprived cells increased the ability of TPA to affect growth, but addition of platelet-derived growth factor (PDGF), transforming growth factor beta (TGF-beta) or retinoic acid (RA) was without effect. Growth arrest caused by bryostatin I, another PKC activator, was equally transitory in serum-supplemented and serum-deprived cells. Cytosol of serum-deprived cells contained only 32% of specific phorbol ester binding sites compared to cells grown with FCS; PKC enzyme activity and immunodectable protein were similarly reduced in cells grown without FCS. There was no difference in rate of TPA-induced down-regulation of PKC activity and cytosolic phorbol ester receptor sites between cells grown with or without serum.

Dissociation between protein kinase C content and biological responsiveness to phorbol esters in tumor promoter-sensitive (MCF-7) and resistant (RPh-4) cells

A cell line (RPh-4) insensitive to the effects of phorbol esters has been isolated from MCF-7 human breast cancer cells. The growth pattern of RPh-4 cells in the presence of 50 ng/mL (80 nM) 12-O-tetradecanoylphorbol 13-acetate (TPA) is similar to that of parental MCF-7 cells in the absence of TPA. While phorbol esters inhibit MCF-7 cell proliferation and increase cell volume and protein content, no such effects are observed in RPh-4 cells. TPA affects MCF-7 but not RPh-4 cell cycle in two ways: a G1 block and a delayed passage through G2 phase. Profound alterations in protein kinase C content and activity are observed in RPh-4 versus MCF-7 cells, i.e. (i) a dramatic decline in the cellular enzyme content; (ii) a loss of the capacity to translocate upon acute TPA stimulation for the remainder enzyme; and (iii) a lack of stimulation by phorbol esters of the endogenous Mr 28,000 substrate. However, these striking changes are only transient and rapidly reverse when RPh-4 cells are subcultured in TPA-free medium, with a 60% and an almost total recovery, respectively, after 15 days and 3 months. By contrast, a much lower rate of reversion is observed in terms of cell growth responsiveness to TPA with a total insensitivity to phorbol ester after 80 days and a 50% inhibition of RPh-4 cell proliferation after 3.5 months. Our data clearly demonstrate an apparent dissociation between the cellular protein kinase C content and the biological responsiveness to phorbol ester in the variant RPh-4 cells. Moreover, they suggest that the Mr 28,000 protein phosphorylation event is not directly related to the cell growth arrest induced by phorbol esters in MCF-7 cells.

Effects of differentiation-inducing agents on maturation of human MCF-7 breast cancer cells

The effects of the differentiation inducing agents (DIAS), sodium butyrate (NaBu), retinoic acid (RA), dimethylformamide (DMF), hexamethylene bisacetamide (HMBA), forskolin, and 12-O-tetradecanoylphorbol-13-acetate (TPA), on the growth, morphology, and estrogen receptor (ER) content and epithelial membrane antigen (EMA) expression on a serumless human breast cancer cell line (MCF-7) were compared. All these agents reversibly caused a concentration-dependent growth inhibition in monolayers and markedly reduced colony-forming efficiency in soft agar. A twofold increase in doubling time was obtained with RA (1 microM), but cell replication ceased with NaBu (1 mM), forskolin (50 microM), DMF (1%), HMBA (5 mM), and TPA (8 nM). Total growth arrest induced by these last compounds was preceded by an accumulation of cells in G0/G1 phase observed at 24 h by flow cytometry and accompanied by a change in cell morphology as seen by light and electronic microscopy. An increase in cell volume and the presence of lipid droplets was noted in treated cells that were spread out, as compared with controls. The acquisition of a more mature phenotype was confirmed by an increased expression of EMA monitored by flow cytometry. A specific reduction in the number of ER without any constant dissociation (Kd) modification was also observed after treatment with the 5 DIAs. No modification of morphological or biochemical characteristics, including EMA expression and ER binding, were observed for RA (1 microM)-treated cells. All these results suggest that induction of a more differentiated phenotype is associated with a block in G1 cell cycle phase, resulting in total growth arrest. Apparently, RA (1 microM)-treated cells did not fulfill these criteria, since only a slight accumulation in G1 and a slowed growth rate were evaluated.

Transcriptional regulation of DF3 gene expression in human MCF-7 breast carcinoma cells

The DF3 gene codes for a high molecular weight human breast tumor-associated glycoprotein. The detection of this antigen in human milk has also suggested that its expression represents a differentiated function of mammary epithelium. The present studies have examined the regulation of DF3 gene expression in human MCF-7 breast carcinoma cells. These cells express two DF3 transcripts of 4.5 and 7.0 kb. Treatment of MCF-7 cells with 12-0-tetradecanoylphorbol-13-acetate (TPA) was associated with increases in levels of both DF3 mRNAs. When nuclear run-on assays were used, DF3 gene transcription was at low to undetectable levels in untreated MCF-7 cells and was increased after TPA exposure. TPA-induced increases in DF3 mRNA levels were also inhibited by actinomycin D (ACT). MCF-7 cells exposed to ACT further demonstrated that the half-lives of the 4.5 and 7.0 kb transcripts are 26 and 11 h, respectively. The results also demonstrate that the protein synthesis inhibitor, cycloheximide (CHX), increases DF3 mRNA levels in MCF-7 cells. These effects of CHX were sensitive to actinomycin D and not associated with stabilization of the DF3 transcripts. Taken together, these findings indicate that DF3 gene expression is controlled at a transcriptional level in TPA- and CHX-treated MCF-7 cells.

Transforming growth factor beta (TGF-beta) reverses phorbol diester resistance of a breast adenocarcinoma (MCF-7) subline

We investigated the effects of TGF-beta on a MCF-7 subline (MCF-7:RPh-4) which is resistant to phorbol diesters with respect to growth inhibition and estrogen receptor content modulation. This biological unresponsiveness of MCF-7:RPh-4 cells to phorbol esters seems to be unrelated to activation of protein kinase C. In the presence of 80 nM PMA (12-O-tetradecanoylphorbol-13-acetate), TGF-beta induced a dose-dependent inhibition of MCF-7:RPh-4 cell proliferation. MCF-7:RPh-4 cells grown in PMA-free medium for at least 28 days remained insensitive to PMA but lost sensitivity to TGF-beta. Under these conditions, addition of 80 nM PMA restored sensitivity to TGF-beta. In the presence of a fixed concentration of TGF-beta, the dose-dependent inhibition of proliferation and the decrease in estrogen receptor content induced by PMA were comparable to those observed in PMA-treated parental MCF-7 cells. These observations indicate that TGF-beta reverses PMA resistance in MCF-7:RPh-4 cells. In addition, TGF-beta does not modify the basal or PMA-stimulated phosphorylation of Mr 28,000 endogenous protein. These results suggest that TGF-beta interferes with the protein kinase C pathway independently of enzyme activation.

Characteristics of the growth inhibition and cytotoxicity of long-chain (sphingoid) bases for Chinese hamster ovary cells: evidence for an involvement of protein kinase C

Long-chain bases are potent inhibitors of protein kinase C and cellular processes mediated by this enzyme. However, when added to cells they usually cause some degree of growth inhibition and cytotoxicity and it is unclear whether this reflects inhibition of protein kinase C or nonspecific detergent effects of these amphipathic compounds. This study examined the effects of sphinganine on Chinese hamster ovary (CHO) cells to gain more insight into these possibilities. Sphinganine concentrations between 0.75 and 4 microM resulted in a combination of growth inhibition and cytotoxicity that correlated with protein kinase C inhibition by five criteria: (1) the effective concentrations were comparable to those for protein kinase C inhibition in vitro and in other intact cells; (2) the structural specificity for the long-chain base moiety paralleled the potency of protein kinase C inhibition; (3) sphinganine blocked changes in protein phosphorylation patterns that occurred in response to phorbol 12-myristate 13-acetate (and vice versa); whereas (4) a mutant cell line that exhibited increased resistance to sphinganine cytotoxicity lacked both phorbol ester- and sphinganine-induced phosphorylation changes and differed somewhat in the behavior of protein kinase C assayed in vitro; and (5) sphinganine did not appear to be acting as a detergent (except at higher concentrations) nor as a lysosomotrophic agent. While the complexity of this cellular behavior mandates caution in interpreting these results, they suggest that the cytotoxicity and growth inhibition may be a consequence of protein kinase C inhibition.

Modulation of estrogen receptor and epidermal growth factor receptor mRNAs by phorbol ester in MCF 7 breast cancer cells

Previous studies have demonstrated an inverse relationship between estrogen receptor (ER) and epidermal growth factor receptor (EGF-R) gene expression in human breast cancer cells. This relationship was further investigated in MCF 7 cells treated with 12-O-tetradecanoylphorbol-13-acetate (TPA). Exposure to 10 nM TPA resulted in a time-dependent increase in EGF-R mRNA, first apparent at 3 h and maximal between 9 and 24 h. There was a concomitant fall in ER mRNA with a maximum decline to 15-20% of control between 12 and 24 h. Although EGF-R mRNA levels declined between 24 and 72 h, both EGF-R mRNA and EGF-R binding remained above control levels and this was accompanied by a sustained depression of ER mRNA. These data support the view that ER and EGR-R gene expression is inversely regulated in human breast cancer and describe for the first time an inhibitory effect of a phorbol ester on steroid hormone receptor gene expression.

Amphiregulin: a bifunctional growth-modulating glycoprotein produced by the phorbol 12-myristate 13-acetate-treated human breast adenocarcinoma cell line MCF-7

A glycoprotein, termed amphiregulin (AR), inhibits growth of several human carcinoma cells in culture and stimulates proliferation of human fibroblasts and certain other tumor cells. It has been purified to apparent homogeneity from serum-free conditioned medium of MCF-7 human breast carcinoma cells that had been treated with phorbol 12-myristate 13-acetate. AR is a single-chain extremely hydrophilic glycoprotein containing cysteines in disulfide linkage(s) that are essential for biological activity; it is stable between pH 2 and pH 12 and after heating for 30 min at 56 degrees C but unstable at 100 degrees C. The apparent molecular weights of AR and N-Glycanase-treated AR are 14,000 and 15,000, respectively, as assessed by gel chromatography, and approximately 22,500 and approximately 14,000, respectively, as determined by polyacrylamide gel electrophoresis. Treatment of AR with N-Glycanase, O-Glycanase, or neuraminidase does not affect its activity. The pI of AR is approximately 7.8. The amino-terminal amino acid sequence of AR has been determined, and no significant sequence homology between AR and other proteins was found. The molecule thus appears to be a distinct growth regulatory protein.

Modulation of estrogen receptors by phorbol diesters in human breast MCF-7 cell line

Treatment of MCF-7 cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) results in an inhibition of cell proliferation and a reduction in the number of estrogen receptors (ER), shown by binding studies and immunoassay. The decrease in ER concentration induced by phorbol ester derivatives parallels their growth inhibitory effect. Moreover, the estrogen receptor of TPA-resistant RPh4 cells (which are insensitive to the antiproliferative and morphological effects of TPA) is not affected by TPA treatment. The reduction in ER concentration appear to be a specific phenomenon since it contrasted with the 2-fold increase in total cell protein content which included an increase in progesterone receptor (PgR). We also found that addition of TPA does not affect estrogen induction of PgR.

Regulation of ribosomal protein S6 kinase in human mammary tumor cells: effect of estrogen, growth factors and phorbol ester

Growth of human mammary tumor cells ZR-75-1 is stimulated by estradiol (E2), epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I). In these cells ribosomal protein S6 kinase is activated by EGF, IGF-I, insulin and phorbol 12-myristate 13-acetate (TPA) but not by E2. The human mammary tumor cell line MDA-MB 231, which is E2-receptor negative, has receptors for EGF, IGF-I and insulin but is unresponsive to these factors in terms of growth and S6 kinase activation. The role of protein kinase C (PKC) in the activation of S6 kinase by growth factors and TPA was investigated in ZR-75-1 cells. Down regulation of PKC activity by treatment with TPA for 48-h blocks the stimulation of S6 kinase by TPA but leaves the activation by EGF, IGF-I and insulin unaffected. In intact ZR-75-1 cells staurosporine blocks activation of S6 kinase by EGF and TPA, however with different IC50. The results show that S6 kinase is not activated by estradiol, that its activation by EGF, IGE-I and insulin does not depend on the presence of PKC activity and that its activation by TPA is mediated by a different (PKC-dependent) pathway.

Epidermal growth factor induces internalization but not degradation of the epidermal growth factor receptor in a human breast cancer cell line

Metabolism of the epidermal growth factor (EGF) receptor was studied in the MDA-MB-231 human breast cancer cell line. As in normal fibroblasts the EGF receptor from MDA-MB-231 cells was synthesized from a Mr = 160,000 precursor and tunicamycin treatment of cells resulted in accumulation of a Mr = 130,000 polypeptide. Unlike normal fibroblasts in which a Mr = 170,000 mature form of the EGF receptor was found, MDA-MB-231 cells contained a Mr = 172,000 mature form. Addition of EGF to MDA-MB-231 cells led to rapid internalization of EGF receptors, however, internalization did not affect receptor half-life and receptors did not recycle to the cell surface. EGF receptors could be visualized by immunofluorescence and remained sequestered in intracellular membranous structures following internalization. EGF was degraded slowly by MDA-MB-231 cells relative to degradation of EGF by normal cells. A high endogenous level of in vivo phosphorylation of threonine 654 of the EGF receptor was found in MDA-MB-231 cells and treatment of cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) further stimulated phosphorylation of this residue. EGF induced receptor internalization resulted in dephosphorylation of threonine 654. The significance of these unusual properties of EGF receptor metabolism in MDA-MB-231 cells is discussed.

TPA induces subcellular translocation and subsequent down-regulation of both phorbol ester binding and protein kinase C activities in MCF-7 cells

Phorbol ester TPA has been previously shown to induce a rapid translocation, followed by a progressive decline of protein kinase C activity in MCF-7 cells (J.M. Darbon et al, 1986, Biochem. Biophys. Res. Comm. 137: 1159-1166). We show now a parallel TPA-induced movement of phorbol ester binding sites from the cytosolic to the particulate fraction with no change in the binding affinities for the (3H) PDBu probe (KD congruent to 2 nM). The subcellular redistribution process is followed by a rapid decrease of the phorbol ester binding capacity at the membrane level. The concomitant decline in both phorbol ester binding and protein kinase C activities that we observed during the course of TPA treatment strongly argues for a real down-regulation of the enzyme in phorbol ester-treated MCF-7 cells. The molecular mechanisms of these events and their relations to the inhibition of cell growth remain to be clarified.

Phorbol esters inhibit the proliferation of MCF-7 cells. Possible implication of protein kinase C

The effect of tumor promoter phorbol esters on cell proliferation was investigated in human breast cancer cell line MCF-7. During a 4-day culture period, the various phorbol ester derivatives TPA, PDD, PDBu, PDBz and PDA inhibited the proliferation of MCF-7 cells in a dose-dependent manner, with respective IC50 of 0.06, 0.75, 2.4, 3.6 and 15 X 10(-9) M. The 4-O-met-TPA, alpha PDD and alph PHR were ineffective at 2 X 10(-7) M, the highest concentration tested. Using a 3H-PDBu probe, we demonstrated the presence of specific, high affinity binding sites in intact cultured cells, with a Kd of about 9 X 10(-9) M. Unlabelled TPA, PDD, PDBU and PDBz competed with 3H-PDBu with respective IC50 of 35, 12.5, 150 and 220 X 10(-9) M. High concentrations of PDA, 4-O-met-TPA and alpha PDD slightly inhibited the 3H PDBu binding, whereas alpha PHR did not until 10(-5) M. The correlation that we observed between the relative potencies of the various phorbol derivatives for inhibiting both PDBu binding and cell proliferation, suggests that tumor promoter phorbol esters may induce growth arrest in MCF-7 cells by the mediation of protein kinase C.

Phorbol esters induce both intracellular translocation and down-regulation of protein kinase C in MCF-7 cells

Exposure of MCF-7 human breast cancer cells to phorbol ester 12-O-tetradecanoyl-13-acetate (TPA) results in a complete inhibition of cell proliferation. We investigated the effects of TPA on protein kinase C activity when cells were exposed to phorbol ester for various lengths of time. TPA induces within 5 min a drastic dose-dependent decrease of the cytosolic protein kinase C activity. The enzyme apparently lost at the cytosolic level was only partially recovered in the particulate fraction. The apparent down-regulation of the translocated enzyme which was only 34% after 1 min reached 72% and 84% after respectively 10 min and 15 min. Moreover, when cells are treated with TPA for longer periods of time, the particulate protein kinase C activity continues to decrease, dropping below control after 1 hour. This progressive decline leads to an almost complete disappearance of protein kinase C activity in MCF-7 cells after 45 hours of TPA treatment. The apparent loss of protein kinase C activity upon short- as well as long-exposure of cells to TPA was not accompanied by a concomitant increase of Ca, PL-independent protein kinase activity. We discuss the implication of these biochemical events in the inhibition of cell proliferation with regard to the respective short- and long-term effects of TPA on protein kinase C activity.

Activation by phorbol esters of protein kinase C in MCF-7 human breast cancer cells

Exposure of MCF-7 human breast cancer cells to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) leads to the inhibition of cell proliferation. We investigate here the short-term effects of TPA on subcellular distribution of protein kinase C, and on protein phosphorylation in cultured MCF-7 cells. We report a rapid and dramatic decrease in cytosolic protein kinase C activity after TPA treatment. Only 30% of the enzymatic activity lost in the cytosol was recovered in the particulate fraction. These data suggest that subcellular translocation of protein kinase C is accompanied by a rapid down-regulation of the enzyme (70%). Furthermore, TPA and other protein kinase C activators rapidly induce the phosphorylation of a 28 kDa protein in intact MCF-7 cells. Phorbol esters devoid of tumor-promoting activity are ineffective both for inducing these early biochemical events and for inhibiting cell proliferation.

Protein kinase C desensitization by phorbol esters and its impact on growth of human breast cancer cells

Active phorbol esters such as TPA (12-0-tetra-decanoylphorbol-13-acetate) inhibited growth of mammary carcinoma cells (MCF-7 greater than BT-20 greater than MDA-MB-231 greater than = ZR-75-1 greater than HBL-100) with the exception of T-47-D cells presumably by interacting with the phospholipid/Ca2+-dependent protein kinase (PKC). The nonresponsive T-47-D cells exhibited the lowest PKC activity. A rapid (30 min) TPA-dependent translocation of cytosolic PKC to membranes was found in the five TPA-sensitive cell without affecting cell growth. However, TPA-treatment of more than 10 hours inhibited reversibly the growth of TPA-responsive cells. This effect coincided with the complete loss of cellular PKC activity due to the proteolysis of the translocated membrane-bound PKC holoenzyme (75K) into 60K and 50K PKC fragments. Resumption of cell growth after TPA-removal was closely related to the specific reappearance of the PKC holoenzyme activity (75K) in the TPA-responsive human mammary tumor cell lines suggesting an involvement of PKC in growth regulation.

Correlation between hormone dependency and the regulation of epidermal growth factor receptor by tumor promoters in human mammary carcinoma cells

The effects of the tumor promoter phorbol 12-tetradecanoate 13-acetate (TPA) on the epidermal growth factor (EGF) receptor levels were investigated in hormone-dependent (MCF-7, T-47-D, and ZR-75-1) and hormone-independent (MDA-MB-231, HBL-100, and BT-20) human mammary carcinoma cell lines. In the absence of TPA, hormone-independent cell lines contained high concentrations of low-affinity EGF receptors (apparent Kd = 8 X 10(-10) M), whereas hormone-dependent cell lines exhibited low concentrations of high-affinity receptors (apparent Kd = 1 X 10(-10) M). TPA causes a change of the receptor from a high- to the low-affinity state in hormone-dependent cell lines (MCF-7, T-47-D, and ZR-75-1), as well as in the hormone-independent HBL-100, whereas the affinity remained unchanged in MDA-MB-231 and BT-20 cells. In addition, progesterone receptor levels are decreased after TPA treatment in the hormone-dependent cell lines MCF-7, T-47-D, and ZR-75-1, whereas the estrogen receptor levels remained unchanged. Tumor promoters such as TPA or teleocidin inhibited the proliferation of these cell lines at concentrations above 10 microM with the exception of the T-47-D cells. The most sensitive cell line towards growth inhibition by tumor promoter was the hormone-dependent MCF-7 cell line. Evaluation of different TPA analogs indicated a positive correlation between the growth-inhibitory effects and their ability to stimulate the subcellular redistribution of protein kinase C activity in MCF-7 cells. These data suggest a protein kinase C-mediated down-regulation of the progesterone receptor concentration and of the EGF receptor affinity, which is supposed to mediate the mitogenic response. Furthermore, these results support the hypothesis that the tumor-derived growth factors induced by estradiol act via the EGF receptor in hormone-dependent mammary carcinoma cells.

The cytosolic phorboid receptor correlates with hormone dependency in six mammary carcinoma cell lines

Potent, structurally different tumor promoters inhibited growth of 6 human mammary carcinoma cell lines (ROOS et al, PNAS in press). This growth inhibition was investigated by measuring the phorboid receptor binding using [3H] PDBu (4 beta-phorbol 12, 13 dibutyrate). Specific, high affinity receptors were found in all six cell lines. [3H] PDBu binding affinities were higher in the cytosolic fractions than in the corresponding intact cells (K alpha = app. 1nM vs K alpha = app. 15nM). The hormone-independent cell lines (BT-20, HBL-100 and MDA-MB-231) exhibited significantly higher levels of cytosolic [3H] PDBu receptors than the hormone-dependent cells (MCF-7, T-47-D and ZR-75-1). The subcellular distribution of the [3H] PDBu binding correlated well with the distribution of the protein kinase C activity (r = 0.95).

Effects of epidermal growth factor and 12-O-tetradecanoylphorbol-13-acetate on metabolism of the epidermal growth factor receptor in normal human fibroblasts

The biosynthesis, phosphorylation, and degradation of the epidermal growth factor (EGF) receptor were examined in normal human fibroblasts. The receptor was initially synthesized as an Mr = 160,000 immature form which matured to an Mr = 170,000 form in a monensin-sensitive manner. Tunicamycin treatment led to the accumulation of an Mr = 130,000 protein. The receptor was phosphorylated on serine and threonine residues in normally growing and quiescent cells, and treatment with EGF or the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a two- to threefold increase in receptor-bound phosphate. EGF increased the amount of phosphoserine and phosphothreonine and caused the appearance of a minor amount of phosphotyrosine. TPA increased the levels of phosphoserine and phosphothreonine exclusively. Prior treatment with TPA inhibited the EGF-dependent appearance of phosphotyrosine in the receptor. Analysis of tryptic phosphopeptides revealed that six of the seven major peptides were common to the receptor from cells treated with EGF or TPA. EGF strongly stimulated [3H]thymidine incorporation in confluent cells, increased final saturation density three to fourfold, and increased whole-cell levels of phosphotyrosine about threefold. Treatment of cells with TPA before addition of EGF inhibited all three of these EGF-dependent responses. EGF also decreased the receptor half-life from 15 h to 1 h, but this was not inhibited by TPA. TPA alone had no detectable effect on the receptor half-life.

Tumor promoters stimulate hyperplasia of microtubule organizing center and inhibit DNA synthesis in cultured cells

Chemical tumor promoters induce significant morphologic changes in several cultured cell models. In this article we describe a new effect of two potent, chemically different tumor promoters, 12-O-tetradecanoylphorbol-13-acetate (TPA) and dihydroteleocidin B (DHTB) on cultured human HeLa and melanoma cells. Using immunofluorescence microscopy, we observed that TPA and DHTB induced a dramatic increase in the size (greater than or equal to 3X normal diameter) of the centrosome, a microtubule-organizing center, within 24 h of incubation. In HeLa cells the effect was serum- and dose-dependent, was observed in 76-92% of cells within 72 h of incubation, and was associated with an increase in cytoplasm-nucleus ratio and proliferation of microtubules from the centrosome. The tumor promoters inhibited serum-induced DNA synthesis in both cell lines. Electron microscopy revealed the presence of clumps of microcentriole bodies or fragments adjacent to the intact centriole.

Effects of epidermal growth factor and 12-O-tetradecanoylphorbol-13-acetate on metabolism of the epidermal growth factor receptor in normal human fibroblasts

The biosynthesis, phosphorylation, and degradation of the epidermal growth factor (EGF) receptor were examined in normal human fibroblasts. The receptor was initially synthesized as an Mr = 160,000 immature form which matured to an Mr = 170,000 form in a monensin-sensitive manner. Tunicamycin treatment led to the accumulation of an Mr = 130,000 protein. The receptor was phosphorylated on serine and threonine residues in normally growing and quiescent cells, and treatment with EGF or the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a two- to threefold increase in receptor-bound phosphate. EGF increased the amount of phosphoserine and phosphothreonine and caused the appearance of a minor amount of phosphotyrosine. TPA increased the levels of phosphoserine and phosphothreonine exclusively. Prior treatment with TPA inhibited the EGF-dependent appearance of phosphotyrosine in the receptor. Analysis of tryptic phosphopeptides revealed that six of the seven major peptides were common to the receptor from cells treated with EGF or TPA. EGF strongly stimulated [3H]thymidine incorporation in confluent cells, increased final saturation density three to fourfold, and increased whole-cell levels of phosphotyrosine about threefold. Treatment of cells with TPA before addition of EGF inhibited all three of these EGF-dependent responses. EGF also decreased the receptor half-life from 15 h to 1 h, but this was not inhibited by TPA. TPA alone had no detectable effect on the receptor half-life.

[Epidermal growth factor: structure, location, phosphorylation and regulation of its receptor]

Epidermal growth factor (EGF) is a Mr 6045 polypeptide first characterized for its ability to stimulate mitogenesis in epidermal and epithelial cells. The first step in the action of the growth factor is its binding to specific, high affinity membrane receptors. These receptors have been studied in a number of tissues and cell culture lines. The level of EGF receptors is modulated by many agents. EGF down-regulates its receptor. In addition, the number of EGF receptors is decreased by other growth factors (platelet-derived growth factor; transforming growth factor), by many tumor promoters and by viral transformation. Several hormones also can regulate EGF binding in its target tissues.

An inhibitory effect of tumour promoters on human epithelial cell growth can be dissociated from an effect on junctional communication

Studies with rodent cells have indicated that the abilities of various tumour promoters to inhibit metabolic cooperation correlate with their potencies as mitogens. Here we have examined the effects of the most potent phorbol ester tumour promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA), on metabolic cooperation and growth of human epidermal cells transformed by SV40 (SVK14 cells). In this system, TPA inhibits junctional communication and at the same concentration also inhibits growth in a reversible fashion. These effects appear to be mediated by binding of phorbol ester to a single class of high affinity binding site with a Kd similar to that reported for rodent cells (Kd = 20.9 nM at 4 degrees C). Further studies on the effects of phorbol esters on other human epithelial cell lines reveal that the inhibitory effects of TPA on growth and metabolic cooperation may be completely dissociated. Alternative mechanisms by which TPA may exert its growth-inhibitory effects are discussed.

Glucocorticoids and prostaglandins inhibit the induction of macrophage DNA synthesis by macrophage growth factor and phorbol ester

The tumor-promoting phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), stimulates starch-elicited mouse peritoneal macrophages to undergo DNA synthesis in vitro, apparently without the generation of an endogenous macrophage growth factor (MGF). No evidence was found for any synergistic interaction between TPA and exogenous colony stimulating factors (CSFs) for macrophage DNA synthesis. Low concentrations of glucocorticoids and also prostaglandins E1 and E2 suppress both the CSF-1-stimulated and the TPA-stimulated macrophage DNA synthesis; these same drugs inhibit the CSF-1-mediated and TPA-mediated enhancement of macrophage plasminogen activator (PA) activity. Thus glucocorticoids and prostaglandins E1 and E2 oppose the action of growth factors and the tumor promoter on macrophage and precursor cell function.