A role for protein kinase C delta in the differential sensitivity of MCF-7 and MDA-MB 231 human breast cancer cells to phorbol ester-induced growth arrest and p21(WAFI/CIP1) induction.

The goal of this study was to investigate the differential sensitivity of estrogen receptor (ER) positive MCF-7 and ER negative MDA-MB 231 breast cancer cells to phorbol myristate acetate (PMA)-dependent growth arrest. MCF-7 cells were growth arrested by 80% while MDA-MB 231 cells were arrested by 20% in response to seven days of treatment with 10 nM PMA. Coincident with the increased sensitivity of MCF-7 cells to be growth arrested by the protein kinase C (PKC) activator PMA, PMA induced 9-fold higher levels of the cyclin dependent kinase (Cdk) inhibitor p21(WAF1/GIP1) in MCF-7 compared to MDA-MB 231 cells. A comparison of the PKC isoforms expressed in MCF-7 versus MDA-MB 231 cells showed that only the PMA-sensitive PKC delta and eta isoforms were expressed at markedly (> or =10-fold) elevated levels in MCF7 versus MDA-MB 231 cells. These results suggested that the differential sensitivity to growth arrest and induction of p2l(WAFl/CIPl) could reflect, at least in part, increased expression of PMA-dependent PKC isoforms delta and/or eta. Direct evidence to support this hypothesis was provided by the ability of transient transfections into MCF-7 cells of constitutively active PKC delta but not of PKC's eta or alpha or epsilon to enhance p21(WAFl/CIP1) promoter activity. These results suggest that PKC delta plays a fundamental role in the regulation of growth in estrogen receptor positive breast cancer cells.

8-chloro-cAMP and 8-chloro-adenosine act by the same mechanism in multiple myeloma cells.

Previous work with 8-chloro-cAMP (8-Cl-cAMP) has raised questions as to whether it works as a cAMP analogue or as a nucleoside analogue after its conversion to 8-chloro-adenosine (8-Cl-Ado). Although degradation of 8-Cl-cAMP to 8-Cl-Ado in culture medium or plasma has been shown, cellular pharmacology data are missing. The purpose of the present study was to identify the cellular metabolism of these drugs and their actions in a human multiple myeloma cell line. The cells were incubated with either 8-Cl-Ado or 8-Cl-cAMP to follow the cellular metabolism of these agents. Both 8-Cl-cAMP and 8-Cl-Ado incubation resulted in the accumulation of 8-Cl-Ado mono-, di-, and tri-phosphate (8-Cl-ATP), however, the triphosphate was the major cytotoxic metabolite. Accumulation of 8-Cl-ATP was dependent on both the exogenous concentration of 8-Cl-Ado and incubation time. At the 10 microM level of 8-Cl-Ado, >400 microM 8-Cl-ATP accumulated in multiple myeloma cells after continuous incubation for 12 h. Similar incubation with 8-Cl-cAMP also resulted in accumulation of 8-Cl-ATP in the cells, albeit at a lower level. The formation of 8-Cl-ATP from 8-Cl-cAMP was inhibited by >80% in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine in the medium, suggesting extracellular conversion of 8-Cl-cAMP to 8-Cl-Ado. Cells lacking Ado kinase did not accumulate 8-Cl-ATP, either from 8-Cl-Ado or 8-Cl-cAMP, and were resistant to these agents. There was also a decline in the endogenous level of the cellular ATP pool parallel to the accumulation of 8-C1-ATP. The elimination of 8-Cl-ATP was biphasic and slow from the cells. The accumulation of 8-Cl-ATP and a decline in the ATP pool inhibited RNA synthesis but did not affect DNA synthesis for up to 12 h of incubation. Taken together, these data demonstrate that the cytotoxic metabolite of 8-Cl-Ado and 8-Cl-cAMP is 8-Cl-ATP. Hence, 8-Cl-cAMP serves as a prodrug and is converted to 8-Cl-Ado in medium with subsequent phosphorylation to accumulate as 8-Cl-ATP in cells. At the cellular level, 8-Cl-ATP is associated with a decrease in the endogenous ATP pool; at the nuclear level, it inhibits RNA synthesis.

Regulation of protein kinase C delta by estrogen in the MCF-7 human breast cancer cell line.

We have previously shown that estrogen up-regulates expression of protein kinase C (PKC) delta in the rat and rabbit corpus luteum as well as in luteinized rat granulosa primary cell cultures. To determine whether a similar regulation of the PKC delta isoform by estrogen occurred in another estrogen responsive system, we investigated the estrogen receptor positive MCF-7 human breast cancer cells. In a characterization of PKC isoforms in MCF-7 cells we determined that PKC delta was the predominant PKC isoform. However in contrast to the effect of estrogen on PKC delta expression in ovarian cells, estrogen treatment of MCF-7 cells resulted in a significant decrease in PKC delta protein and mRNA expression in a time and dose dependent manner. Treatment of MCF-7 cells with 10(-10)-10(-8) M estrogen for 7 days down-regulated specifically PKC delta mRNA and protein while expression of other PKC isoforms was unchanged. The opposite regulation of PKC delta expression in ovarian and breast cancer cells prompted us to evaluate the type of estrogen receptor present in both cell types. Results showed that luteinized rat granulosa cells expressed predominantly estrogen receptor beta while the MCF-7 cells expressed predominantly estrogen receptor alpha and barely detectable levels of estrogen receptor beta. These results suggest that the differential ability of estrogen to regulate PKC beta expression could potentially be a result of differential signaling through the two estrogen receptor subtypes.

8Cl-cAMP cytotoxicity in both steroid sensitive and insensitive multiple myeloma cell lines is mediated by 8Cl-adenosine.

We have examined the cytotoxic effects of cyclic adenosine-3', 5'-monophosphate (cAMP) derivatives on multiple myeloma cells lines and determined that the 8-Chloro substituted derivative (8Cl-cAMP) is one of the most potent. We report here that 8Cl-cAMP is cytotoxic to both steroid sensitive and insensitive myeloma cells with a half maximal concentration of approximately 3 micromol/L. 8Cl-cAMP toxicity in myeloma cells is dependent on phosphodiesterase activity in the serum of cell culture medium. A metabolite of 8Cl-cAMP, 8-Chloro-adenosine (8Cl-AD), kills myeloma cells as effectively as 8Cl-cAMP. Adenosine deaminase (ADA) converts 8Cl-AD into 8Cl-inosine and abrogates the cytotoxic effects of 8Cl-cAMP, 8Cl-AMP, and 8Cl-AD, as does 5-(p-Nitrobenzyl)-6-Thio-Inosine (NBTI), an inhibitor of nucleoside uptake. These data suggest that 8Cl-cAMP must be converted to 8Cl-AD and that 8Cl-AD is the compound that enters the cell. Contrary to glucocorticoid-mediated cell death in myeloma cells, the pathway of 8Cl-AD-mediated cell death appears to be independent of interleukin-6 (IL-6) actions. Although the exact mode of action for this agent is currently unknown, its ability to kill steroid sensitive and insensitive multiple myeloma cells in an IL-6 independent fashion may offer exciting new therapeutic options.

Association of PKC delta and active Src in PMA-treated MCF-7 human breast cancer cells.

Phorbol ester treatment of MCF-7 cells led to the tyrosine phosphorylation and activation of PKC delta. However, through Western blot analysis and in vitro immunecomplex kinase assays, we detected a differential localization of tyrosine-phosphorylated PKC delta and catalytically active PKC delta. Catalytically active PKC delta was concentrated in Triton X-100 solubilized-membrane fractions while tyrosine-phosphorylated PKC delta was localized to the cytosol fraction. Phorbol ester treatment of MCF-7 cells stimulated both the time-dependent in vivo association of Src with PKC delta, evidenced in Src immunoprecipitates by the co-immunoprecipitation of PKC delta, and activation of Src, evidenced in Src immunoprecipitates as an increase in reactivity with a Src antibody (clone 28) reactive only with active Src (dephosphorylated on residue 530) and in Src and PKC delta immunoprecipitates by an increase in Src kinase activity. While our data are consistent with reports in the literature showing the activator/stimulus-dependent tyrosine phosphorylation of PKC delta, our data show that the tyrosine phosphorylation of PKC delta is not essential for kinase activity. These results are the first to demonstrate an in vivo association between PKC delta and active Src in the absence of over-expression of either PKC delta or Src, and support the association of Src and PKC delta towards a physiological function.

Cyclic adenosine-3',5'-monophosphate-mediated cytotoxicity in steroid sensitive and resistant myeloma.

Multiple myeloma is a neoplastic proliferation of plasma cells. Glucocorticoids are among the most effective agents against multiple myeloma, acting through the glucocorticoid receptor to induce programmed cell death. However, some patients do not respond to glucocorticoids, and those that do respond eventually develop resistance to this therapy. Alternative strategies using drugs that mediate cytotoxicity through complementary pathways have theoretical appeal. Cyclic adenosine-3',5'-monophosphate (cAMP) derivatives are cytotoxic to a number of cell lines of lymphocytic origin. cAMP analogues activate protein kinase A, affecting cell growth and differentiation. The cascade of events initiated by cAMP derivatives and glucocorticoid, although distinct, may share some distal molecular targets. We have found that pharmacological concentrations of 8-chloro-cAMP, dibutyryl-cAMP, and 8-bromo-cAMP are cytotoxic to multiple myeloma cells, enhance glucocorticoid effects, and can kill glucocorticoid-resistant clones. cAMP analogues induce apoptosis as demonstrated by the fragmentation of myeloma DNA chromatin in a distinctive ladder pattern. In contrast to glucocorticoids, cAMP growth inhibition cannot be reversed by exogenous interleukin 6. cAMP derivatives have activity against multiple myeloma and are appropriate candidates for clinical trials.

Cell-specific expression of activin and its two binding proteins in the rat decidua: role of alpha 2-macroglobulin and follistatin.

Rat decidual tissue is formed by two distinct decidual cell populations located either antimesometrially or mesometrially in the uterus. They differ in morphology, the genes they express, the proteins they secrete, and the role they play during pregnancy. Recently, we have shown that rat decidua expresses follistatin and alpha 2-macroglobulin (alpha 2-MG), two binding proteins to activin. In the present study, we determined whether the decidua of pregnant and pseudopregnant rats also expresses activin, whether activin messenger RNA (mRNA) is confined to a particular cell population, and whether it is regulated by its binding proteins. Decidual and placental tissues were collected at different stages of pseudopregnancy or pregnancy. mRNA expression was examined by in situ hybridization, reverse transcription-polymerase chain reaction, and Northern analysis. Developmental studies revealed that activin A became highly expressed in the antimesometrial decidua only from day 11 at a time when this tissue was undergoing extensive degeneration. Very little activin A mRNA could be detected in the mesometrial decidua. However, late in pregnancy, significant expression of activin A mRNA was detected in the mesometrial decidua undergoing extensive cell death at this stage. Developmental study revealed that activin A mRNA became expressed in the antimesometrial decidua only when follistatin mRNA disappeared from this tissue. Furthermore, mesometrial decidua expressing the most alpha 2-MG mRNA had reduced levels of activin A mRNA. These data suggest that follistatin and alpha 2-MG may, by binding to activin, prevent activin A from stimulating the expression of its own gene. To examine this possibility we first established that the rat decidua expresses activin receptor II at a constant level between days 11-15. Then we examined whether follistatin and alpha 2-MG down-regulate activin expression in a simian virus 40-transformed decidual cell line (GG-AD). These cells express activin A mRNA in abundance, very little follistatin, and no alpha 2-MG. Follistatin and alpha 2-MG caused a dose-related decrease in activin A mRNA levels in these cells. The same inhibitory effect was observed with activin A-blocking antibody. In summary, the results of this investigation demonstrate that rat decidual tissue expresses mRNAs for activin A and its two binding proteins; follistatin and alpha 2-MG. The expression of each mRNA is cell specific and developmentally regulated. The finding that both activin-binding proteins and antibody inhibit the expression of activin A in cultured decidual cells suggests that activin regulates its own gene expression in the decidua.(ABSTRACT TRUNCATED AT 400 WORDS)

A variant glucocorticoid receptor messenger RNA is expressed in multiple myeloma patients.

In multiple myeloma cells resistant to glucocorticoids, we have previously identified a variant glucocorticoid receptor (GR) transcript (P. A. Moalli et al., Cancer Res., 53: 3877-3879, 1993). Here, we report a reverse transcription-PCR assay to assess whether this aberrant GR transcript is present in myeloma patients. We detected both the wild-type and variant GR transcripts in the patient isolate that was the source of our myeloma cell lines, in patients refractory to steroid treatment, and in healthy control subjects. Simultaneous amplification of wild-type and variant GR mRNAs indicates that the variant GR is more highly expressed in cells that are resistant to glucocorticoids. We hypothesize that the variant GR is a normal mRNA transcript that acts to modulate glucocorticoid responsiveness, and increased expression contributes to a resistant phenotype.

Alternatively spliced glucocorticoid receptor messenger RNAs in glucocorticoid-resistant human multiple myeloma cells.

Glucocorticoids are highly effective chemotherapeutic agents used in the treatment of hematological malignancies including multiple myeloma. However, the clinical usefulness of this class of drugs is limited by the problem of resistance. In the following study, we have isolated two alternatively spliced transcripts of the glucocorticoid receptor from a complementary DNA library generated from the glucocorticoid-resistant myeloma cell line MM.1Re. In each of the clones, specific exons of the hormone binding domain are precisely deleted. Our data implicate alternate splicing as a mechanism by which a cell generates different receptor isoforms and as a consequence evades the effects of hormone.

T47DCO cells, genetically unstable and containing estrogen receptor mutations, are a model for the progression of breast cancers to hormone resistance.

We postulate that one mechanism for the progression of breast cancers to hormone resistance involves the acquisition of mutant estrogen receptors (ER)4 by genetically unstable cell subpopulations. The T47D human breast cancer cell line may be a model for such progression, having sublines that are ER positive and estrogen responsive, ER positive and estrogen resistant, or ER negative. Also, T47D cells can be either hyperdiploid (HD) or hypertetraploid (HT) or persistently alternate between these states. T47DCO cells are a HD and ER-positive, but estrogen-resistant, subline of T47D cells that undergoes spontaneous tetraploidization. Such a stable variant, designated T47Dv, is 85% HT (Cancer Res., 49: 3943, 1989). We now show that single-cell clones derived from the mixed HD/HT T47Dv can be either HD or HT, and can be either estrogen responsive or estrogen resistant, for growth and for progesterone receptor regulation. To begin the study of ER in this model system of T47DCO and their derivatives, we have generated complementary DNA libraries from the parental HD T47DCO cells and have isolated three ER complementary DNA mutants. These include two frame-shift/termination mutants that would encode ERs truncated in the DNA-binding domain and in the hormone-binding domain and a third mutant with a large in-frame deletion spanning the hinge region and a part of the hormone-binding domain. If expressed, these mutant ERs would lack hormone-binding capacity and would be undetected by the anti-ER antibodies currently in clinical use. Genetic instability, when associated with mutant ERs in subpopulations of breast tumor cells, may provide the selective pressure leading to hormone resistance. T47DCO cells and their clonal derivatives provide a model for the systematic study of ER mutations and other mechanisms of hormone resistance in Stage IV breast cancer.

Human progesterone receptor transformation and nuclear down-regulation are independent of phosphorylation.

We have studied the phosphorylation of progesterone receptors (PR) in T47Dco human breast cancer cells using a monoclonal antibody directed against human PR called AB-52. This antibody recognizes both the A- (Mr approximately 94,000) and B- (Mr approximately 120,000) hormone binding proteins of PR, and was used to immunoprecipitate phosphorylated receptors isolated from cells incubated in vivo with [32P]orthophosphate. The specific activity, or phosphorylation levels, relative to protein levels was quantified by combined immunoblotting and autoradiography followed by densitometry. We find that immunopurified untransformed hormone-free receptors, which have a characteristic triplet B, singlet A structure, are phosphoproteins with similar levels of phosphate incorporation in all protein bands. If PR are first transformed to the nuclear binding form by treatment of cells with progesterone, and then labeled with [32P]orthophosphate, the receptor proteins are additionally phosphorylated. These chromatin-bound hormone occupied receptors incorporate five to 10 times more labeled phosphate per total receptor protein than do PR from untreated cells during the same [32P]incubation time. The second round of phosphorylation may also account for mobility shifts of transformed A- and B-receptors observed in sodium dodecyl sulfate-polyacrylamide gels. Both untransformed and transformed species of A- and B-receptors are phosphorylated only on serine residues, and neither the extent of phosphorylation, nor the phosphoamino acids, are affected by treatment of the cells with epidermal growth factor or insulin. We previously reported that after hormone binding and transformation of receptors to the tight chromatin binding state, PR undergo processing, or nuclear down-regulation. AB-52 was used to compare PR protein and phosphorylation levels when cells were treated for 0.5-48 h with progesterone or the synthetic progestin R5020. Both agonists lead to hyperphosphorylation of nuclear PR before phosphorylation levels decrease, in parallel with the drop in protein levels as receptors down-regulate. Treatment of cells with RU 486, an antiprogestin, leads to PR transformation as determined by immunoblotting, but subsequent down-regulation does not occur. After transformation, chromatin-bound RU 486-occupied receptors become intensely phosphorylated however, with specific activities 15 times greater than those of untransformed PR. Since these receptors are phosphorylated but not processed, the hormone-induced nuclear phosphorylation of PR is unlikely to be a signal for receptor processing.(ABSTRACT TRUNCATED AT 400 WORDS)

Human progesterone A-receptors can be synthesized intracellularly and are biologically functional.

In order to investigate the origin and functional independence of the human progesterone receptor A binding protein, we have expressed a truncated human progesterone receptor cDNA in both gene transfer and in vitro translation assays. Proteins identical in size and antigenicity to the A-receptors found naturally in human progesterone target cells are synthesized from this cDNA that lacks the putative B receptor initiator methionine codon of the complete cDNA. The functional independence of A-receptors is suggested by their ability to bind hormone and to stimulate transcription from the progestin responsive mouse mammary tumor virus promoter.

Multiple human progesterone receptor messenger ribonucleic acids and their autoregulation by progestin agonists and antagonists in breast cancer cells.

We have used AB-52, a monoclonal antibody which recognizes both the A (94,000 daltons) and B (120,000 daltons) proteins of human progesterone receptors (hPR), and hPR-50, a PR complementary DNA probe isolated from a T47D-pcD library, to study the structure and hormonal regulation of the hPR mRNAs and proteins in human breast cancer cells. RNA blot hybridization analysis of poly(A+) RNA shows that T47DCO, an estrogen resistant human breast tumor cell line in which PR are constitutively expressed, contain at least six PR mRNAs ranging in size from 2.5 to 11.4 kilobases. All six are mature cytoplasmic messages that are also present in normal human endometrium and in PR-positive MCF-7 breast cancer cells, but not in PR-negative cells. Using hPR-50 RNA synthesized in vitro as a 1.3 kilobase standard, we calculate that MCF-7 cells contain approximately 16 message molecules per cell which are increased to approximately 45 by estradiol treatment; T47DCO cells contain approximately 90 message molecules per cell constitutively expressed. Treatment of T47DCO cells with progesterone leads to down-regulation of immunoreactive A- and B-receptors in the first 8-12 h, followed by their replenishment during the next 48 h. In parallel, hPR message levels initially decrease and then return to pretreatment levels. The synthetic progestin R5020 chronically down-regulates A- and B-receptors; the proteins are profoundly suppressed for at least 48 h, while PR mRNAs fall to less than 15% of control. However, with both hormones, parallel changes in protein and message levels are observed, suggesting that progestational agonists autoregulate the levels of their own receptors by inhibiting transcription of the PR gene. Antagonists appear to have different effects. With the antiprogestin RU 486 there is discordance between hPR protein and message levels which may be due to an ineffective inhibitory interaction between the antagonist-occupied receptors and PR genes, thereby disrupting the negative feedback loop.

Immunologic analysis of human breast cancer progesterone receptors. 2. Structure, phosphorylation, and processing.

We have used a monoclonal antibody (MAb) directed against chick oviduct progesterone receptors (PR), that cross-reacts with human PR, to analyze PR structure and phosphorylation. This MAb, designated PR-6, interacts only with B receptors (Mr 120,000) of T47D human breast cancer cells; it has no affinity for A receptors (Mr 94,000) or for proteolytic fragments from either protein. The antibody immunoprecipitates native B receptors and was used to study the structure of native untransformed 8S and transformed 4S receptors, using sucrose density gradient analysis, photoaffinity labeling, and gel electrophoresis. On molybdate-containing low-salt gradients, PR-6 complexes with 8S B receptors, causing their shift to the bottom of the gradient while A receptors remain at 8 S. Therefore, A and B receptors form separate 8S complexes, and we conclude that A and B do not dimerize in the holoreceptor. Similar gradient studies using salt-containing, molybdate-free buffers show that there are two forms of salt-transformed 4S receptors, comprising either A proteins or B proteins, suggesting that A and B are also not linked to one another in transformed PR. The independence of A- and B-receptor complexes was confirmed by the finding that purified, transformed B receptors bind well to DNA-cellulose. Since PR-6 cross-reacts with nuclear PR, it was used to analyze nuclear PR processing--a down-regulation step associated with receptor loss as measured by hormone binding. Insoluble nuclear receptors and soluble cytosol receptors were measured by immunoblotting following treatment of T47D cells for 5 min to 48 h with either R5020 or progesterone. From 8 to 48 h after R5020 treatment, immunoassayable receptors decreased in nuclei and were not recovered in cytosols. Nuclear receptors also decreased after progesterone treatment but replenished in cytosols between 8 and 24 h after the start of treatment. Thus, processing involves a true loss of nuclear receptor protein, and not just loss of hormone binding activity, and occurs after progesterone or R5020 treatment. This loss is chronic, however, only in R5020-treated cells. Additional studies focused on the covalent modifications of receptors. We previously described shifts in apparent molecular weight of nuclear PR following R5020 treatment using in situ photoaffinity labeling. To show whether these shifts can be explained by receptor phosphorylation, untreated cells and hormone-treated cells were metabolically labeled with [32P]orthophosphate, and the B receptors were isolated by immunoprecipitation with PR-6 and analyzed by sodium dodecyl sulfate (SDS) gel electrophoresis.(ABSTRACT TRUNCATED AT 400 WORDS)

Mediation of insulin-like growth factor actions by the insulin receptor in H-35 rat hepatoma cells.

We have used H-35 rat hepatoma cells to test whether the type II insulin-like growth factor (IGF) receptor mediates metabolic responses to IGF-II. On the basis of both affinity cross-linking experiments and competition binding experiments, H-35 cells display insulin and type II IGF receptors, but not type I IGF receptors. IGF-II and multiplication-stimulating activity (MSA; the rat homolog of IGF-II) stimulate tyrosine aminotransferase, amino acid transport, and glycogen synthase activities to the same magnitude as insulin. However, MSA and IGF-II stimulate these metabolic responses only at high concentrations, indicating that these peptides are acting through the insulin receptor. Incubation of H-35 cells with MSA also induces a state of unresponsiveness to the further actions of both MSA and insulin. There is no associated loss of either insulin or IGF-II binding, indicating that desensitization occurs at a postbinding step in hormone action. The high concentration of MSA necessary to induce desensitization is also consistent with MSA acting through the insulin receptor. We conclude that in H-35 cells, the insulin receptor, rather than the type II IGF receptor, mediates the metabolic responses stimulated by MSA and IGF-II as well as the MSA-induced desensitization to insulin and MSA action.

Regulation of insulin and insulin-like growth factor (IGF) responsiveness by IGFs in rat hepatoma cells.

We have previously reported that insulin induces a complete and reversible desensitization to the induction of tyrosine aminotransferase by insulin and insulin-like growth factors (IGFs) in HTC rat hepatoma cells. This loss of responsiveness cannot be accounted for by down-regulation of cell surface receptors, but occurs at a postbinding step in hormone action. Here we present evidence that IGF-I and IGF-II also induce desensitization to the actions of both IGFs and insulin and that this effect is mediated by IGF receptors. First, the concentration dependence for this effect is similar to that for the IGF-I and IGF-II induction of tyrosine aminotransferase, which has been shown to be mediated by IGF receptors. Second, antibody to the insulin receptor, which blocks insulin, but not IGF-II, binding to HTC cells, causes a rightward shift in the concentration dependence for insulin induction of desensitization, but does not significantly change the concentration dependence for IGF-II. These results indicate that IGF-II-induced desensitization to IGF-II is not mediated by the insulin receptor, but presumably by an IGF receptor. Although the IGFs do cause a moderate decrease in the binding of IGFs and insulin (to approximately 50-75% of the control value), this cannot account for the virtually complete desensitization to their actions. We conclude that IGFs, like insulin, induce a nearly complete loss of responsiveness to insulin and IGFs, that this effect is mediated via IGF receptors, and that desensitization occurs at a step distal to hormone binding which may be common to the actions of insulin and the IGFs.

Madin-Darby canine kidney cells display type I and type II insulin-like growth factor (IGF) receptors.

Using affinity cross-linking techniques, we report the presence of type I IGF and type II IGF receptors in Madin-Darby canine kidney cells, a line of cells lacking insulin receptors. The IGF receptors were further characterized by competition binding studies and found to be similar to IGF receptors in other tissue types. In Madin-Darby canine kidney cells, the type I IGF receptor binds IGF-I greater than IGF-II greater than insulin and the type II IGF receptor binds IGF-II and IGF-I with approximately the same affinity, but does not bind insulin.

Insulin regulation of insulin-like growth factor action in rat hepatoma cells.

We have reported previously that insulin causes a complete but reversible desensitization to insulin action in rat hepatoma HTC cells in tissue culture, and that this insulin resistance is mediated by postbinding mechanisms rather than receptor down-regulation (Heaton, J. H., and Gelehrter, T. D. (1981) J. Biol. Chem. 256, 12257-12262). We report here that insulin causes a similar desensitization to the induction of tyrosine aminotransferase by the insulin-like growth factors IGF-I and IGF-II isolated from human plasma, and by multiplication-stimulating activity, the rat homologue of IGF-II. The results of both competition-binding studies and affinity cross-linking experiments indicate that insulin-like growth factors (IGFs) bind primarily to IGF receptors rather than to insulin receptors. The low concentrations at which these factors induce transaminase is consistent with their acting primarily via IGF receptors. This is confirmed by experiments utilizing anti-insulin receptor antibody which both inhibits 125I-insulin binding and shifts the concentration dependence of insulin induction of tyrosine aminotransferase to the right. This same immunoglobulin does not inhibit 125I-multiplication-stimulating activity binding and only minimally inhibits 125I-IGF-I binding. Anti-insulin receptor antibody also does not significantly shift the concentration dependence for the IGFs, suggesting that IGFs induce transaminase by acting via IGF receptors. Although insulin down regulates insulin receptors, it does not decrease IGF-I or IGF-II binding. We conclude that insulin causes desensitization of HTC cells to IGFs by affecting a postbinding step in IGF action, which may be common to the actions of both insulin and insulin-like growth factors.

Insulin resistance in H-35 rat hepatoma cells is mediated by post-receptor mechanisms.

Incubation of H-35 cells with 300 ng/ml (50 nM) of insulin causes a 3-4-fold induction of tyrosine aminotransferase at 4-6 h of incubation. At 24 h the activity of transaminase returns to basal levels despite the presence of sufficient insulin to stimulate a maximal response. Furthermore, addition of 300 ng/ml of fresh insulin fails to stimulate the induction of transaminase. In contrast, the addition of 0.1 microM dexamethasone to insulin-treated cells stimulates the induction of tyrosine aminotransferase, indicating that the loss of responsiveness is specific to insulin action. Incubation of H-35 cells with insulin also causes a 25-30% decrease in insulin binding. This modest decrease in receptor binding is not sufficient to explain the virtually complete loss of insulin responsiveness. Hence, in H-35 hepatoma cells insulin-induced desensitization to insulin action is mediated primarily by post-receptor events.