Mechanism of inhibition of MMTV-neu and MMTV-wnt1 induced mammary oncogenesis by RARalpha agonist AM580.

We hypothesized that specific activation of a single retinoic acid receptor-alpha (RARalpha), without direct and concurrent activation of RARbeta and gamma, will inhibit mammary tumor oncogenesis in murine models relevant to human cancer. A total of 50 uniparous mouse mammary tumor virus (MMTV)-neu and 50 nuliparous MMTV-wnt1 transgenic mice were treated with RARalpha agonist (retinobenzoic acid, Am580) that was added to the diet for 40 (neu) and 35 weeks (wnt1), respectively. Among the shared antitumor effects was the inhibition of epithelial hyperplasia, a significant increase (P<0.05) in tumor-free survival and a reduction in tumor incidence and in the growth of established tumors. In both models, the mechanisms responsible for these effects involved inhibition of proliferation and survival pathways, and induction of apoptosis. The treatment was more effective in the MMTV-wnt1 model in which Am580 also induced differentiation, in both in vivo and three-dimensional (3D) cultures. In these tumors Am580 inhibited the wnt pathway, measured by loss of nuclear beta-catenin, suggesting partial oncogene dependence of therapy. Am580 treatment increased RARbeta and lowered the level of RARgamma, an isotype whose expression we linked with tumor proliferation. The anticancer effect of RARalpha, together with the newly discovered pro-proliferative role of RARgamma, suggests that specific activation of RARalpha and inhibition of RARgamma might be effective in breast cancer therapy.

CRBP suppresses breast cancer cell survival and anchorage-independent growth.

We showed earlier that cellular retinol-binding protein (CRBP) expression is downregulated in a subset of human breast cancers. We have now investigated the outcome of ectopic CRBP expression in MTSV1-7 cells, a SV40 T antigen-transformed human breast epithelial cell line devoid of endogenous CRBP expression. We found that: (i) CRBP did not inhibit adherent cell growth but suppressed foci formation in post-confluent cultures and colony formation in soft agar; (ii) this effect was due to CRBP inhibition of cell survival, as demonstrated by viability and TUNEL assays of cells in soft-agar or plated on polyHEMA-coated dishes; (iii) CRBP inhibited protein kinase B/Akt activation in cells in suspension but not in adherent cells and the CRBP suppression of anchorage-independent growth was mimicked by cell treatment with the phosphatidylinositol-3 kinase (PI3K) inhibitor LY294002; (iv) CRBP enhanced retinyl ester formation and storage but did not regulate retinoic acid synthesis or retinoic acid receptor activity. Ectopic CRBP-mediated inhibition of anchorage-independent cell survival and colony formation in the absence of significantly altered responses to either retinol or retinoic acid was also documented in T47D human breast cancer cells. In conclusion, the data suggest two novel and linked CRBP functions in mammary epithelial cells: inhibition of the PI3K/Akt survival pathway and suppression of anchorage-independent growth.

Retinol conversion to retinoic acid is impaired in breast cancer cell lines relative to normal cells.

The bioactivity of retinol (vitamin A) is in part dependent on its metabolism to retinoic acid (RA). We investigated the ability of breast epithelial cells to synthesize RA when challenged with a physiological retinol dose (2 microM). Normal human mammary epithelial cells (HMEC) cultured from reduction mammoplasties were competent in RA synthesis and the ability to synthesize RA was retained by immortal, nontumorigenic breast epithelial cell lines (MTSV1.7, MCF-10F, and 184B5). In contrast, most (five of six) breast cancer cell lines could not synthesize RA or did so at low rates relative to normal cells. A notable exception was the MDA-MB-468 cell line, which was fully competent in RA synthesis. Most (>/=68%) of the RA synthesized by breast cells was recovered from the culture medium. Cellular retinol binding protein and cellular RA binding protein II, both expressed in HMEC, had various expression patterns in the cell lines that did not correlate with the observed differences in RA synthesizing ability. Strong RA induction of the RA hydroxylase P450RAI (CYP26) was confined to ERalpha-positive T47D and MCF-7 breast cancer cells and did not appear to explain the lack of detectable RA levels in these cells since RA remained undetectable when the cells were treated with 5-10 microM liarozole, a P450RAI inhibitor. We hypothesize that retinol bioactivity is impaired in breast cancer cells that cannot synthesize RA. In preliminary support of this hypothesis, we found that retinol (0.5-2 microM) inhibited MCF-10F but not T47D or MCF-7 cell growth.

Methylation of conserved CpG sites neighboring the beta retinoic acid response element may mediate retinoic acid receptor beta gene silencing in MCF-7 breast cancer cells.

We investigated the mechanism of retinoic acid receptor (RAR) beta2 gene silencing in breast cancer cells. Transfection experiments indicated that MCF-7 cells transactivate an exogenous beta2 promoter (-1470/+156) to the same extent as MTSV1.7 breast epithelial cells, which express endogenous RARbeta2. This was true even in the context of replicated chromatin, suggesting a cis-acting rather than a trans-acting defect. Cytosine methylation, a cis-acting DNA modification, has been implicated in RARbeta2 silencing in cancer cells. Upon bisulfite genomic sequencing, we found that 3 CpG sites in the beta2 RARE region were variably methylated in MCF-7 cells but were not methylated in MTSV1.7 cells or in 2 MDA-MB-231 subclones that differed in RARbeta2 expression (high in clone A2, low in clone A4). However, the 5'-UTR region was hypermethylated in clone A4 relative to clone A2 cells. Following 5-azacytidine treatment, RA and trichostatin A markedly induced RARbeta2 expression in MCF-7 cells but not in MDA-MB-231 clone A4 cells. A beta2 RARE reporter construct in which the methylation-susceptible cytosines in the sense strand were replaced by thymine displayed marked loss of activity in a replicated chromatin-dependent manner. We conclude that cytosine methylation contributes to RARbeta2 gene silencing in MCF-7 cells and that methylation of the RARE region may be particularly important. Oncogene (2000) 19, 4066 - 4070.

Cellular retinol-binding protein expression and breast cancer.

BACKGROUND: The biologic activity of vitamin A depends, in part, on its metabolism to active nuclear receptor ligands, chiefly retinoic acid. The cellular retinol-binding protein (CRBP) binds vitamin A with high affinity and is postulated to regulate its uptake and metabolism. In this report, we analyze the expression of CRBP in normal and malignant breast tissues. METHODS: We evaluated CRBP expression by in situ hybridization in six reduction mammoplasty specimens and 49 human breast carcinoma specimens by use of digoxigenin-labeled RNA probes and in nine cultured mammoplasty specimens by northern or western blot analysis. Statistical significance was evaluated with the chi(2) test or Fisher's exact test if the sample sizes were small. All P values are from two-sided tests. RESULTS: CRBP was expressed in all 15 mammoplasty specimens (normal breast tissue) and in 33 of 35 available specimens of normal tissue adjacent to carcinoma. In contrast, 12 (24%) of 49 carcinoma lesions were uniformly negative for CRBP (P =.023 for comparison with adjacent normal breast tissue). The loss of CRBP expression was as frequent in ductal carcinoma in situ (six [27%] of 22) as in invasive lesions (six [22%] of 27), suggesting that it is a relatively early event in carcinogenesis and not associated with patient age, tumor grade, and expression of steroid receptors or c-Myc. Preliminary experiments did not find an association between CRBP and retinoic acid receptor beta loss, but most (four of five) CRBP-negative tumors were also retinoic acid receptor beta negative. CONCLUSION: CRBP is underexpressed in 24% (95% confidence interval = 12.5%-36.5%) of human breast carcinomas, implying a link between cellular vitamin A homeostasis and breast cancer. We hypothesize that the loss of CRBP restricts the effects of endogenous vitamin A on breast epithelial cells.

Synergistic transcriptional activation of the mouse urokinase plasminogen activator (uPA) gene and of its enhancer activator protein 1 (AP1) site by cAMP and retinoic acid.

We have investigated the mechanism whereby all-trans retinoic acid (tRA) potentiates the 8-bromo-cAMP (8-BrcAMP)-dependent transcription of the urokinase plasminogen activator (uPA) gene in SC115 mouse mammary carcinoma cells. Photoaffinity labelling experiments showed that tRA did not alter the cellular content of cAMP-dependent protein kinase regulatory subunits I and II. In agreement with this, nuclear run-on analysis in the presence of the translational inhibitor puromycin demonstrated that the effect of 8-BrcAMP and its potentiation by tRA were independent of protein synthesis. A transiently transfected 6.6 kb uPA 5'-flanking region-chloramphenicol acetyltransferase (CAT) fusion gene mimicked the response of the endogenous uPA gene. Thus 1 mM 8-BrcAMP induced a 100-200% increase in CAT content, 100 nM tRA had no effect and 100 nM tRA+1 mM 8-BrcAMP induced a 300-500% increase in cells co-transfected with tRA receptor and/or 9-cis-RA receptor. Analysis of 5'-deleted constructs showed that the tRA effect required at least two cis regions: -2657 to -2186, encompassing the 100 bp uPA enhancer, and -709 to -324, which exhibited silencing activity. Neither region contained a tRA-response element-like motif. Because tRA receptor and 9-cis-RA receptor interact with activator protein 1 (AP1), we tested whether tRA regulated the uPA enhancer AP1 site in the presence of 8-BrcAMP. We found that a dimer of this site fused to a minimal uPA-CAT fusion gene was responsive to 1 mM 8-BrcAMP (100% CAT increase), not responsive to 100 nM tRA, and synergistically responsive to 100 nM tRA+1 mM 8-BrcAMP (240% CAT increase) in cells co-transfected with Fos and Jun. Synergistic activation of the same construct and of the 6.6 kb uPA-CAT fusion gene was also obtained using tRA and 100 nM PMA. We conclude that multiple cis elements, probably including the uPA enhancer AP1 site, mediate the tRA potentiation of uPA transcription.

Laminin and estradiol regulation of the plasminogen-activator system in MCF-7 breast-carcinoma cells.

We have investigated the effects of laminin, on the plasminogen-activator system of MCF-7 breast-carcinoma cells. MCF-7 cells were incubated on plastic or laminin-coated wells, and medium and cell lysate aliquots were assayed for tissue-type (tPA) and urokinase-type plasminogen activator (uPA) by a chromogenic assay in combination with anti-uPA antibodies. Cells cultured on laminin displayed a 5-fold increase in tPA activity and a 2-fold decrease in uPA activity relative to cells on plastic. These effects could be mimicked by laminin fragment P1 but not by collagen I or fibronectin. tPA activity of cells treated with estradiol (10 nM) was 3-fold higher, that of cells on laminin treated with estradiol was 15-fold higher, than that of control. Northern-blot analysis showed that tPA mRNA levels were up-regulated by estradiol and laminin, whereas PAI-1 mRNA levels were down-regulated by laminin and not affected by E2. Concomitant treatment with laminin and estradiol, decreased PAI-1 mRNA and increased tPA mRNA levels, accounting for the synergistic increase in tPA activity. Laminin exerted only a modest (approx. 2-fold) inhibitory effect on uPA mRNA levels. In the breast-carcinoma cell line MDA-MB-231, down-regulation of PAI-1 and uPA mRNA by laminin was not observed. Adhesion assays indicated that alpha2beta1 is the predominant receptor for laminin in MCF-7 cells. MDA-MB-231 cells expressed alpha2 (54%) but this integrin is not used as a laminin receptor. These results support a role for alpha2beta1 in mediating interactions of MCF-7 with LN.

The cellular retinoic acid binding protein II is a positive regulator of retinoic acid signaling in breast cancer cells.

The role of the cellular retinoic acid binding protein type II (CRABPII) in the retinoic acid (RA) signaling pathway is poorly understood. Northern blot analysis of 12 breast cell lines showed that CRABPII mRNA content correlated with growth inhibition by RA, suggesting that this binding protein enhances cellular response to RA. Ectopic CRABPII expression supported dose-dependent growth inhibition by RA in SC115-resistant but not MDA-MB-231-resistant cells, indicating that CRABPII is sufficient to rescue RA antiproliferation in a permissive background. In both cell lines, ectopic binding protein enhanced gene activation by RA. Thus, induction of tissue transglutaminase by all-trans-RA and, surprisingly, 9-cis-RA was enhanced 5-fold over and above the level of induction in control cells (SC115), and activation of a RA response element reporter was enhanced 3-fold (MDA-MB-231). A 5-fold enhancement of RA induction of RA receptor beta expression as a result of ectopic binding protein expression was also demonstrated (SC115). These findings indicate that CRABPII is a positive regulator of RA signaling in breast cells.

Loss of an estrogen receptor isoform (ER alpha delta 3) in breast cancer and the consequences of its reexpression: interference with estrogen-stimulated properties of malignant transformation.

Comparison of mRNA ratios of a non-DNA-binding estrogen receptor (ER(alpha)) isoform, missing exon 3 (ER(alpha)delta3), to the full-length ER(alpha), in normal breast epithelium to that in primary breast cancers and breast cancer cell lines revealed a 30-fold reduction of this ratio in cancer cells (P < 0.0001). To test what functions may have been affected by the loss of ER(alpha)delta3, stable clones of MCF-7 cells expressing ectopic ER(alpha)delta3 protein, at the range of physiological ER(alpha), were generated. In vector-transfected controls the ER(alpha)delta3-mRNA and protein were less than 10% while in the ER(alpha)delta3-expressing clones, ER(alpha)delta3-mRNA and protein ranged from 36-76% of the total ER(alpha). Estrogen (E2) stimulated the expression of pS2-mRNA in pMV7 vector control cells, but the stimulation was reduced by up to 93% in ER(alpha)delta3-expressing clones. In addition, several properties associated with the transformed phenotype were also strongly affected when ER(alpha)delta3 protein was reexpressed. Compared with vector-transfected control cells, the saturation density of the ER(alpha)delta3-expressing clones was reduced by 50-68%, while their exponential growth rate was only slightly (14.5 +/- 5%) lower. The in vivo invasiveness of the ER(alpha)delta3-expressing cells was significantly reduced (P = 0.007) by up to 79%. E2 stimulated anchorage-independent growth of the pMV7 vector control cells, but reduced it to below baseline levels in ER(alpha)delta3 clones. The reduction of the pS2 response to E2 in the ER(alpha)delta3-expressing clones and the E2 block of anchorage-independent growth to below baseline were more pronounced than expected from the dominant negative function of ER(alpha)delta3. These observations suggest that E2 may activate an additional ER(alpha)delta3-dependent inhibitory pathway. The drastic reduction of ER(alpha)delta3 to ER(alpha) ratio in breast cancer, and the fact that when present in breast cancer cells this isoform leads to a suppression, rather than enhancement, of the transformed phenotype by E2 suggests that the regulation of ER(alpha)-mRNA splicing may need to be altered for the breast carcinogenesis to proceed.

Human breast carcinoma slice cultures retain retinoic acid sensitivity.

We have shown earlier that surgical human breast cancer tissue can be maintained in culture as in culture as intact tissue slices (organ culture). Because tumor organ culture ostensibly preserves the interacting network of tumor cells, stromal fibroblasts, endothelial cells and extracellular matrix, it represents a rather complex culture system. Such a system may be especially useful in preclinical trials, where the objective is to make extrapolations to the even more complex in vivo situation. A classical therapeutic target in breast cancer is the estrogen receptor, and we showed earlier that human breast cancer slices retain expression of this receptor in culture. Retinoic acid, the active form of vitamin A, is also an important (negative) growth regulator in breast cancer. In the present communication, we used in situ hybridization to monitor the expression of retinoic acid receptors in tumor slices cultured for 4 days. We show that both members of the all-trans retinoic acid and 9-cis retinoic acid receptor family (RAR and RXR, respectively) are expressed. Moreover, RNase protection analysis showed that expression of the cellular retinoic acid-binding protein type II gene, a known retinoic acid target gene, is upregulated by treatment with 1 microM all-trans retinoic acid for 2 days. These findings attest to the feasibility of using tumor organ cultures as a preclinical model for the evaluation of synthetic vitamin A derivatives (retinoids).

Defective expression of cellular retinol binding protein type I and retinoic acid receptors alpha2, beta2, and gamma2 in human breast cancer cells.

Because the retinoic acid (RA) signaling pathway regulates cell proliferation and differentiation, inactivation of genes integral to the pathway represents a potential mechanism of carcinogenesis. We have studied in human breast cancer cells (T47D, MCF-7, ZR75-1, MDA-MB-231, and BT20) the expression of a subset of retinoid signaling genes that are themselves transcriptionally up-regulated by RA, the cellular retinol binding protein type I (CRBPI) and the RA receptors (RARs) alpha2, beta2, and gamma2. We find that constitutive expression of these genes is low or undetectable, and that expression levels are seldom responsive to 24 h treatment with 1 microM all-trans or 9-cis RA (Northern blot analysis). This is in contrast to breast fibroblasts, which show RA-dependent expression of all four genes under the same conditions. Moreover, normal human breast epithelial cells express CRBPI and RARbeta2 at the mRNA level, suggesting that loss of expression of these genes is tied to malignant transformation. RARbeta2, but not CRBPI, was also expressed in RA-treated MTSV1-7 cells, an immortalized but nontumorigenic luminal epithelial cell line. Lack of CRBPI and RARbeta2 expression in cancer cells was not due to general impairment of RA signaling, as shown by RA activation of a RARE3-tk-CAT reporter in a subclone of MDA-MB-231 cells that did not express either CRBPI or RARbeta2. These results suggest that at least two independent defects in the expression of proteins that function in retinoid signaling may be involved in breast carcinogenesis.

Upregulation of cytokeratins 8 and 18 in human breast cancer T47D cells is retinoid-specific and retinoic acid receptor-dependent.

The mamary gland is chiefly composed of luminal epithelial cells expressing cytokeratins (K) 8, 18 and 19, and basal/myoepithelial cells expressing cytokeratins 5 and 14. Human breast cancer T47D cells have a luminal phenotype and are growth-inhibited by retinoids, a class of compounds known to regulate cytokeratin expression. To extend our knowledge of retinoid action in breast cancer, we have studied the retinoid regulation of cytokeratin expression in the T47D model. We found that retinoid inhibition of T47D cell growth was accompanied by increases in K8, K18 and K19 mRNA steady-state levels (Northern blot analysis). The effect on K8 was studied in greater detail. This effect was seen with as low as 1 nM all-trans retinoic acid (tRA) and was maximal (up to 7 fold over control) with 1 microM tRA (the highest dose tested). Time-course studies revealed a detectable effect at 1 h and a maximal effect at 8-24 h. Non-retinoidal growth inhibitors (tamoxifen, BrcAMP and genistein) did not modulate K8 expression, demonstrating that the effect of tRA was specific, K8 mRNA upregulation was blocked by actinomycin D and cycloheximide, suggesting, in accordance with other studies, that tRA exerted a transcriptional effect that was secondary to de novo protein synthesis. Five retinoids known to activate retinoic acid receptor (RAR) and/or retinoid X receptor (RXR) - tRA; 9-cis-retinoic acid, 9cRA; 13-cis RA, 13cRA; retinyl acetate; and N-(4-hydroxyphenyl) retinamide 4HPR - inhibited T47D cell growth and increased K8 expression, whereas an arotinoid (Ro-40-8757) that is not a RAR activator caused growth inhibition but did not upregulate K8. Activation of RAR alpha contributed to K8 upregulation, since this effect was partially blocked by the RAR alpha-selective antagonist Ro-41-5253. Analogous results were obtained throughout when blots were reprobed with K18 cDNA. Western blot and immunocytochemistry experiments demonstrated that protein levels of K8 and K18 increased by 2 days of treatment with 1 microM tRA. These results show that retinoids enhance the expression of cognate cytokeratin markers of luminal differentiation in T47D breast cancer cells.

Redundant regulation of urokinase plasminogen activator transcription by the two major isozymes of cAMP-dependent protein kinase.

The regulation of urokinase plasminogen activator (uPA) gene expression by the two major cAMP-dependent protein kinase isozymes was studied in SC115 mouse mammary carcinoma cells using the site-selective cAMP analog approach. SC115 cells expressed both type I and type II cAMP-dependent protein kinase holoenzyme (at a ratio of 2:3), and selective, partial activation of each holoenzyme could be demonstrated in vitro using appropriate combinations of cAMP analogs. When cells were exposed to the same analog combinations, uPA expression was upregulated 2- to 4-fold when either holoenzyme I or holoenzyme II was targeted. For comparison, a high concentration (1 mM) of 8-bromo-cAMP, an analog that does not discriminate between kinase isoforms, up-regulated uPA 10-fold. These findings suggest that there are two pathways of cAMP-dependent regulation of uPA, one mediated by holoenzyme I, the other by holoenzyme II, and that the end result of activation of each pathway is the same. Differences in the mechanism whereby each pathway regulates uPA were searched for but not found. Both pathways were shown to be dependent on catalytically active enzyme, to be potentiated by retinoic acid treatment, and to regulate uPA transcriptionally. The most likely interpretation of these findings is that uPA transcription is mediated solely by the action of the common catalytic subunit, regardless of whether it originated from holoenzyme I or holoenzyme II.

Estradiol modulation of plasminogen activator production in organ cultures of human breast carcinomas: correlation with clinical outcome of anti-estrogen therapy.

We have studied the estradiol sensitivity of primary human breast carcinomas in organ culture in a prospective pilot series of 109 tumors. The effect on plasminogen activator (PA) production was used as the end-point of estrogen action. We found that: (i) All tumors secreted detectable levels of urokinase-type PA (uPA); the level of basal uPA production was markedly heterogeneous but showed a weak association with the level of estrogen receptor positivity (p = 0.049). (ii) Only 23.5% of the tumors secreted tissue-type PA (tPA) in addition to uPA; a higher proportion of these tumors had histological characteristics indicative of good prognosis (18% vs. 3% of tumors secreting only uPA). (iii) Estradiol modulated uPA production and this effect was receptor-mediated. (iv) Responsiveness to estradiol was limited to a subset (25 of 60 or 41.7%) of estrogen and progesterone-receptor-positive tumors. (v) Of 20 evaluable patients with lymph-node and receptor-positive breast cancer who received adjuvant anti-estrogen therapy, 11 were identified as estradiol-sensitive by the in vitro PA assay; of these, 10 had no evidence of disease after a median follow-up period of 3+ years. In contrast, of 9 patients with tumors identified as estradiol-insensitive, 4 developed metastases within 3+ years of follow-up. (vi) Consistent with the previously reported inhibitory effect of corticosteroids on uPA production in organ cultures of human tumors, the basal culture level of uPA produced by tumors from patients receiving corticosteroids at the time of surgery was significantly lower than the level of uPA in the remaining tumors (p = 0.029). Also, tumors from patients receiving thyroid hormone, known to stimulate uPA in vitro, showed a slight trend toward increased production of uPA. These results show that hormone effects on tumor PA production are qualitatively similar in organ culture and in the host. This and the emerging individual correlation between sensitivity to estradiol in vitro, as determined by PA, and the clinical effectiveness of anti-estrogen therapy, underscore the potential usefulness of the organ culture approach.

Retinoic acid priming potentiates the induction of urokinase-type plasminogen activator by cyclic adenosine monophosphate in mouse mammary carcinoma cells.

Interactive regulation of gene expression by retinoic acid (RA) and adenosine monophosphate (cAMP) in mammary tumor cells was explored using Shionogi mouse mammary carcinoma cells (SC115) as a model and urokinase-type plasminogen activator (uPA) as a target gene product. Twenty-four hour treatment of SC115 cells with 100 nM RA, 1 mM 8-bromo-cAMP (BrcAMP), and 100 nM RA + 1 mM BrcAMP resulted in extracellular uPA activity increases of 1.4-fold, sevenfold, and 20-fold, respectively. These effects were dose-dependent with regard to both interacting members. Similar responses were obtained if 1 nM cholera toxin or 10 microM forskolin was used instead of the cAMP analog. Retinoids lacking the carboxylic acid function were inactive. The changes in uPA activity were accompanied by similar changes in uPA antigen concentration, as seen via Western blot analysis, and uPA mRNA abundance, as seen via Northern blot analysis. Actinomycin D, an inhibitor of RNA synthesis, blocked uPA stimulation by BrcAMP, suggesting that mRNA levels were transcriptionally regulated. The effect of BrcAMP on extracellular uPA activity was first evident at 2 h and peaked at approximately 6 h; the effect of RA alone and the synergistic response to joint treatment, however, followed a slower time course, requiring at least 12 h for initial expression and increasing gradually with time up to at least 48 h. Priming with RA for 48 h followed by extensive washing of the cells resulted in a threefold enhancement of the stimulatory effect of BrcAMP on uPA. Experiments utilizing the casein/plasminogen overlay method for the detection of uPA secretion by increased rate of uPA secretion per cell rather than to an increased fraction of uPA-secreting cells. Initial investigation of the mechanism of RA potentiation of cAMP responsiveness showed that RA did not alter cellular cAMP levels or total cAMP-dependent protein kinase A activity. Finally, the tumor promoter phorbol myristate acetate, an activator of protein kinase C, also increased SC115 cell uPA activity and synergized with RA. This raised the possibility that the enhancement of cAMP responsiveness by RA was indirectly mediated via an effect on protein kinase C. Experiments with protein kinase C-depleted cells, however, showed that this was not the case. In conclusion, RA treatment of SC115 cells potentiates the effect of cAMP on uPA expression at the single cell level via a partially irreversible mechanism independent of protein kinase C. The molecular target of RA and whether SC115 cell differentiation underlies the effect of RA remain to be established.

Preservation of steroid hormone receptors in organ cultures of human breast carcinomas.

In a study of human breast carcinomas in short-term organ culture, in which plasminogen activator modulation by estrogen was used as a test of estrogen sensitivity (R. Mira-y-Lopez and L. Ossowski, Cancer Res., 47: 3558-3564, 1987), we found that the number of estrogen and progesterone receptor-positive cancers showing estrogen sensitivity was less than anticipated from reported rates of antiestrogen-induced clinical remission. Since in these experiments the estrogen receptor (ER) content of the tumor cultures was only inferred from determinations carried out before culture, we postulated that the apparent estrogen insensitivity of some tumors resulted from poor ER preservation. We have now measured ER levels directly in cultured tissue and found that (a) ER levels in slices of human breast cancers decreased 78% (median) after 1-4 days; 4 of 16 (25%) ER-positive breast cancers had no detectable estradiol binding activity after culture; (b) the drop in ER level was a result of net receptor loss rather than inactivation of binding activity; (c) loss of cell viability could be definitively ruled out as a cause of decreased receptor level; (d) cortisol receptor levels in human breast cancers and ER levels in other hormone-responsive cancers also decreased in culture, and to a similar extent. Higher ER levels (sometimes equal to preculture levels) were preserved by culture at subphysiological temperature or in slices of controlled thickness, not exceeding 0.6 mm. These findings should be considered when organ culture is used to predict tumor hormone responsiveness.

Hormonal modulation of plasminogen activator: an approach to prediction of human breast tumor responsiveness.

We have determined that the primary reason for the frequently encountered poor survival of human scirrhous breast carcinomas in short-term (4 days) organ culture is mechanical injury to the tumor tissue during explant preparation. It was possible to minimize this injury by preparing 0.5-mm-thick slices using very sharp blades. This resulted in much improved preservation of tissue structure and function, as assessed by histology, DNA content, and enzyme synthesis and secretion. With the exception of insulin, which was always present in the culture medium, exogenous hormones, including estrogen, or serum did not further improve explant preservation. In rodent mammary tumors, growth in vivo and production of the serine protease plasminogen activator (PA) in organ culture are coordinately regulated by hormones, suggesting that PA may be a valuable indicator of tumor hormone responsiveness. We have now tested the effect of estrogen and other hormones on PA secretion in organ cultures of primary human breast carcinomas. We found that: modulation of PA by 17-beta-estradiol (10-8) M) occurred only in carcinomas which were positive for both estrogen and progesterone receptors; of 21 such tumors, 11 (52%) were responsive. Plasminogen activator was not modulated by estradiol in any of the 22 tumors which were negative for one or both receptors; hydrocortisone (10(-7) M) effectively inhibited, and 3,5,3'-L-triiodothyronine (10(-8) M) and adenylate cyclase activators effectively stimulated PA in most breast tumors, regardless of their estrogen and progesterone receptor status. Prolactin (5 micrograms/ml) had no effect when tested alone; urokinase-type PA was found to be the principal PA produced by human breast tumors. Changes in its rate of synthesis and secretion and not in the content of PA inhibitors appeared to be the prevailing mechanism of enzyme regulation by hormones. In summary, short-term organ culture coupled with the use of PA as an index of response appears to be a promising approach to the study of hormone sensitivity of primary human breast carcinomas.

Identification of a pituitary factor responsible for enhancement of plasminogen activator activity in breast tumor cells.

Sheep pituitary glands contain a protein that stimulates plasminogen activator (PA) activity 3- to 20-fold in serum-free cultures of T47D, MTW9/PL, and SC115 breast tumor cells. This protein was found to be similar to basic fibroblast growth factor (bFGF) in size, cationic nature, and affinity for heparin. Purified human placental bFGF, a homologue of human and bovine pituitary bFGF, was effective in stimulating mammary tumor cell PA at a concentration of 1 ng/ml. Antibodies to placental bFGF blocked the PA stimulatory activity of sheep pituitary extracts. Because of these properties, the active protein in sheep pituitary glands was identified as bFGF. This represents another function of bFGF, indicating that its spectrum of target cells is wider than previously thought. Because of previously established correlations between PA production in vitro and tumor growth in vivo, we suggest that bFGF may contribute to the growth of breast carcinomas in vivo. Other types of carcinomas may also be affected by bFGF.

Coordinate inhibition of plasminogen activator and tumor growth by hydrocortisone in mouse mammary carcinoma.

A body of evidence has suggested that hormones which modulate plasminogen activator production by cultured tumor explants in vitro may have a qualitatively comparable effect on the growth of the same tumors in vivo. As a test of this correlation and to explore its potential for predicting the in vivo response of tumors to hormones, we have studied here the effect of hydrocortisone on the growth of primary and first generation transplants of mouse mammary tumor virus-determined mammary tumors in BALB/c X DBA/8 F1 (hereafter called CD8F1) mice; hydrocortisone had been found previously to inhibit plasminogen activator production by explants of these tumors. The results were: (a) hydrocortisone reversibly blocked the growth of palpable primary tumors; growth resumed at control rates following withdrawal of exogenous hormone; (b) hydrocortisone inhibited the growth of first-generation tumor transplants when administered either before or after the appearance of palpable tumors; (c) pretreatment with hydrocortisone both delayed the appearance of primary tumors and greatly reduced tumor incidence in susceptible mice; a substantial part of the decrease in tumor incidence was apparently irreversible; (d) hydrocortisone reduction of tumor growth was accompanied by inhibition of tumor plasminogen activator content, and these effects displayed a similar dose dependence (enzyme content of tumor lysates was measured by the 125I-fibrin plate assay); the enzyme present in control and hormone-treated tumors was predominantly of the urokinase type. These findings suggest that plasminogen activator production and mammary tumor growth in CD8F1 mice are coordinately regulated and thus encourage the view that plasminogen activator might be useful as an in vitro marker for predicting the in vivo response of tumors to hormones.