The BA-BCS 2021: An Initial "Trial" for Integrating Basic Science and Medical Progress on Breast Cancer in a Latin-American Country

The first Buenos Aires Breast Cancer Symposium (BA-BCS) was held in a virtual format, between the 17^th and the 21^st of May 2021. The main goal of the meeting was to facilitate the interaction among physicians and basic researchers from South America and with peers from the rest of the world. To embrace their different interests and concerns, the congress included not only talks on basic, translational and clinical research, but also round tables to discuss diagnostic methods, research financing and biobank management, as well as virtual poster sessions in which the youngest fellows presented their recent findings. This report provides a brief overview of the talks delivered during the meeting, which addressed a wide variety of vital issues for breast cancer research mostly focused on the accurate diagnosis, prevention and treatment of this illness. The presentations included a wide spectrum of themes including hormone receptors and the relevance of their mutations, immunotherapy, cancer stem cells, mouse models, environmental hazards, genetics and epigenetics, local and systemic therapies, liquid biopsies, the metastatic cascade, therapy resistance and dormancy, among others.

Buenos Aires Breast Cancer Symposium BA-BCS 2021: Tribute to Dr. Christiane Dosne Pasqualini in her 101 birthday, May, 17-21, 2021

Session 1: Tumor heterogeneity and breast cancer therapy. Session 2: From hormone receptors to the immune system: the evolution of therapeutic targets in breast cancer. Session 3: Cancer stem cells and de-differentiated phenotype. Session 4: Mouse models for studying breast cancer initiation and progression. Session 5: Round Table 1 - Genomics Platforms. Session 6: Genetics and Epigenetics of Breast Cancer. Session 7: Understanding the metastatic cascade to learn how to inhibit tumor progression. Session 8: Round Table 2 - Biorepositories and sample management. Session 9: Estrogen receptors: their involvement in endocrine resistance and dormancy. Session 10: Novel targets in the era of precision medicine. Session 11: Round Table 3 - Interaction among government, non-government agencies, and industry for funding and promoting breast cancer translational research. Session 12: Local and systemic therapies. Session 13: New developments in diagnosis and epidemiology of breast cancer.

Identification of an AP1-ZFP36 Regulatory Network Associated with Breast Cancer Prognosis

It has been established that ZFP36 (also known as Tristetraprolin or TTP) promotes mRNA degradation of proteins involved in inflammation, proliferation and tumor invasiveness. In mammary epithelial cells ZFP36 expression is induced by STAT5 activation during lactogenesis, while in breast cancer ZFP36 expression is associated with lower grade and better prognosis. Here, we show that the AP-1 transcription factor components, i.e. JUN, JUNB, FOS, FOSB, in addition to DUSP1, EGR1, NR4A1, IER2 and BTG2, behave as a conserved co-regulated group of genes whose expression is associated to ZFP36 in cancer cells. In fact, a significant down-modulation of this gene network is observed in breast, liver, lung, kidney, and thyroid carcinomas compared to their normal counterparts. In breast cancer, the normal-like and Luminal A, show the highest expression of the ZFP36 gene network among the other intrinsic subtypes and patients with low expression of these genes display poor prognosis. It is also proposed that AP-1 regulates ZFP36 expression through responsive elements detected in the promoter region of this gene. Culture assays show that AP-1 activity induces ZFP36 expression in mammary cells in response to prolactin (PRL) treatment thorough ERK1/2 activation. These results suggest that JUN, JUNB, FOS and FOSB are not only co-expressed, but would also play a relevant role in regulating ZFP36 expression in mammary epithelial cells.

CtBP1 associates metabolic syndrome and breast carcinogenesis targeting multiple miRNAs

Metabolic syndrome (MeS) has been identified as a risk factor for breast cancer. C-terminal binding protein 1 (CtBP1) is a co-repressor of tumor suppressor genes that is activated by low NAD+/NADH ratio. High fat diet (HFD) increases intracellular NADH. We investigated the effect of CtBP1 hyperactivation by HFD intake on mouse breast carcinogenesis. We generated a MeS-like disease in female mice by chronically feeding animals with HFD. MeS increased postnatal mammary gland development and generated prominent duct patterns with markedly increased CtBP1 and Cyclin D1 expression. CtBP1 induced breast cancer cells proliferation. Serum from animals with MeS enriched the stem-like/progenitor cell population from breast cancer cells. CtBP1 increased breast tumor growth in MeS mice modulating multiple genes and miRNA expression implicated in cell proliferation, progenitor cells phenotype, epithelial to mesenchymal transition, mammary development and cell communication in the xenografts. These results define a novel function for CtBP1 in breast carcinogenesis.

Abstract 1160: CtBP1 is the molecular link that associates breast cancer and metabolic syndrome

Breast cancer is still one of the most important public health problems in the entire world. Obesity and metabolic syndrome (MS) increases the incidence and aggressiveness of breast cancer. C-Terminal Binding Protein (CtBP1) is a transcriptional corepressor of tumor suppressor genes and is considered a molecular sensor of cell metabolic state due to is activated in high energy conditions (high NADH). In this work we studied the effects of the activation of CtBP1 pathway by metabolic syndrome on breast tumor development and progression. We generated a murine model of MS by chronic high fat diet (HFD) administration. By histological and whole mount methods, we found that breast tissue of animals receiving HFD presented higher levels of immature adipose tissue and an increased glandular area with more generation of lateral branches and terminal end buds of mammary ducts. Breast tissue of HFD animals also showed higher expression of the proliferation markers (cyclin D1) and epithelial markers (E-cadherin). Interestingly, HFD induced CtBP1 expression in the mammary ducts. Furthermore, the number and size of mamospheres generated with LM38-LP breast cancer cells were significantly increased when cells were incubated with serum from HFD fed mice compared to the serum of animals under control diet (CD). In addition, to investigate CtBP1 role in tumor progression we performed xenografts in nude mice fed with CD or HFD by subcutaneous injecton of breast tumor cells MDA MB 231 with depleted CtBP1 expression (shRNA CtBP1) or control cells (shRNA scramble). We found that CtBP1 depletion dramatically decreased tumor growth and KI67 expression relative to control tumors. Furthermore, xenografts developed in HFD fed mice were less differentiated compared to CD. Finally, CtBP1 diminished expression tumors showed lower mesenchymal markers expression, progenitor cells markers and markers involved in mammary development. Our studies demonstrated for the first time that gene transcription regulation by CtBP1 provides an important molecular link among MS, CtBP1 function and tumor growth. Hence, these results suggest an association to understand metabolism and breast cancer.

Citation Format: Paola De Luca, Nicolás Dalton, Cristian Pablo Moiola, Carolina Flumian, Georgina Scalise, Juliana Porretti, Cintia Massillo, Edith Kordon, Laura Todaro, Elba Vazquez, Roberto Meiss, Adriana De Siervi. CtBP1 is the molecular link that associates breast cancer and metabolic syndrome. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1160. doi:10.1158/1538-7445.AM2015-1160

Abstract 3121: Study of leukemia inhibitory factor (LIF) system in a murine mammary tumor comprising luminal and myoepithelial cells

The presence of LIF, a cytokine that activates JAK/STAT and ERK pathways, its receptor (LIFRb) and the constitutive activation of Stat3 (a molecule downstream LIFRb) have been described in tumors, primary cultures and several cancer-derived cell lines, suggesting a role for this pathway in tumor progression. Also, Stat3 activation was widely related to an immunosuppressive phenotype in many tumor models.

From the murine mammary adenocarcinoma M38 comprising luminal (LEP) and myoepithelial (MEP) cells (both transformed) we derived LM38-LP (LEP & MEP), LM38-HP (epithelioid) and LM38-D2 (MEP) cell lines. While the mixed cell line presents a malignant behavior, the last two show a lower growth rate and metastatic potential than the parental tumor. Previously we demonstrated that this lower malignancy is partially caused by their inability to suppress the host's immune system, underscoring the importance of cellular interactions in complex tumor behaviors.

Our purpose was to evaluate LIF and LIFRb mRNA expression levels and to study LIF effects on Stat3 and ERK activation, as well as on the proliferation and clonogenic capacity in the LM38 family of cell lines.

Using RT-PCR we could see that LM38-LP cell line expresses high LIF and LIFRb mRNA levels, LM38-HP expresses LIF mRNA but not it receptor's mRNA and LM38-D2 has low levels of both molecules. By Western blot we studied LIF (80 ng/ml) effect over Stat3 and ERK activation. While in LM38-LP and LM38-D2 cells Stat3 and ERK phosphorylation was rapidly induced at 5 min, in LM38-HP cells, that don't express LIFRb, no response was detected. Then, we evaluated the effect of a prolonged 48 h treatment with LIF (1, 5, 20 and 80 ng/ml) on cell viability using MTS assay. While LIF slightly reduced cell viability (20%) in LM38-LP and LM38-D2 cell lines, LM38-HP cells were not affected. Finally, since LIF is also involved in stem cell maintenance, the effect of LIF in a low density/clonogenic setting was evaluated. While in LM38-LP and LM38-D2 cell lines LIF significatively (p<0.05) increased the number of colonies, LM38-HP cells were unaffected.

We can conclude that LIF could be involved in LEP-MEP interaction and although it induced a subtle viability reduction, LIF could be nevertheless promoting tumor progression by increasing the number of clonogenic cells. In addition, LIF-induced Stat3 activation could be the responsible of the immunosuppresive characteristics of the mix tumor cell line, a proposal that deserves future studies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3121. doi:10.1158/1538-7445.AM2011-3121

Abstract C47: 21-OH 6,19-epoxyprogesterone: A new dissociated glucocorticoid with anti-inflammatory properties but without antiapoptotic effects on chemotherapeutically treated mouse mammary tumor cells

Glucocorticoids (GCs) are widely used as coadyuvants in the treatment of solid tumors with chemotherapics such as paclitaxel (PXL) and doxorubicin (DOXO). However, clinical evidence supports the idea that GCs may induce resistance to these antitumor therapies. To improve cancer treatment quality, intense efforts are being made to design new dissociated steroids (steroid receptor glucocorticoid modulators (SRGMs). These compounds should mimic only the beneficial aspects of glucocorticoid therapy without inducing the undesired side effects in cancer patients. We have previously shown that dexamethasone (Dex), a well-known synthetic GC, inhibits PXL or DOXO induced apoptosis in breast tumor cells through a mechanism involving Bcl-XL and Bcl-2 induction. Here, our goal was to compare Dex effects with those triggered by 21-OH-6,19-epoxyprogesterone (21-OH-6,19-OP), which is a rigid steroid considered a passive GC antagonist. Molecular dynamic simulation studies predicted that 21-OH-6,19OP is unable to induce GR/21-OH-6,19-OP complex homodimerization and/or binding to coactivators, making these complex incapable to directly activate gene transcription (Alvarez et al J Med Chem. 2008. 13:1352). The effects of 21-OH-6,19-OP on apoptosis induced by DOXO or PXL in the epithelial mouse mammary tumor cell line LM3 was determined by analyzing Caspase-3 activity and gene and protein expression of antiapoptotic members of the Bcl family, i.e. Bcl-2 and Bcl-XL. The results show that, contrary to Dex, 21-OH-6,19-OP cannot reverse apoptosis induced by DOXO or PXL in LM3 cells. In fact, 21-OH-6,19-OP and Dex differentially regulate Bcl-2 and Bcl-XL expression. Dex's reversion on chemotherapic induced apoptosis (Dex+DOXO o Dex+PXL) correlates with an increase in Bcl-XL levels, while 21-O-H-6, 19-OP does not affect neither cell death nor Bcl-XL levels. The relevance of Bcl-XL involvement in DOXO resistance of cells treated with Dex was confirmed by knocking-down Bcl-XL with specific siRNA. Silencing Bcl-XL expression directly implicated loss of antiapoptotic GC effects. On the other hand, 21-OH-6,19-OP antiinflammatory action was also evaluated on the lung epithelial tumor cell line A549 by determining Interleukin-8 (IL-8) and Cyclooxygenase-2 (COX-2) expression in the presence of the Tumor Necrosis Factor-alpha (TNF-α). Expression levels of IL-8 and COX-2, measured by qRT-PCR, indicate that 21-OH-6,19-OP may have antiinflammatory effects as it reverses these inflammation markers expression induced by TNF-α in lung epithelial cells. Together, these findings suggest that 21-OH-6,19-OP is a SRGM that keeps glucocorticoid antiinflammatory properties without antiapoptotic effects, thus becoming a novel candidate for treatment of solid tumors as breast cancer.

Citation Information: Cancer Res 2009;69(23 Suppl):C47.

STAT5 transcriptional activity is impaired by LIF in a mammary epithelial cell line

Gene regulation mediated by STAT factors has been implicated in cellular functions with relevance to a variety of processes. Particularly, STAT5 and STAT3 play a crucial role in mammary epithelium displaying reciprocal activation kinetics during pregnancy, lactation and involution. Here, we show that LIF treatment of mammary epithelial HC11 cells reduces the phosphorylation levels and transcriptional activity of p-STAT5 in correlation with STAT3 phosphorylation. We have also found that STAT5 activity is negatively modulated by this cytokine, both on a gene whose expression is induced, as well as on a promoter repressed by STAT5. Besides, our results show that lactogenic hormones increase LIF effect on gene induction without modifying STAT3 phosphorylation state. Our findings strongly suggest that there is crosstalk between STAT5 and STAT3 pathways that could modulate their ability to regulate gene expression.

Evidence for the transforming activity of a truncated Int6 gene, in vitro

Int6/eIF3-p48 was first identified as a common integration site for MMTV in mouse mammary tumors. In all cases, the MMTV integration event resulted in an interruption of the normal Int6 transcript from one allele leaving the second allele intact and operative. We hypothesize that insertion of MMTV into Int6 results in a mutated allele that encodes a shortened Int6 mRNA and protein (Int6sh), which either modifies normal Int6 function or possesses a new independent function. To confirm the transforming potential of the mutation and its dominant function, we transfected two mammary epithelial cell lines, MCF10A (human), and HC11 (mouse), with Int6sh under the control of the elongation factor-1alpha (eEF1A) promoter. Expression of Int6sh in MCF10A and HC11 mammary epithelial cells leads to anchorage-independent growth in soft agar indicative of a transformed phenotype. Colonies selected from agar exhibited high levels of mutated Int6sh and wild type Int6 RNA transcripts by RT-PCR and Northern blot analysis. In addition, Int6sh transformed MCF10A and HC11 cells formed nodular growths, in vivo, in immune compromised hosts. NIH3T3 cells, mouse embryo fibroblasts, were also transformed to anchorage-independent growth in vitro by Int6sh expression. These observations provide direct evidence that the Int6 mutations observed in MMTV-induced tumors and hyperplasia contribute to the malignant transformation of the mammary epithelial cells.

Isotopic labelling studies on far-infrared spectra of nickel-histamine complexes

Nickel-histamine (hm) complexes type [Ni(hm)Cl2] and [Ni(hm)3]X2 (Where X= Cl, Br, I, ClO4) were investigated in the far-infrared region. Metal isotope labelling and deuteration effects were employed for observed band assignments. Metal-ligand vibrations were discussed and correlated with the structures of the complexes.

Isotopic studies of the metal-ligand vibrations in histamine complexes with copper(II)

Two known copper complexes of the formulae Cu(hm)2(ClO4)2, Cu(hm)Cl2 and new, Cu(hm)Br2 (hm = histamine) have been investigated in the 600-50 cm(-1) far-infrared region. Assignments of vibrations related to metal-ligand bonds have been made based on metal isotope substitution, partial deuteration and halogen sensitivity. Copper-hm stretching vibrations have been localised at 420, 417 and 411 cm(-1) for modes involving amine nitrogen and at 282, 270, 259 cm(-1) for those of imidazole nitrogen. Vibrational coupling between some metal-ligand modes has been postulated.

Expression of a truncated Int3 gene in developing secretory mammary epithelium specifically retards lobular differentiation resulting in tumorigenesis

Insertional mutation of the Int3 gene, a member of the Notch gene family, is frequently associated with primary mouse mammary tumors induced by the mouse mammary tumor virus (MMTV). A major consequence of these mutations is the production of a shortened 2.4-kb tumor specific Int3 RNA transcript that encodes the entire intracellular domain of the Int3 protein. Previous studies have demonstrated that mammary gland development and function was severely impaired in transgenic mice expressing the truncated Int3 gene product from the MMTV viral promoter. Both mammary ductal growth and secretory lobule development were curtailed in these mice. These results were attributed to a gain of function modification of the Int3 gene, which led to a restriction of cell fate selection in the affected mammary epithelial cells. To confirm and extend these findings, truncated Int3 was expressed from the whey acidic protein (WAP) promoter, the activity of which, unlike that of the MMTV long terminal repeat, is restricted to the secretory mammary epithelial population. In transgenic mice carrying the WAP/Int3 construct, mammary ductal growth was unaffected in virgin females, but growth and differentiation of secretory lobules during gestation was profoundly inhibited. Coincidental with the block in lobular secretory differentiation, mammary dysplasia and tumorigenesis occurred in all breeding females by 25 weeks of age. In nonbreeding WAP/Int3 females, mammary tumor incidence also reached 100%, but only after 70 weeks. The WAP/Int3 mammary tumors were highly malignant, and most tumor-bearing females, irrespective of breeding history, developed metastatic lung lesions. These results suggest that WAP promotor-targeted Int3 function is associated with mammary secretory cell differentiation and maintenance in this transgenic model. Consistent with the conclusion that WAP-driven truncated Int3 expression influenced only lobular differentiation and not ductal growth and extension during mammary gland development, transplants of WAP/Int3 gland into nontransgenic mammary fat pads produced complete mammary ductal outgrowths in virgin FVB/N mice but failed to develop secretory lobules when the females were impregnated.

Detection and location of amphiregulin and Cripto-1 expression in the developing postnatal mouse mammary gland

Amphiregulin (Ar) and Cripto-1 (Cr-1) are growth promoting peptides that share amino acid sequence homology with epidermal growth factor (EGF). The present study examined Ar and Cr-1 mRNA and protein expression during various stages of C57BL/6 mouse mammary morphogenesis. Reverse transcription-polymerase chain reaction (RT-PCR) was used to detect transcripts for Ar and Cr-1 at all stages of mammary development. Immunocytochemical (ICC) localization demonstrated that in virgin 4-week to mature 12-week-old mouse fourth inguinal mammary gland, Ar and Cr-1 are expressed in the stromal cells, luminal epithelial cells, and myoepithelial cells of the branching ducts. Ar, and to lesser extent Cr-1, were also found in the epithelial cap cells and in the luminal epithelial cells of the advancing terminal end bud (TEB) from virgin 4-week and 6-week-old mice. Western blot analysis demonstrated that both Ar (28 and 26 kDa) and Cr-1 (90, 67, 56, and 21 kDa) proteins are expressed in virgin, 13.5 day midpregnant and in the 14 day lactating mammary gland. In addition, Ar and Cr-1 are associated with developing alveolar structures as determined by ICC. These results imply that together with EGF and transforming growth factor alpha (TGF alpha), Ar and Cr-1 may play salient roles as modifiers in the morphogenesis and differentiation of the mammary gland.

Estrogen inhibition of MPA-induced mouse mammary tumor transplants

Estrogen compounds were used to treat mice bearing syngeneic transplants of medroxyprogesterone acetate(MPA)-induced BALB/c mammary adenocarcinomas. Both MPA-dependent and MPA-independent tumor lines were used. These lines expressed estrogen (ER) and progesterone receptors (PR). We demonstrate that different doses of estradiol benzoate (EB) and 17-beta-estradiol (E2) inhibit tumor growth and induce tumor regression in both MPA-independent and -dependent tumors, even in the presence of MPA or progesterone (P). EB was unable to induce regression of (ER-) hormone-independent tumor lines. A few MPA-dependent tumors became resistant to the estrogenic treatment; in subsequent passages some of these tumors retained their MPA-responsiveness, although estrogen sensitivity was not recovered.

Hormone dependence of a mouse mammary tumor line induced in vivo by medroxyprogesterone acetate

The administration of MPA to virgin female BALB/c mice led to the development of mammary adenocarcinomas, which in further in vivo transplants gave rise to both MPA-dependent and MPA-independent lines. In this paper we chose one of the MPA-dependent lines with high contents of estrogen (ER) and progesterone (PR) receptors, and were able to demonstrate that a) the growth of these tumors could be manipulated by the administration or the withdrawal of the hormonal supply; b) PR were down-regulated in MPA-treated mice; c) progesterone had the same stimulatory effect as MPA on tumor growth; d) tumors did not grow in estrogen-treated mice; e) tumor growth was much lower in males than in females; f) the presence of the ovaries had a positive influence on tumor growth, even in the presence of MPA; g) the withdrawal of progestin pellets in ovariectomized mice usually led to complete remissions followed by regrowth of the tumors after several weeks; and h) the regrowing tumors maintained their steroid receptor pattern and (in 3 out of 4 cases) their hormone-dependent behavior in further passages.

Transforming growth factor-beta activities in 'in vivo' lines of hormone-dependent and independent mammary adenocarcinomas induced by medroxyprogesterone acetate in BALB/c mice

We have determined the presence of transforming growth factor-beta (TGF-beta)-like polypeptides in mammary adenocarcinomas induced by medroxyprogesterone acetate (MPA) in BALB/c mice. In hormone-dependent tumors (HD) from nontreated and MPA-treated mice a high molecular weight (43 kDa) transforming activity was purified by Bio-Gel P-60 chromatography. This TGF was able to confer the neoplastic phenotype on NRK-49F cells without the addition of epidermal growth factor (EGF), though its activity was potentiated by EGF. It did not compete for binding to the EGF receptor, had no mitogenic activity on monolayer cultures of NRK fibroblasts, and was a potent inhibitor of DNA synthesis induced in these cells by EGF and insulin. In HD and hormone-independent tumors (HI) another TGF with a Mr of 13 kDa was isolated. This transforming activity showed the same biological properties as 43 kDa TGF, with the exception that in the absence of EGF it did not stimulate soft agar growth of NRK-49F cells. The synthesis of both factors in 'in vivo' HD tumors seems to be under MPA control, since it is much lower in HD tumors from MPA-treated mice. Further purification of the 13 and 43 kDa TGFs by hydrophobic interaction HPLC demonstrated that each one eluted in a different position, and that their elution profile differed from the TGF-beta from human platelets. The biological activity of the 13 and 43 kDa TGFs was not neutralized by a specific anti-TGF-beta antibody.

Progesterone receptors in estrogen-induced fibromatosis of guinea pigs

The presence of estrogen and progesterone receptors was investigated in estrogen-induced guinea pig fibromatosis. These lesions were composed of fibroblast-like cells embedded in abundant collagen which stained positively for desmin and vimentin suggesting muscler and fibroblast differentiation. Cytosolic progesterone receptors ranging from 13 to 2045 fmol/mg protein were present in 10/13 samples while nuclear progesterone receptors were detected in 7/10 samples with values ranging from 8 to 4639 fmol/mg DNA. The Kd of the cytosolic progesterone receptor measured by a Scatchard analysis yielded a value of 3.35 X 10(-9) M with a sedimentation coefficient of 9S as determined by a sucrose gradient. Estrogen nuclear binders were found in 9/10 samples ranging from 6 to 282 fmoles/mg DNA and cytosolic estrogen binders in 10/13 with values of 9 to 349 fmoles/mg protein. The Kd of the cytosolic estrogen binder was 22 X 10(-9) M. This binder was not detected by a commercially available monoclonal antibody. Estrogen cytosolic binders and progesterone cytosolic receptors were also detected in some samples of non tumoral fibrotic tissue. It is concluded that this experimental model is appropriate for the investigation of the effects of estrogen and progesterone on proliferative fibroblastic lesions.

Mammary adenocarcinomas induced by medroxyprogesterone acetate: hormone dependence and EGF receptors of BALB/c in vivo sublines

Mammary adenocarcinomas were induced by medroxy-progesterone acetate (MPA) in female BALB/c mice. From 5 primary tumors, 9 different sublines were established by s.c. transplantation into syngeneic female mice; these developed after a long latent period (4-12 months). Each subline was transplanted both into 4 mice treated with 40mg of MPA depot (s.c. contralaterally to the tumor inoculum) and into 4 non-treated mice. Of the 9 sublines, 6 proved to be hormone-dependent (MPA-D) and 3 hormone-independent or autonomous (MPA-I). However, even the autonomous lines, when treated with MPA, showed a slight increase in growth. All MPA-D lines had a high content of ER (20-254 fmoles/mg of protein), PR (63-710), PRL-R (44-74) and low or non-detectable EGF-R. Of the 3 MPA-I sublines that were studied, 2 showed a high content of ER (16-125), PR (27-708), PRL-R (19-70) and EGF-R (29-65) while the other one had a low content of ER (0-36), PR (0-13), no EGF-R and moderate PRL-R (15-52). Spontaneous mammary tumors of BALB/c and C3H origin, which also showed an MPA-I pattern of tumor growth, had high levels of EGF-R. We postulate that MPA has a direct effect on mammary tumor cells in MPA-D lines and that the expression of EGF-R is correlated with an autonomous pattern of growth.