Retinoblastoma function is a better indicator of cellular phenotype in cultured breast adenocarcinoma cells than retinoblastoma expression

Novel anti-apoptotic effect of the retinoblastoma protein: implications for polyamine analogue toxicity

The retinoblastoma protein (pRb) pathway is frequently altered in breast cancer cells. pRb is involved in the regulation of cell proliferation and cell death. The breast cancer cell line L56Br-C1 does not express pRb and is extremely sensitive to treatment with the polyamine analogue N(1),N(11)-diethylnorspermine (DENSPM) which causes apoptosis. Polyamines are essential for the regulation of cell proliferation, cell differentiation and cell death. DENSPM depletes cells of polyamines, e.g., by inducing the activity of the polyamine catabolic enzyme spermidine/spermine N(1)-acetyltransferase (SSAT). In this study, L56Br-C1 cells were transfected with human pRb-cDNA. Overexpression of pRb inhibited DENSPM-induced cell death and DENSPM-induced SSAT activity. This suggests that the pRb protein level is a promising marker for polyamine depletion sensitivity and that there is a connection between pRb and the regulation of SSAT activity. We also show that SSAT protein levels and SSAT activity do not always correlate, suggesting that there is an unknown regulation of SSAT.

Regulation of RB transcription in vivo by RB family members

In cancer cells, the retinoblastoma tumor suppressor RB is directly inactivated by mutation in the RB gene or functionally inhibited by abnormal activation of cyclin-dependent kinase activity. While variations in RB levels may also provide an important means of controlling RB function in both normal and cancer cells, little is known about the mechanisms regulating RB transcription. Here we show that members of the RB and E2F families bind directly to the RB promoter. To investigate how the RB/E2F pathway may regulate Rb transcription, we generated reporter mice carrying an eGFP transgene inserted into a bacterial artificial chromosome containing most of the Rb gene. Expression of eGFP largely parallels that of Rb in transgenic embryos and adult mice. Using these reporter mice and mutant alleles for Rb, p107, and p130, we found that RB family members modulate Rb transcription in specific cell populations in vivo and in culture. Interestingly, while Rb is a target of the RB/E2F pathway in mouse and human cells, Rb expression does not strictly correlate with the cell cycle status of these cells. These experiments identify novel regulatory feedback mechanisms within the RB pathway in mammalian cells.

Cell Cycle Control Related Proteins (p53,p21, and Rb) and Transforming Growth Factorβ(TGFβ) in Benign and Carcinomatous (In Situ and Infiltrating) Human Breast: Implications in Malignant Transformations

A comparative study of the products of the cell cycle control genes p53 (mutated form), p21, Rb (nonphosphorylated and phosphorylated form) and TGFbeta was performed by immunohistochemistry and Western blot, in benign breast disorders and breast cancer (in situ and infiltrating tumors). For the five proteins studied, the relative numbers of positively stained cells were higher in in situ carcinoma than in benign breast diseases. In infiltrating breast tumors, the relative numbers of positively stained cells were even higher than in in situ tumors except for the percentage of pRb immunostained cells, which decreased slightly in infiltrative tumors. For the other four proteins, the percentages of positively stained cases were similar to those found in in situ tumors. In the three groups of patients, TGFbeta immunoreaction appeared in the cytoplasm while immunoreactions to p53, p21, Rb, and pRb were found always in the nucleus except for p21 in in situ tumors, which showed cytoplasmic immunoreaction. Present results suggest that accumulation of mutated p53, cytoplasmic p21, and pRb in breast gland epithelium might be a crucial point in the development of in situ adenocarcinoma. In the infiltrating tumors, the expression of p21 in the nuclei and the decrease in pRb expression suggest an insufficient attempt to hinder cell proliferation.

Combination effect of oncolytic adenovirotherapy and TRAIL gene therapy in syngeneic murine breast cancer models

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) gene therapy and oncolytic adenovirotherapy have been investigated extensively in xenografic human tumor models established in immunocompromised nude mice. However, the effects of these therapies on syngeneic murine tumors in immunocompetent settings were not well documented. We hypothesized that TRAIL gene therapy used with an oncolytic adenovirus would overcome the weaknesses of the two therapies used individually. In this study, we evaluated the antitumor effects of an oncolytic adenovirus, Delta24, in both human and murine breast cancer cell lines. We also analyzed the effects of TRAIL gene therapy combined with oncolytic virotherapy in these cancer cells. Our results showed that Delta24 can replicate and help the E1-deleted adenovector replicate in murine cancer cells. We also found that these two therapies combined had greater antitumor activity than either one alone in both human and murine breast cancer cells lines and in the syngeneic breast cancer models established in immunocompetent mice. Moreover, Delta24 virotherapy alone and combined with TRAIL gene therapy dramatically reduced the spontaneous liver metastasis that originated in the subcutaneous 4T1 tumor established in Balb/c mice. These findings provide important considerations in the development and preclinical assessments of oncolytic virotherapy.

Human cytochrome c enters murine J774 cells and causes G1 and G2/M cell cycle arrest and induction of apoptosis

Cytochrome c is well known as a carrier of electrons during respiration. Current evidence indicates that cytochrome c also functions as a major component of apoptosomes to induce apoptosis in eukaryotic cells as well as an antioxidant. More recently, a prokaryotic cytochrome c, cytochrome c(551) from Pseudomonas aeruginosa, has been shown to enter in mammalian cells such as the murine macrophage-like J774 cells and causes inhibition of cell cycle progression. Much less is known about such functions by mammalian cytochromes c, particularly the human cytochrome c. We now report that similar to P. aeruginosa cytochrome c(551), the purified human cytochrome c protein can enter J774 cells and induce cell cycle arrest at the G(1) to S phase, as well as at the G(2)/M phase at higher concentrations. Unlike P. aeruginosa cytochrome c(551) which had no effect on the induction of apoptosis, human cytochrome c induces significant apoptosis and cell death in J774 cells, presumably through inhibition of the cell cycle at the G(2)/M phase. When incubated with human breast cancer MCF-7 and normal mammary epithelial cell line MCF-10A1 cells, human cytochrome c entered in both types of cells but induced cell death only in the normal MCF-10A1 cells. The ability of human cytochrome c to enter J774 cells was greatly reduced at 4 degrees C, suggesting energy requirement in the entry process.

Altered structure and expression of RB1 gene and increased phosphorylation of pRb in human vestibular schwannomas

Tumor-specific alterations at the RB1 gene locus in 30 human vestibular schwannomas including 10 NF2 and 20 sporadic cases were analysed. Southern blot analysis of DNA from these samples revealed loss of heterozygosity (LOH) at the RB1 locus in 6 of 24 informative cases (25%) compared to normal blood DNAs from the same patients. Northern blot analysis showed normal size RB1 mRNA in all the tumor samples. However, there was a 2-5-fold increase in the level of expression of the RB1 gene in all the tumor samples compared to the WI38 cell line which was used as control. Western blot analysis of the RB1 protein, pRb showed a 2.5-5-fold increase in the level of total pRb as compared to normal WI38 cell line. Sixty five to seventy five percent of the total pRb were in phosphorylated form in most tumors. The LOH at the RB1 gene locus suggests genetic instability in these patients. Further, increased levels of RB1 mRNA, total pRb and the phosphorylated form of pRb suggests that RB1 gene in these tumors may have anti-apoptotic function. These results suggest that the RB1 gene has a major role in the development of human vestibular schwannomas.

Apoptosis or growth arrest: Modulation of tumor suppressor p53's specificity by bacterial redox protein azurin

The tumor suppressor protein p53 is known to induce either apoptosis or growth arrest depending on cellular background. We have previously reported that a bacterial redox protein azurin induces apoptosis in J774 cell line-derived macrophages whereas a site-directed mutant M44KM64E azurin shows very little cytotoxicity and fails to induce apoptosis in J774 cells. We now report that purified M44KM64E mutant azurin protein can enter both J774 cells as well as the human breast cancer MCF-7 cells. Entry of M44KM64E mutant azurin in J774 cells causes strong inhibition of cell-cycle progression at the G1 to S phase and a higher level of transcription of the p21 gene. Corresponding to high p21 levels, the levels of cyclins and cyclin-dependent kinases were greatly lowered in M44KM64E mutant azurin-treated J774 cells. Interestingly, M44KM64E mutant azurin protein failed to elicit inhibition of cell-cycle progression in MCF-7 cells, presumably because of mutation at the retinoblastoma tumor suppressor protein that allows functional E2F formation in MCF-7 cells even in the presence of high intracellular p21 level. Thus, the WT azurin induces apoptosis but little inhibition of cell-cycle progression whereas the M44KM64E mutant azurin is deficient in the induction of apoptosis but mediates strong inhibition of cell-cycle progression, demonstrating the role of a single bacterial protein and its hydrophobic patch in modulating two important functions of p53.

Myc protein is differentially sensitive to quinidine in tumor versus immortalized breast epithelial cell lines

Quinidine regulates growth and differentiation in human breast tumor cells, but the immortalized mammary epithelial MCF-10A cell line is insensitive to quinidine. We found that a morphologically similar differentiation response was evoked by quinidine and c-myc antisense oligonucleotides in MCF-7 cells and this prompted us to investigate the actions of quinidine on c-myc gene expression. Myc protein levels were suppressed in human breast tumor cell lines, but not in MCF-10A cells, an observation that supports the hypothesis that suppression of c-myc gene expression is involved in the preferential growth and differentiation response of breast tumor cells to quinidine. Quinidine reduced c-myc mRNA levels in MCF-7 cells. Acute induction of c-myc mRNA by estradiol, as well as the c-myc response to sub-cultivation in fresh serum and H-ras driven elevations in c-myc mRNA were depressed by 50-60% in the presence of quinidine. Quinidine decreased c-myc promoter activity in MCF-7 cells in a transient reporter gene assay and a 168 bp region of human c-myc promoter (-100 to +68 with respect to the P1 promoter) was sufficient to confer responsiveness to quinidine. Quinidine is a potential lead compound for developing pharmacological agents to regulate Myc. In addition, the study of quinidine-regulated events is a promising approach to unravel differentiation control pathways that become disrupted in breast cancer.

Rb localization and phosphorylation kinetics correlate with the cellular phenotype of cultured breast adenocarcinoma cells

Retinoblastoma protein (Rb) expression has been correlated with state of differentiation, proliferation rate, and metastatic potential in breast adenocarcinomas and established cell lines. These observations, based on immunoreactivity of total Rb rather than hypophosphorylated protein, do not address the relationship between functional Rb and indicators of an aggressive transformed cellular phenotype. We hypothesized that the distribution of functional Rb and the kinetics of Rb phosphorylation would differ between cell lines representing immortalized mammary epithelium (MCF10A), differentiated nonmetastatic mammary adenocarcinoma (MCF-7), and poorly differentiated, highly metastatic mammary adenocarcinoma (MDA-MB-231) and that these differences would be informative of the cellular phenotype. Direct immunofluorescence microscopy was used to compare qualitatively the subcellular localization of total and hypophosphorylated Rb protein in synchronized and asynchronous cells. This technique was also used to quantitatively assess the amounts of hypophosphorylated Rb throughout the cell cycle in these representative cell lines. Total Rb stained more prominently than hypophosphorylated Rb in the nucleus of all asynchronous cells. Rb phosphorylation was more rapid in MCF-7 cells than in MCF10A cells, whereas Rb dephosphorylation appeared deregulated in MDA-MB-231 cells. We conclude that assessment of hypophosphorylated Rb may be more useful than assessment of total Rb for the evaluation of transformed breast adenocarcinoma phenotypes.