The retinoblastoma gene regulates somatic growth during mouse development

Overexpression of the retinoblastoma gene (Rb) in mice leads to the dwarf phenotype. To explore the potential mechanism of Rb effects on the somatic growth, bitransgenic mice with tetracycline-regulated Rb expression were generated, and their phenotypes were compared with those of previously established Rb mouse models. By gestational day 12.5, embryos lacking Rb and those expressing twice the regular amount of Rb are 15% larger and 10-30% smaller, respectively, compared with their wild-type littermates. The dwarf phenotype is associated with increased plasma levels of insulin-like growth factor-I (IGF-I) but not with growth hormone and glucose concentrations. Down-regulation of the Rb transgene expression results in a reduction of the IGF-I plasma concentrations to normalcy and an increase of somatic growth prenatally and postnatally. Consistent with the in vivo results, cells overexpressing Rb require higher thresholds of IGF-I to stimulate proliferation. Thus, Rb plays an integral role for mouse somatic growth and maintenance during ontogenesis, and IGF-I pathway is likely to be a target for such regulation.

K-ras exon 2 point mutations in human endometrial cancer

In the present study, we screened for the K-ras exon 2 point mutations in a group of 87 gynecological neoplasms (82 endometrial carcinomas, four carcinomas of the uterine cervix and one uterine carcinosarcoma) using the non-isotopic PCR-SSCP-direct sequencing techniques. Direct sequencing analysis revealed CAA-->CAC (Gln-->His) K-ras codon 61 point mutations in two (2.4%) of the 82 endometrial carcinomas mentioned above. These two cases were endometrial endometrioid carcinomas at an early clinical stage of disease (stage IB and IC due to FIGO). Those endometrial carcinomas that showed K-ras exon 2 point mutations revealed a strong positivity for heterogeneous nuclear retinoblastoma protein staining; none of these, however, have had the K-ras codon 12 point mutation. In addition, there were no K-ras gene point mutations in three endometrial carcinomas lacking the Rb protein immunohistochemically. None of the cervical carcinomas tested had K-ras gene point mutations, whereas one carcinosarcoma harbored K-ras codon 61 point mutation (CAA-->CAC). In conclusion, our data support the view that K-ras exon 2 point mutations are rare events in human endometrial cancer. Rb and K-ras gene abnormalities may occur independently of each other during endometrial carcinogenesis in humans.

Rb and E2F-1 regulate telomerase activity in human cancer cells

The ends of human chromosomes (telomeres) lose up to 200 bp of DNA per cell division. Chromosomal shortening ultimately leads to senescence and death in normal cells. Many human carcinoma lines are immortal in vitro, suggesting that these cells have a mechanism for maintaining the ends of their chromosomes. Telomerase is a ribonucleoprotein complex that synthesizes telomeric DNA onto chromosomes using its RNA component as a template. Recent studies have shown that inactivation of the retinoblastoma gene product pRb and the cyclin dependent kinase inhibitor p16(INK4A) is required for telomerase activity in epithelial cells. We have demonstrated previously that restoration of functional retinoblastoma (Rb) expression is sufficient to downregulate telomerase activity in carcinoma cells. To determine mechanisms by which Rb regulates telomerase expression, we examined the effects of cyclin dependent kinase (cdk) mediated Rb inactivation and the release of E2F-1 on telomerase activity in human carcinoma cells. Overexpression of cdk2 and cdk4 but not a dominant negative cdk2 rescued Rb mediated downregulation of telomerase activity. Overexpression of the cdk regulatory subunit cyclin D1 also rescued telomerase downregulation and p16 expression alone was sufficient to ablate activity. E2F-1 overexpression was sufficient to rescue Rb mediated reduction of telomerase activity, but an E2F-1 mutant defective in DNA and Rb binding activities failed to produce this effect. Tumor tissue from E2F-1 -/- mice was negative for telomerase activity, indicating a key regulatory role for this transcription factor.

The cyclin-dependent kinase inhibitor (CDKI) flavopiridol disrupts phorbol 12-myristate 13-acetate-induced differentiation and CDKI expression while enhancing apoptosis in human myeloid leukemia cells

Interactions between the cyclin-dependent kinase inhibitor (CDKI) flavopiridol (FP) and phorbol 12-myristate 13-acetate (PMA) were examined in U937 human leukemia cells in relation to differentiation and apoptosis. Simultaneous, but not sequential, exposure of U937 cells to 100 nM FP and 10 nM PMA significantly increased apoptosis manifested by characteristic morphological features, mitochondrial dysfunction, caspase activation, and poly(ADP-ribose) polymerase cleavage while markedly inhibiting cellular differentiation, as reflected by diminished plastic adherence and CD11b expression. Enhanced apoptosis in U937 cells was associated with an early caspase-independent increase in cytochrome c release and accompanied by a substantial decline in leukemic cell clonogenicity. Moreover, PMA/FP cotreatment significantly increased apoptosis in HL-60 promyelocytic leukemia cells and in U937 cells ectopically expressing the Bcl-2 protein. In U937 cells, coadministration of FP blocked PMA-induced expression and reporter activity of the CDKI p21WAF/CIP1 and triggered caspase-mediated cleavage of the CDKI p27KIP1. Coexposure to FP also resulted in a more pronounced and sustained activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase cascade after PMA treatment, although disruption of this pathway by the mitogen-activated protein kinase kinase 1 inhibitor U0126 did not prevent potentiation of apoptosis. FP accelerated PMA-mediated dephosphorylation of the retinoblastoma protein (pRb), an event followed by pRb cleavage culminating in the complete loss of underphosphorylated pRb (approximately Mr 110,000) by 24 h. Finally, gel shift analysis revealed that coadministration of FP with PMA for 8 h led to diminished E2F/pRb binding compared to the effects of PMA alone. Collectively, these findings indicate that FP modulates the expression/activity of multiple signaling and cell cycle regulatory proteins in PMA-treated leukemia cells and that such alterations are associated with mitochondrial damage and apoptosis rather than maturation. These observations also raise the possibility that combining CDKIs and differentiation-inducing agents may represent a novel antileukemic strategy.

Differential expression and regulation of the retinoblastoma family of proteins during testicular development and spermatogenesis: roles in the control of germ cell proliferation, differentiation and apoptosis

Normal spermatogenesis is highly dependent on well-balanced germ cell proliferation, differentiation, and apoptosis. However, the molecular mechanisms that govern these processes are largely unknown. Retinoblastoma family proteins (pRb, p107 and p130) are potentially important regulators of cell growth, differentiation and apoptosis. pRb has been shown to be expressed in the rat testis and involved in the regulation of spermatogenesis. In the present study, the expression and localization of the other two pRb family members, p107 and p130, were analysed at both mRNA and protein levels during testicular development and spermatogenesis using Northern, Western blotting, immunohistochemistry, and in situ hybridization. Furthermore, changes of levels and phosphorylation status of pRb family proteins in response to growth suppression and/or apoptosis induction were investigated using a seminiferous tubule culture system and three animal models. Our data suggest that: (1) pRb family proteins are differentially expressed in the rat testis and they function in a cell-type-specific manner during testicular development and spermatogenesis; (2) they participate in the control of germ cell cycle and act in a cell cycle-phase-specific fashion during germ cell proliferation, and (3) they are also involved in the regulation of apoptosis of germ cells and Leydig cells.

Loss of p16 and/or pRb protein expression in NSCLC. An immunohistochemical and prognostic study

The disruption of p16/pRb pathway has been reported in several types of human cancer including lung cancer. However, the etiological and prognostic involvement of this pathway remains poorly defined in lung tumorigenesis in Taiwan. Therefore, we investigated the expressions of p16 and pRb in 107 non-small cell lung cancer (NSCLC) patients by immunohistochemical analysis. The associations of altered p16 and pRb expressions with prognosis were also examined. Sixty-four specimens (59.8%) exhibited alteration of p16 and/or pRb expression, with 49.5 and 22.4% of the tumors failing to express p16 and pRb, respectively. However, there was no reciprocally correlated expression of p16 and pRb. Negative p16 and pRb expression did not correlate well with patients' clinicopathological parameters such as age, sex, smoking habit, tumor type, and tumor stage. In addition, negative p16 and pRb expression was not statistically associated with poor prognosis (P=0.10 and P=0.74, respectively). Our results suggested that the disruption of the p16/pRb pathway plays a significant role in NSCLC tumorigenesis whereas it may not have significant importance in the prognostic evaluation of NSCLC in Taiwan.

Alteration of cell growth and morphology by overexpression of transforming growth factor beta type II receptor in human lung adenocarcinoma cells

TGF-beta is a potent inhibitory regulator of cell growth, which is transduced through interaction between type I (RI) and type II (RII) receptors that form heteromeric kinase complexes. Abnormal expression of these receptors has been identified in several human epithelial cancers and has been shown to be highly associated with resistance to TGF-beta. In this study, we investigated the expression of RI and RII in 13 human non-small cell lung cancer cell lines (NSCLCs) and demonstrated decreased or loss of RII expression in five lung cancer cell lines, but not of RI. Of these cell lines, the role of RII in NCI-H358 adenocarcinoma, which lacks RII and is insensitive to TGF-beta, was investigated by transducing this cell line with a recombinant retrovirus expressing full-length TGF-beta RII. Stably transfected cells showed significant increase in RII mRNA and protein expression. These cells responded to exogenous TGF-beta1 with suppressed proliferation in a dose-dependent manner and G1 arrest accompanied by morphological change distinct from control cells. We also investigated whether overexpression of dominant-negative RII (dnRII) in NCI-H441 adenocarcinoma, which is sensitive but expresses low levels of RII, could block signaling through the receptor complex. The overexpression of this kinase-domain-truncated RII by expressing the retroviral dnRII construct led to loss of the ability to respond to TGF-beta1 and an exhibition of uncontrolled growth. These results suggest a close association between the loss of the expression of wild-type TGF-beta RII and carcinogenesis in human lung cancer cells.

Transcriptional down-regulation of the retinoblastoma protein is associated with differentiation and apoptosis in human colorectal epithelial cells

The aim of this study was to investigate the regulation of Rb protein expression in relation to increased differentiation and induction of apoptosis in colonic epithelial cells. In vivo, Rb protein expression was found to be down-regulated towards the top of the normal colonic crypt, coincident with the region of differentiation and apoptosis, but highly expressed in colonic carcinoma tissue. Using in vitro models to study the regulation of Rb expression in pre-malignant colonic epithelial cells, we have been able to show for the first time that Rb protein expression is transcriptionally down-regulated in differentiated pre-malignant cells (in post-confluent cultures) but not in malignant colorectal epithelial cells. Furthermore, suppression of rb protein function by the HPV-E7 viral oncoprotein increased both spontaneous and DNA damage-induced apoptosis. These results suggest that Rb is able to act as a survival factor in colonic epithelial cells by suppressing apoptosis, and that over-expression of pRb in colorectal tumour cells can cause a loss of sensitivity to apoptotic signalling, resulting in aberrant cell survival and resistance to therapy.

Intranuclear compartmentalization of cyclin E during the cell cycle: disruption of the nucleoplasm-nucleolar shuttling of cyclin E in bladder cancer

Cyclin E/cyclin-dependent kinase 2 complexes are essential during the cell cycle for entrance into S phase. Cyclin E expression starts in mid-G1, reaches a maximum at S-phase entrance, and undergoes proteolysis mediated by the ubiquitin pathway as the cell progresses through S phase. Laser scanning cytometry, a microscope-based cytofluorometer combining the advantages of both flow and image analysis, allowed the determination of subcellular localization of cyclin E, p27, and retinoblastoma protein during cell cycle progression in normal human fibroblasts and nine bladder cancer cell lines. We observed that in normal fibroblasts and most tumor cell lines, cyclin E localizes in the nucleoplasm during mid-G1, and is translocated to the nucleolus during G1-S-phase transition, and its levels are undetectable in G2-M phase. Neither levels nor subcellular localization of p27 and retinoblastoma protein was cell cycle dependent in normal or tumor cells. However, four of nine bladder cancer cell lines continued to express cyclin E in all phases of the cycle, and image analysis revealed that it was localized to nucleoli. These observations suggest that the nucleolus mediates a cyclin E "shuttling" between the nucleus and the cytoplasm that is probably involved in its regulation and that this mechanism could be disrupted in bladder cancer.

Loss of p16(INK4a) expression correlates with decreased survival in pediatric osteosarcomas

Abnormalities of the G1 cell-cycle checkpoint are commonly reported in cancers at various anatomic sites. pRB, p16(INK4a) and cyclin D1 are critical G1-checkpoint proteins responsible for maintaining the balance of cellular proliferation. We examined a series of 38 pediatric osteosarcomas for abnormal expression of pRB, p16(INK4a) and cyclin D1 by immunohistochemical analysis of archival biopsy specimens. Overall, 17/38 (45%) osteosarcomas showed evidence of G1-checkpoint abrogation, including 11/38 (29%) with loss of pRB expression and 6/38 (16%) with loss of p16(INK4a) expression. Cyclin D1 over-expression was not detected. There was an inverse correlation between loss of pRB and p16(INK4a) expression (p = 0.07). pRB and p16(INK4a) abnormalities were independent of site of disease, presence of metastasis at diagnosis and percentage of tumor necrosis in the resection specimen. Clinical follow-up was available on all patients (median 31.6 months, range 5.9-116 months). Absence of p16(INK4a) expression significantly correlated with decreased survival in univariate analysis (p = 0.03), while loss of pRB expression did not affect survival. Immunohistochemical analysis of p16(INK4a) expression in pediatric osteosarcomas may be a useful adjunctive marker of prognosis.

Prognostic value of immunohistochemical expression of topoisomerase alpha II, MIB-1, p53, E-cadherin, retinoblastoma gene protein product, and HER-2/neu in adrenal and extra-adrenal pheochromocytomas

No reliable pathologic criteria have been identified that predict clinical behavior in adrenal and extra-adrenal pheochromocytomas (PHEOs). Reliable prognostic markers for the prediction of clinical outcome are needed to assign optimal treatment for potentially malignant tumors. In this report, we evaluated several molecular markers (topoisomerase II alpha, E-cadherin, HER-2/neu, and retinoblastoma (RB) gene protein) that have not been previously studied in PHEOs. Paraffin-embedded, formalin-fixed tissue blocks from 50 cases of PHEO (30 benign and 20 malignant, 31 adrenal and 19 extra-adrenal) were obtained from University of Utah Health Sciences Center, Salt Lake City, and the Medical College of Wisconsin, Milwaukee. Gross (tumor size, weight, local extension, cyst formation, hemorrhage, necrosis), microscopic (pleomorphism, hyaline globules, intranuclear inclusion, mitotic count, capsular and vascular invasion, ganglionic/neuronal differentiation), and immunohistochemical features (topoisomerase II alpha, p53, MIB-1, E-cadherin, RB, and HER-2/neu) were studied. With the exception of vascular invasion (P = 0.025), there were no unequivocal gross or microscopic characteristics that distinguished benign from malignant lesions (P approximately = 0.11-0.71). Topoisomerase III and MIB-1 indices in malignant lesions were significantly higher than those observed in benign lesions (P = 0.012 and 0.019). Differences in p53 expression were not statistically significant (P = 0.082). Loss in RB protein product expression was significantly more common in malignant lesions (P = 0.001), E-cadherin loss and HER-2/-neu overexpression were not observed in any of the benign or malignant lesions. We studied the immunohistochemical expression of topoisomerase II alpha, MIB-1, p53, RB gene protein product, E-cadherin, and HER-2/neu in a series of adrenal and extra-adrenal PHEOs. Overexpression of topoisomerase II alpha and MIB-1 and loss of RB protein product were more common in malignant lesions, whereas p53, E-cadherin, and HER-2/neu do not seem to have diagnostic utility in the prediction of biologic behavior in these neoplasms.

In vitro sensitivity of T-cell lymphoblastic leukemia to UCN-01 (7-hydroxystaurosporine) is dependent on p16 protein status: a Pediatric Oncology Group study

p16 regulates the cell cycle pathway by inhibiting the cyclin Ds-cyclin-dependent kinase (CDK) 4/6-mediated phosphorylation of retinoblastoma protein (pRb). Previously, we reported that most primary T-cell acute lymphoblastic leukemia (T-ALL) harbored p16 inactivation and hyperphosphorylated pRb without cyclin Ds or CDK4/6 alterations. Therefore, inhibiting CDK4/6 may be an ideal therapeutic approach for p16 (-) T-ALL. UCN-01 (7-hydroxystaurosporine) is a potent antitumor agent that exerts its effects through the inhibition of CDKs. We now report that p16 protein expression status of T-ALL cells influences their sensitivity to UCN-01. In 36 primary T-ALL cells, the IC50s of UCN-01 in the 27 p16 (-) cells (43+/-52 nM) was significantly lower than that in the 9 p16 (+) cells (258+/-260 nM). Our results suggest that agents like UCN-01 may be useful as a p16-selective therapy for T-ALL.

Frequent alterations in the expression of tumor suppressor genes p16INK4A and pRb in esophageal squamous cell carcinoma in the Indian population

PURPOSE

Alterations in the cell cycle regulatory p16INK4a/Cyclin D1/pRb pathway play a pivotal role in tumorigenesis. Knowledge of alterations in the tumor suppressor protein pRb and its negative regulator, p16CDKN2/MTS1/INK4a in esophageal squamous cell carcinoma (ESCC) from the Indian subcontinent is meager. To gain insight into the mechanisms underlying tumorigenesis and to search for diagnostic molecular markers for ESCC, we analyzed the expression of p16INK4a and pRb in ESCCs in the Indian population.

METHODS

Immunohistochemical analysis of pRb and p16INK4a proteins was carried out in paraffin-embedded sections from 61 surgically resected ESCCs and matched normal tissues, and the results correlated with clinicopathological parameters using chi square and Fisher's exact tests. Dual immunohistochemical analysis has been carried out to demonstrate the concomitant loss of expression of p16INK4a and pRb.

RESULTS

Fifty-nine of 61 (97%) cases showed aberration(s) in either or both of these proteins confirming their critical role in esophageal tumorigenesis. Loss of pRb was observed in 51 of the 61 (84%) and loss of p16INK4a was observed in 35 of 61 (57%) cases. Loss of pRb showed significant association with dedifferentiation of the tumor (P = 0.004). p16-/pRb-, and p16+/pRb- phenotypes were significantly associated with nodal metastasis (P = 0.017 and 0.027, respectively), while p16-/pRb+ phenotype was associated with dedifferentiation of the tumor (P = 0.012).

CONCLUSION

pRb/p16INK4a pathway plays a critical role in esophageal tumorigenesis in the Indian population. The dual hits (concomitant loss) of pRb and p16INK4a expression suggest that these two components are not mutually exclusive, and can both be altered in a significant proportion of primary ESCCs serving as putative diagnostic markers for esophageal cancer. However, the impact of dual hit on tumor behavior and disease prognosis remains to be determined.

Patterns of expression of Rb and p16 in astrocytic gliomas, and correlation with survival

The retinoblastoma pathway is a key cell cycle regulatory complex that controls the passage of cells through the G1 checkpoint and is a frequent target of genetic alterations in gliomas. In this study, we examined the expression of Rb and p16 in 170 primary astrocytic gliomas by immunohistochemical techniques, and correlated the expression with overall survival to determine their prognostic value as immunomarkers. There were 130 patients with glioblastoma multiforme (GBM) and 40 with anaplastic astrocytoma (AA). Alterations in the levels of Rb or p16 expression were seen in the majority (>90%) of the gliomas studied. The expression of Rb was completely absent or low in 47.5% of the GBM and 67.5% of the AA. The remainder of the tumors was immunopositive for Rb to varying degrees. Immunoreactivity for p16 was absent in 56% of the GBM and 77.5% of the AA. Kaplan-Meier survival plots (log-rank test) and Cox proportional hazards regression analysis, adjusted for age and histology, showed that neither Rb nor p16 expression independently predicted survival. The results of our study suggest that although genetic alterations of Rb and p16 are common in gliomas, immunohistochemical analysis of these markers correlates poorly with prognosis.

Loss of heterozygosity at 13q14 correlates with RB1 gene underexpression in human breast cancer

Loss of heterozygosity (LOH) on the long arm of chromosome 13 is common in human breast tumors, pointing to the existence of several suppressor genes in this region. LOH at 13q14 has been implicated in alterations of retinoblastoma gene (RB1) expression. However, attempts to identify a link between the absence of retinoblastoma protein expression and LOH at the RB1 locus by means of immunohistochemical techniques have produced conflicting results. Therefore, we quantified RB1 mRNA by means of reverse transcriptase-polymerase chain reaction in a large series of human sporadic primary breast tumors. RB1 gene underexpression was observed in 28 (21.7%) of 129 breast-tumor RNAs. Allelic loss at this locus correlated with RB1 mRNA underexpression (P < 10(-7)), demonstrating a causal link. These data, based on a technique other than immunohistochemistry, confirm that RB1 is the main target of the 13q14 LOH observed in human breast cancer. We also found that RB1 underexpression correlated with Scarff-Bloom-Richardson (SBR) histopathologic grade III (P = 0.033), negative estrogen-receptor status (P = 0.026) and large tumor size (P = 0.010). The latter correlation was due mainly to a high mitotic index (one of the three components comprising SBR grade), suggesting that RB1 influences the proliferation rate of breast tumors. RB1 status (underexpression vs. normal expression) was not associated with subsequent relapse or with shorter relapse-free survival. This study shows a major role of the RB1 gene in the pathogenesis of breast cancer. RB1 gene underexpression promotes breast-tumor aggressiveness and rapid tumor-cell proliferation, making RB1 an outstanding target for future gene-based breast-cancer therapy.

Cell cycle regulators p27 and pRb in lymphomas - correlation with histology and proliferative activity

The cell cycle is a complex event in which multiple regulator-proteins participate. The G(1)/S checkpoint of the cell cycle is controlled by pRb protein, which functions in its hypophosphorylated form as a negative regulator of growth. p27 (Kip1), a member of CIP/KIP family of cyclin inhibitory proteins, participates in inhibition of forming complexes that allow pRb to phosphorylate and lead the cell into mitosis. The expression of these important cell cycle regulator proteins was studied in a total of 96 non-Hodgkin's lymphoma (NHL) samples, which were classified according to the REAL classification. The expression of p27, pRb and the cell proliferation marker Ki-67 (MIB-1) was evaluated in lymphomas using immunohistochemistry. This study showed that there were coordinate changes in the expression of p27 and pRb in NHL. When compared to low-grade lymphomas, high-grade lymphomas showed significantly reduced expression of p27 and inversely pRb expression was increased (P< 0.001). Increase in expression of Ki-67 was parallel with pRb expression, and was mainly seen in cells that lacked p27 expression (P< 0.0001). This study suggests that changes in the control of the cell cycle closely relate to the pathobiology of NHL.

BRG1, a component of the SWI-SNF complex, is mutated in multiple human tumor cell lines

Human BRG1 is a component of the evolutionarily conserved SWI-SNF chromatin remodeling complex. BRG1 has been implicated in growth control through its interaction with the tumor suppressor pRb and may consequently serve as a negative regulator of proliferation. Postulating that BRG1 may itself be a tumor suppressor gene, we screened a panel of tumor cell lines to determine whether the gene is targeted for mutation. We report that the COOH-terminal region of BRG1 is homozygously deleted in two carcinoma cell lines, prostate TSU-Pr1 and lung A-427. In addition, biallelic inactivations of BRG1 were observed in four other cell lines derived from carcinomas of the breast, lung, pancreas, and prostate; their mutations in BRG1 included three frameshift lesions and one nonsense lesion. Point mutations were also discovered in a number of other cell lines, however in most cases any effect of these mutations on BRG1 function remains to be established. A variety of different mutations within BRG1, in several cell lines, suggest that BRG1 may be targeted for disruption in human tumors. Significantly, reintroduction of BRG1 into cells lacking BRG1 expression was sufficient to reverse their transformed phenotype inducing growth arrest and a flattened morphology. These data strongly support the model that BRG1 may function as a tumor suppressor and strengthen the hypothesis that the regulation of gene expression through chromatin remodeling is critical for cancer progression. It will be important to confirm these observations in primary tumors.

[The expression of P16 and Rb proteins in 106 cases of lung cancer]

OBJECTIVE

To observe the relationship between the expression of P16 and Rb proteins and the proliferation of lung cancer cells.

METHODS

The expressions of P16 and Rb proteins were measured in 66 cases of NSCLC and 40 cases of SCLC by means of immunohistochemistry technique.

RESULTS

The inverse correlation of Rb and P16 proteins expression was confirmed in 66 cases of NSCLC (chi 2 = 10.52, P < 0.01). No relationship between the expression of P16 and differentiation, stage, lymph node metastasis or pathological type was found (P > 0.05). There was no correlation between the expression of Rb and differentiation, stage, lymph node metastasis or pathological pattern (P > 0.05). The frequency of the loss of Rb protein expression in SCLC (78%) was significantly higher than in NSCLC (21%, chi 2 = 32.3, P < 0.01).

CONCLUSIONS

Loss of Rb protein expression in SCLC may be an important marker of the diagnosis of SCLC. The expression of P16 adversely correlates with that of Rb. This study provides strong confirmation about that there might be a negative feedback between P16 and Rb in phase G1.

Overexpression of retinoblastoma protein predicts decreased survival and correlates with loss of p16INK4 protein in gallbladder carcinomas

This study was designed to determine whether the level of retinoblastoma protein (pRb) expression predicts tumor progression and prognosis in gallbladder carcinomas (GBCs) and the relationship between pRb and pl6INK4 protein expression. The expression of these two proteins was evaluated immunohistochemically in 37 tumors from 36 patients with GBC. pRb loss and overexpression were observed in 5 (13.5%) and 18 (48.6%) of the 37 tumors, respectively. Both pRb loss and overexpression were significantly correlated with advanced TNM stage, lymph node metastasis, and tumor perineural invasion. Moreover, pRb overexpression was significantly associated with decreased overall survival (P = 0.001; log-rank test). Further analysis indicated that the influence of pRb overexpression on survival was independent of TNM stage and lymph node metastasis. Loss of p16INK4 protein was observed in 28 of the 37 GBCs (75.7%), but was not significantly associated with any clinicopathological factors or survival. pRb overexpression was significantly associated with the loss of p161NK4 protein (P < 0.0001). These results suggest that pRb overexpression significantly predicts decreased survival in GBCs.

Aryl hydrocarbon receptor is required for p300-mediated induction of DNA synthesis by adenovirus E1A

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological responses to environmental contaminants such as 2,3,7, 8-tetrachlorodibenzo-p-dioxin. Embryonic fibroblast (EF) isolated from AHR-null mice exhibited slow cell growth compared with wild-type EF. Reintroduction of AHR into AHR-null EF increased cell growth, suggesting that AHR is involved in cell cycle control. The role of the AHR in cell cycle control was examined using the adenovirus oncoprotein E1A. EF, derived from wild-type and AHR-null mice, were transfected with two mutant E1A expression plasmids that inactivate either p300/CBP or retinoblastoma protein (pRb). Although DNA synthesis of wild-type EF was induced by both E1A mutants, DNA synthesis in the AHR-null EF was induced only by the mutant that binds pRb, not by the mutant to p300/CBP. These data show that both pRb and p300/CBP were the target of E1A-induced DNA synthesis in wild-type EF. In AHR-null mice, however, only pRb was the target of E1A-induced DNA synthesis and p300/CBP cannot be inactivated by E1A in the absence of AHR. Immunoprecipitation revealed that AHR directly bound to p300, thus suggesting the intriguing possibility that AHR is involved in control of the cell cycle via interaction with p300.

E2F-Rb complexes assemble and inhibit cdc25A transcription in cervical carcinoma cells following repression of human papillomavirus oncogene expression

Expression of the bovine papillomavirus E2 protein in cervical carcinoma cells represses expression of integrated human papillomavirus (HPV) E6/E7 oncogenes, followed by repression of the cdc25A gene and other cellular genes required for cell cycle progression, resulting in dramatic growth arrest. To explore the mechanism of repression of cell cycle genes in cervical carcinoma cells following E6/E7 repression, we analyzed regulation of the cdc25A promoter, which contains two consensus E2F binding sites and a consensus E2 binding site. The wild-type E2 protein inhibited expression of a luciferase gene linked to the cdc25A promoter in HT-3 cervical carcinoma cells. Mutation of the distal E2F binding site in the cdc25A promoter abolished E2-induced repression, whereas mutation of the proximal E2F site or the E2 site had no effect. None of these mutations affected the activity of the promoter in the absence of E2 expression. Expression of the E2 protein also led to posttranscriptional increase in the level of E2F4, p105(Rb), and p130 and induced the formation of nuclear E2F4-p130 and E2F4-p105(Rb) complexes. This resulted in marked rearrangement of the protein complexes that formed at the distal E2F site in the cdc25A promoter, including the replacement of free E2F complexes with E2F4-p105(Rb) complexes. These experiments indicated that repression of E2F-responsive promoters following HPV E6/E7 repression was mediated by activation of the Rb tumor suppressor pathway and the assembly of repressing E2F4-Rb DNA binding complexes. Importantly, these experiments revealed that HPV-induced alterations in E2F transcription complexes that occur during cervical carcinogenesis are reversed by repression of HPV E6/E7 expression.

Presence of human papilloma virus in tumor tissue from children with retinoblastoma: an alternative mechanism for tumor development

Epidemiological studies have shown that the use of barrier methods of contraception is associated with a decreased incidence of papilloma virus infection and reduced risk of having a child with retinoblastoma. Thirty-nine primary retinoblastomas were analyzed for the presence of papilloma virus sequences. Tumor tissue sections were also used to assess the expression of the retinoblastoma protein and proliferative index. Papilloma sequences were detected in 14 of 39 (36%) tumors. Tumors in which viral sequences were detected were associated with a lower proliferative index (68% versus 78%; P = 0.015). Children with tumors containing viral sequences had a lower risk of extraocular disease (odds ratio, 9.0; 95% confidence interval, 1.6-49; P = 0.008) and a lower birth weight (2.9 versus 3.5 kg; P = 0.030). Based on these data, it is our hypothesis that papilloma viruses may play a role in the development of sporadic retinoblastoma. Detection of papilloma virus sequences and retinoblastoma protein in certain primary lesions suggests an alternative mechanism of tumor development for sporadic retinoblastoma.

Protein expression of the RB-related gene family and SV40 large T antigen in mesothelioma and lung cancer

Mutational inactivation of the RB-related gene RBL2/p130 has been reported as a common and important prognostic factor in human lung cancer. To examine the role of the RB-related gene family in lung cancer we analysed the protein expression of the RB gene in cell lines obtained from 83 patients with small cell lung cancer (SCLC) and 114 patients with non-SCLC that included 21 novel lung tumor samples. While we detected five new SCLC with mutant RB expression (RB inactivation in 75/83; 90.4%), we did not detect any RB mutations in the new non-SCLC cell lines (RB inactivation in 13/114 non-SCLC and mesothelioma; 11.4%). In addition, we detected expression of a full-length RBL1/p107 and RBL2/p130 species in every sample tested (RBL1 or RBL2 inactivation in 0/69) and confirmed that both RB-related gene products retain functional binding activity to the E1A viral oncoprotein. Since expression of SV40 Large T antigen (Tag) has been reported in a subset of human lung tumors where it may inactivate RBL1 and RBL2, we also examined mesothelioma and non-mesothelioma lung tumors for Tag expression. Although we detected a faint 85 kDa protein species using specific anti-Tag antibodies, this signal migrated slightly faster than Tag extracted from Cos7 cells and did not exhibit binding activity to the RB or RBL1 proteins. Finally, we subjected 11 lung cancer cell lines to nucleotide sequencing and did not detect mutations within the C-terminal RBL2 exons 19-22 as recently reported. While the RB/p16 tumor suppressor pathway is targeted for mutations in 100% of lung cancers, mutational inactivation of the related RBL1 and RBL2 genes is a rare event.

Retrovirus-mediated delivery of HPV16 E7 antisense RNA inhibited tumorigenicity of CaSki cells

OBJECTIVE

In cervical cancer, high-risk human papillomavirus (HPV) genes are expressed solely in cancerous cells and have been proposed to be the most important etiological factors for cervical cancer, thus making them suitable targets for gene therapy. In this study, we aim to inactivate the HPV16 E7 in CaSki cells and test the possibility of reducing the tumorigenicity of these cells.

METHODS

The full-length HPV16 E7 cDNA was cloned in the pBabe-puro or pWZL-Hygro retrovirus vector in reverse orientation and was stably transfected into CaSki cells by replication-defective retrovirus infection giving rise to CaSki-E7AS and CaSki-E7AS2X cells. Immunoprecipitation/Western analysis and real-time RT-PCR were performed to document the levels of HPV16 E7 gene product. Flow cytometry was performed to study changes in the cell cycle in response to reduced E7 protein. The expression of bcl-2, RB, and E2F-1 was studied using Western blot analysis. Tumorigenicity of CaSki, CaSki-E7AS, and CaSki-E7AS2X cells was assayed with subepidermal tumor growth in nude mice.

RESULTS

We have documented that the delivery of the antisense gene construct resulted in the reduction of HPV16 E7 protein expression and cell proliferation in CaSki cells. Furthermore, we demonstrated that these changes were accompanied by cell cycle arrest, up-regulation of RB, and down-regulation of E2F-1 and bcl-2 proteins. More importantly, dose-dependent transduction of the antisense HPV16E7 construct was able to inhibit and/or retard the tumorigenicity of CaSki cells in vivo.

CONCLUSIONS

Down-regulation of HPV16 E7 with antisense RNA is beneficial in reducing the tumorigenicity of CaSki cells and can potentially be useful for HPV-associated malignancy gene therapy.