Markers associated with genomic instability, immunogenicity and immune therapy responsiveness in Metaplastic carcinoma of the breast: Expression of γH2AX, pRPA2, P53, PD-L1 and tumor infiltrating lymphocytes in 76 cases

BACKGROUND

Metaplastic breast cancer (MpBC) is an aggressive subtype of breast carcinoma that is often resistant to conventional chemotherapy. Therefore, novel treatment strategies are urgently needed. Immune check point inhibitors have shown activity in programmed death-ligand 1 (PD-L1) - positive metastatic triple negative breast carcinoma (TNBC), which raises the possibility that immunotherapy may also be effective in MpBC as most of the MpBCs are triple negative. The aim of the present study was to assess genomic instability and immunogenicity in tumor specimens of patients with MpBC.

METHODS

A total of 76 patients diagnosed with MpBC over a 15-year period were included in the study. We performed immunohistochemical analyses for tumor cell PD-L1, immune cell PD-L1 and p53 on tissue microarrays (TMAs), analyzed stromal and intratumoral tumor infiltrating lymphocytes (TILs) from hematoxylin and eosin-stained (H&E) slides and scored gamma-H2AX (γH2AX) and phosphorylated-RPA2 (pRPA2) from whole tissue sections. We correlated marker expression with clinicopathologic features and clinical outcome.

RESULTS

All tumors expressed γH2AX and pRPA2 with median expressions of 43% and 44%. P53- (68%), tumor cell PD-L1- (59%) and immune cell PD-L1-positivity (62%) were common in MpBCs. Median stromal TIL and intratumoral TIL counts were 5% and 0. The spindle and squamous cell carcinomas expressed the highest levels of PD-L1 and TILs, and carcinoma with mesenchymal differentiation the lowest.

CONCLUSIONS

MpBC appears to be an immunogenic cancer with high genomic instability and frequent PD-L1-positivity, implying that check point inhibitors might be effective in MpBC. Expression levels of PD-L1 and TILs varied across different histologic subtypes, suggesting that immunotherapy might be less effective in carcinoma with mesenchymal differentiation.

Successful Treatment of Lower Limb Burn Wounds With Long-Term Survived Human Skin Allograft in an Immunosuppressed Patient: A Case Report

Early excision and autografting have been the principles in managing acute burn wounds. Despite the known benefits of early autografting, there are situations in which the placement of autograft is unsafe or even unavailable. In these clinical situations, skin substitutes like artificial dressings and human skin allografts are considered as useful for temporary wound coverage. We present an immunosuppressed patient with deep lower limb burn wound who received human skin allograft for wound management. The applied human skin allograft persisted for a longer period without infection or rejection and successfully improved her wound healing. Large and well-designed prospective studies are needed to confirm the encouraging results of the present case report.

SETD2 loss-of-function promotes renal cancer branched evolution through replication stress and impaired DNA repair

Defining mechanisms that generate intratumour heterogeneity and branched evolution may inspire novel therapeutic approaches to limit tumour diversity and adaptation. SETD2 (Su(var), Enhancer of zeste, Trithorax-domain containing 2) trimethylates histone-3 lysine-36 (H3K36me3) at sites of active transcription and is mutated in diverse tumour types, including clear cell renal carcinomas (ccRCCs). Distinct SETD2 mutations have been identified in spatially separated regions in ccRCC, indicative of intratumour heterogeneity. In this study, we have addressed the consequences of SETD2 loss-of-function through an integrated bioinformatics and functional genomics approach. We find that bi-allelic SETD2 aberrations are not associated with microsatellite instability in ccRCC. SETD2 depletion in ccRCC cells revealed aberrant and reduced nucleosome compaction and chromatin association of the key replication proteins minichromosome maintenance complex component (MCM7) and DNA polymerase δ hindering replication fork progression, and failure to load lens epithelium-derived growth factor and the Rad51 homologous recombination repair factor at DNA breaks. Consistent with these data, we observe chromosomal breakpoint locations are biased away from H3K36me3 sites in SETD2 wild-type ccRCCs relative to tumours with bi-allelic SETD2 aberrations and that H3K36me3-negative ccRCCs display elevated DNA damage in vivo. These data suggest a role for SETD2 in maintaining genome integrity through nucleosome stabilization, suppression of replication stress and the coordination of DNA repair.

Immortalised breast epithelia survive prolonged DNA replication stress and return to cycle from a senescent-like state

Mammalian cells have mechanisms to counteract the effects of metabolic and exogenous stresses, many of that would be mutagenic if ignored. Damage arising during DNA replication is a major source of mutagenesis. The extent of damage dictates whether cells undergo transient cell cycle arrest and damage repair, senescence or apoptosis. Existing dogma defines these alternative fates as distinct choices. Here we show that immortalised breast epithelial cells are able to survive prolonged S phase arrest and subsequently re-enter cycle after many days of being in an arrested, senescence-like state. Prolonged cell cycle inhibition in fibroblasts induced DNA damage response and cell death. However, in immortalised breast epithelia, efficient S phase arrest minimised chromosome damage and protected sufficient chromatin-bound replication licensing complexes to allow cell cycle re-entry. We propose that our observation could have implications for the design of drug therapies for breast cancer.

Replication stress and oxidative damage contribute to aberrant constitutive activation of DNA damage signalling in human gliomas

Malignant gliomas, the deadliest of brain neoplasms, show rampant genetic instability and resistance to genotoxic therapies, implicating potentially aberrant DNA damage response (DDR) in glioma pathogenesis and treatment failure. Here, we report on gross, aberrant constitutive activation of DNA damage signalling in low- and high-grade human gliomas, and analyze the sources of such endogenous genotoxic stress. Based on analyses of human glioblastoma multiforme (GBM) cell lines, normal astrocytes and clinical specimens from grade II astrocytomas (n=41) and grade IV GBM (n=60), we conclude that the DDR machinery is constitutively activated in gliomas, as documented by phosphorylated histone H2AX (gammaH2AX), activation of the ATM-Chk2-p53 pathway, 53BP1 foci and other markers. Oxidative DNA damage (8-oxoguanine) was high in some GBM cell lines and many GBM tumors, while it was low in normal brain and grade II astrocytomas, despite the degree of DDR activation was higher in grade II tumors. Markers indicative of ongoing DNA replication stress (Chk1 activation, Rad17 phosphorylation, replication protein A foci and single-stranded DNA) were present in GBM cells under high- or low-oxygen culture conditions and in clinical specimens of both low- and high-grade tumors. The observed global checkpoint signaling, in contrast to only focal areas of overabundant p53 (indicative of p53 mutation) in grade II astrocytomas, are consistent with DDR activation being an early event in gliomagenesis, initially limiting cell proliferation (low Ki-67 index) and selecting for mutations of p53 and likely other genes that allow escape (higher Ki-67 index) from the checkpoint and facilitate tumor progression. Overall, these results support the potential role of the DDR machinery as a barrier to gliomagenesis and indicate that replication stress, rather than oxidative stress, fuels the DNA damage signalling in early stages of astrocytoma development.

DNA damage signalling guards against activated oncogenes and tumour progression

DNA damage response (DDR), the guardian of genomic integrity, emerges as an oncogene-inducible biological barrier against progression of cancer beyond its early stages. Recent evidence from both cell culture and animal models as well as analyses of clinical specimens show that activation of numerous oncogenes and loss of some tumour suppressors result in DNA replication stress and DNA damage that alarm the cellular DDR machinery, a multifaceted response orchestrated by the ATR-Chk1 and ATM-Chk2 kinase signalling pathways. Such activation of the DDR network leads to cellular senescence or death of oncogene-transformed cells, resulting in delay or prevention of tumorigenesis. At the same time, the ongoing chronic DDR activation creates selective pressure that eventually favours outgrowth of malignant clones with genetic or epigenetic defects in the genome maintenance machinery, such as aberrations in the ATM-Chk2-p53 cascade and other DDR components. Furthermore, the executive DDR machinery is shared by at least two anticancer barriers, as both the oncogene-induced DNA replication stress and telomere shortening impact the cell fate decisions through convergence on DNA damage signalling. In this study, we highlight recent advances in this rapidly evolving area of cancer research, with particular emphasis on mechanistic insights, emerging issues of special conceptual significance and discussion of major remaining challenges and implications of the concept of DDR as a tumorigenesis barrier for experimental and clinical oncology.

The DNA damage signalling kinase ATM is aberrantly reduced or lost in BRCA1/BRCA2-deficient and ER/PR/ERBB2-triple-negative breast cancer

The ataxia-telangiectasia-mutated (ATM) kinase is a key transducer of DNA damage signals within the genome maintenance machinery and a tumour suppressor whose germline mutations predispose to familial breast cancer. ATM signalling is constitutively activated in early stages of diverse types of human malignancies and cell culture models in response to oncogene-induced DNA damage providing a barrier against tumour progression. As BRCA1 and BRCA2 are also components of the genome maintenance network and their mutations predispose to breast cancer, we have examined the ATM expression in human breast carcinomas of BRCA1/2 mutation carriers, sporadic cases and familial non-BRCA1/2 patients. Our results show that ATM protein expression is aberrantly reduced more frequently among BRCA1 (33%; P=0.0003) and BRCA2 (30%; P=0.0009) tumours than in non-BRCA1/2 tumours (10.7%). Furthermore, the non-BRCA1/2 tumours with reduced ATM expression were more often estrogen receptor (ER) negative (P=0.0002), progesterone receptor (PR) negative (P=0.004) and were of higher grade (P=0.0004). In our series of 1013 non-BRCA1/2 cases, ATM was more commonly deficient (20%; P=0.0006) and p53 was overabundant (47%; P<0.0000000001) among the difficult-to-treat ER/PR/ERBB2-triple-negative subset of tumours compared with cases that expressed at least one of these receptors (10 and 16% of aberrant ATM and p53, respectively). We propose a model of 'conditional haploinsufficiency' for BRCA1/2 under conditions of enhanced DNA damage in precancerous lesions resulting in more robust activation and hence increased selection for inactivation or loss of ATM in tumours of BRCA1/2 mutation carriers, with implications for genomic instability and curability of diverse subsets of human breast cancer.

DNA damage response in human testes and testicular germ cell tumours: biology and implications for therapy

DNA damage response (DDR) is emerging as a physiological anti-cancer barrier in early stages of cancer development, as shown for several types of solid cancers derived from somatic cells. Here we discuss our recently published and unpublished results on the exceptional paucity of such constitutive activation of the DDR machinery in human testicular germ cell tumours (TGCTs), including their common pre-invasive stage of carcinoma in situ (CIS). Our conclusions are supported by immunohistochemical analyses of multiple markers of activated DNA damage signalling, such as the phosphorylated ATM and Chk2 checkpoint kinases and phosphorylated histone H2AX. We propose that the unique lack of DDR activation in TGCTs reflects the biology of their cell of origin, the gonocyte. Furthermore, we propose that the lack of DDR activation avoids the pressure to select for mutations in DDR genes such as p53 or ATM, and the resulting intact DDR machinery may have implications for the exceptional curability of TGCTs by DNA damaging therapies.

DNA damage response mediators MDC1 and 53BP1: constitutive activation and aberrant loss in breast and lung cancer, but not in testicular germ cell tumours

MDC1 and 53BP1 are critical components of the DNA damage response (DDR) machinery that protects genome integrity and guards against cancer, yet the tissue expression patterns and involvement of these two DDR adaptors/mediators in human tumours remain largely unknown. Here we optimized immunohistochemical analyses of human 53BP1 and MDC1 proteins in situ and identified their virtually ubiquitous expression, both in proliferating and quiescent, differentiated tissues. Focus formation by 53BP1 and/or MDC1 in human spermatogenesis and subsets of breast and lung carcinomas indicated physiological and 'pathological' activation of the DDR, respectively. Furthermore, aberrant reduction or lack of either protein in significant proportions of carcinomas supported the candidacy of 53BP1 and MDC1 for tumour suppressors. Contrary to carcinomas, almost no activation or loss of MDC1 or 53BP1 were found among testicular germ-cell tumours (TGCTs), a tumour type with unique biology and exceptionally low incidence of p53 mutations. Such concomitant presence (in carcinomas) or absence (in TGCTs) of DDR activation and DDR aberrations supports the roles of MDC1 and 53BP1 within the ATM/ATR-regulated checkpoint network which, when activated, provides an early anti-cancer barrier the pressure of which selects for DDR defects such as p53 mutations or loss of 53BP1/MDC1 during cancer progression.

Centrosome abnormalities are frequently observed in non-small-cell lung cancer and are associated with aneuploidy and cyclin E overexpression

Centrosome abnormalities are observed in human cancers and have been associated with aneuploidy, a driving force in tumour progression. However, the exact pathways that tend to cause centrosome abnormalities have not been fully elucidated in human tumours. Using a series of 68 non-small-cell lung carcinomas and an array of in vitro experiments, the relationship between centrosome abnormalities, aneuploidy, and the status of key G1 to S-phase transition cell-cycle molecules, involved in the regulation of centrosome duplication, was investigated. Centrosome amplification and structural abnormalities were common (53%), were strongly related to aneuploidy, and, surprisingly, were even seen in adjacent hyperplastic regions, suggesting the possibility that these are early lesions in lung carcinogenesis. Cyclin E and E2F1 overexpression, but not p53 mutation, was observed to correlate with centrosome abnormalities in vivo (p = 0.029 and p = 0.015, respectively). This was further strengthened by the observation that cyclin E was specifically present in the nucleus and/or cytoplasm of the cells that contained centrosome aberrations. The cytoplasmic cyclin E signal may be attributed, in part, to the presence of truncated low-molecular-weight isoforms of cyclin E. In order to isolate the effect of cyclin E on the appearance of centrosome abnormalities, a U2OS tetracycline-repressible cyclin E cell line that has a normal centrosome profile by default was used. With this system, it was confirmed in vitro that persistent cyclin E overexpression is sufficient to cause the appearance of centrosome abnormalities.

The immunohistochemical expression pattern of Chk2, p53, p19INK4d, MAGE-A4 and other selected antigens provides new evidence for the premeiotic origin of spermatocytic seminoma

AIMS

Spermatocytic seminoma is a rare germ cell derived tumour of the testis that occurs mainly in older men. We analysed the expression of recently discovered markers for germ cell differentiation and the mitosis-meiosis transition in order to define the antigen profile for diagnostic purposes and to clarify the biology and histogenesis of spermatocytic seminoma.

METHODS AND RESULTS

Twenty-five spermatocytic seminomas were examined for immunohistochemical expression of germ cell-specific onco-fetal antigens and proteins involved in regulation of germ cell division, DNA repair and differentiation. The panel included Chk2, p19INK4d, p53, MAGE-A4, KIT, TRA-1-60, neurone-specific enolase and placental-like alkaline phosphatase. Four of these proteins/antigens have never before been investigated in spermatocytic seminoma. Proteins highly expressed in gonocytes and spermatogonia, such as Chk2, MAGE-A4 and neurone-specific enolase, were consistently present in spermatocytic seminoma. Antigens expressed in embryonic germ cells but not in the normal adult testis, e.g. TRA-1-60, were undetectable, with the exception of p53 protein, which was demonstrated in 80% of cases. A proto-oncogene p19INK4d, which is involved in the transition from mitotic to meiotic division in germ cells, was not detected in spermatocytic seminoma.

CONCLUSIONS

The investigation provided new information concerning the expression of Chk2, MAGE-A4, neurone-specific enolase and p19INK4d in spermatocytic seminoma. The pattern of expression is highly consistent with the origin of spermatocytic seminoma from a premeiotic germ cell, which has lost embryonic traits and has committed to spermatogenic lineage but has not yet passed the meiotic checkpoint, most probably from the spermatogonium of the adult testis.

Chk2 tumour suppressor protein in human spermatogenesis and testicular germ-cell tumours

Chk2 is a transducer of DNA damage signals and a tumour suppressor whose germ-line mutations predispose to diverse tumour types. Unlike its downstream targets such as the p53 tumour suppressor, the expression patterns of Chk2 in tissues and tumours remain unknown. As DNA breaks occur commonly during gametogenesis, and p53 is wild-type and overexpressed in testicular cancer, we examined abundance and localisation of the Chk2 protein during normal development of human testes, and at various stages of germ-cell tumour (GCT) pathogenesis. Our results show that Chk2 is abundant in foetal germ cells and adult spermatogonia, yet only weakly expressed or lacking during the meiotic and later stages of spermatogenesis. High levels of Chk2 are detected in the majority of GCTs including all pre-invasive carcinoma-in-situ lesions, contrary to variable expression and even lack of Chk2 in subsets of invasive GCTs and some teratoma structures, respectively. Together with our analyses of cell culture models, these results indicate that downmodulation or lack of Chk2 is not simply attributable to quiescence or differentiation, they suggest a role for Chk2 in mitotic rather than meiotic divisions, support the concept of foetal origin of GCTs, and have implications for protein-based screening for tumour-associated aberrations of Chk2.

p21/WAF1/Cip1 expression in invasive ductal breast carcinoma: relationship to p53, proliferation rate, and survival at 5 years

The p21/WAF1/Cipl antibody, DCS-60, was characterized by means of immunoblotting and immunofluorescence on a variety of human breast cancer cell lines. Heterogeneous staining of nuclei was observed with strong staining of cells in early G1. p21/WAF1/Cipl expression in invasive ductal, not otherwise specified breast carcinomas was determined using immunohistochemistry with this antibody and computerized image analysis. Two hundred and twenty-two tumors, including 130 from patients with no axillary node involvement, were examined. p21-positive tumor cell nuclei were found in 30% of the breast carcinomas. The percentage of tumor cell nuclei that were positive ranged from less than 1% to greater than 10%. In the whole cohort of patients, p21 expression was significantly associated with a low histological grade. In the node-negative group, there was a significant negative correlation between p21 positivity and a high (>10%) MIB-1 score. The mean MIB-1 score was significantly lower in p21-positive tumors in the whole cohort of patients (P=0.03) and in the nodenegative group (P=0.02). No association was found between p21 expression and overall survival at 5 years. With respect to p21/p53 phenotype, the significant difference in survival was noted only for the group of patients treated with adjuvant chemotherapy. The p21- p53+ phenotype had the worst survival (58% surviving 5 years), while the p21+ p53- phenotype had good survival (83% surviving 5 years; P<0.05). The results seem to suggest a correlation between p21/p53 phenotype and response to adjuvant chemotherapy.

DNA damage-activated kinase Chk2 is independent of proliferation or differentiation yet correlates with tissue biology

The Chk2 kinase is a tumor suppressor and key transducer of DNA-damage checkpoints. We show that the human Chk2 protein is relatively stable, nuclear, and responding to gamma-radiation throughout the cell cycle. Contrary to the retinoblastoma protein-regulated, labile Chk1 kinase restricted to S-G(2) phases, Chk2 remains activatable even in quiescent and differentiating cells. In human tissues, Chk2 is homogeneously expressed in renewing cell populations such as epidermis or intestine, heterogeneous in conditionally renewing tissues, and absent or cytoplasmic in static tissues such as muscle or brain. These data highlight striking differences between Chk2 and Chk1 and show unexpected correlation of Chk2 expression with tissue biology.

Aberrant expression of G1-phase cell cycle regulators in flat and exophytic adenomas of the human colon

BACKGROUND & AIMS

The G1/S-phase controlling mechanism known as the RB pathway is commonly deregulated in human malignancies. Here, the abundance and localization of key components of the retinoblastoma (RB) pathway were determined in exophytic and flat colorectal adenomas.

METHODS

Samples of normal colonic mucosa (n = 41) and flat (n = 45) and exophytic (n = 26) adenomas were examined immunohistochemically using antibodies to cyclins D1, D2, D3, cyclin-dependent kinase (CDK) 4, retinoblastoma protein (pRB), and the CDK inhibitors p16INK4a, p18INK4c, and p19INK4d.

RESULTS

In normal colonic epithelium, cyclin D2 was undetectable; expression of cyclin D1, CDK4, and pRB correlated with proliferation; and p16, p18, p19, and cyclin D3 were most abundant in quiescent, differentiated cells. Adenomas showed elevated expression of cyclin D1 and pRB, frequent induction of cyclin D2, and absence of p16. No obvious abnormalities were found for p18, p19, or cyclin D3. Overexpressed cyclin D2 was more common among exophytic and pRB among flat adenomas, respectively. Elevated cyclin D1, D2, and CDK4 correlated with enhanced dysplasia.

CONCLUSIONS

Aberrant expression of cyclins D1, D2, CDK4, p16, and pRB occur in significant subsets of exophytic and flat adenomas, particularly among cases with high-grade dysplasia. Such defects of the RB pathway may perturb cell-cycle control and thereby contribute an early step in colorectal tumorigenesis.

Aberrant p27Kip1 promoter methylation in malignant melanoma

p27Kip1 is a regulator of the mammalian cell cycle and a putative tumor suppressor. Distinct altered patterns of p27Kip1 protein expression are found in a variety of human carcinomas, and p27Kip1 expression levels usually correlate directly with disease-free survival. The mechanism(s) by which p27Kip1 expression is reduced or lost during tumorigenesis remains unclear. Specific alterations of the p27Kip1 gene, including mutations and homozygous deletions, are exceedingly rare in human cancers. We have used methylation-specific PCR and bisulfite genomic sequencing to examine the methylation status of p27Kip1 in 61 primary and metastatic tumors and 35 cell lines from patients with malignant melanoma. Dense methylation of a CpG island in the promoter region of p27Kip1 was detected in four of 45 metastatic tumors (9%) and three of the cell lines (9%), including two cell lines established from two different metastases from the same patient. Examination of a naturally occurring, allele-specific sequence variant demonstrated that p27Kip1 methylation is associated with transcriptional silencing in situ. Cell lines with p27Kip1 methylation showed retention of the wild-type allele and detectable p27Kip1 protein whose abundance was reduced compared with normal melanocytes. Collectively, our data suggest that DNA methylation may be a cause of monoallelic p27Kip1 silencing in malignant melanoma, which would support a role for p27Kip1 haploinsufficiency in human cancer.

Lack of p19INK4d in human testicular germ-cell tumours contrasts with high expression during normal spermatogenesis

p19INK4d, a member of the INK4 family of cyclin-dependent kinase inhibitors, negatively regulates the proto-oncogenic cyclin D/CDK4(6) complexes whose ability to phosphorylate the retinoblastoma tumour suppressor (RB) promotes G1/S transition. In contrast to the related p16INK4a tumour suppressor, expression patterns of 19INK4d in human tissues and tumours remain unknown. As the RB pathway is commonly targeted in cancer, and mouse models suggest a role for p19INK4d in spermatogenesis, we examined the abundance and localization of p19INK4d in the human testis, both during normal development and at various stages of germ-cell tumour pathogenesis. Our data show that the p19INK4d protein is abundant in spermatocytes of normal human adult testes, whereas virtually no p19INK4d is detectable in testicular cancer, including the preinvasive carcinoma in situ stage. Together with the lack of p19INK4d in human foetal germ cells, these results support the concept of foetal origin of the testicular germ-cell tumours, and help better understand the emerging role of the RB pathway in spermatogenesis and tumorigenesis in the human testis. Oncogene (2000) 19, 4146 - 4150

Distinct versus redundant properties among members of the INK4 family of cyclin-dependent kinase inhibitors

p16(INK4a), p15(INK4b), p18(INK4c) and p19(INK4d) comprise a family of cyclin-dependent kinase inhibitors and tumor suppressors. We report that the INK4 proteins share the ability to arrest cells in G1, and interact with CDK4 or CDK6 with similar avidity. In contrast, only p18 and particularly p19 are phosphorylated in vivo, and each of the human INK4 proteins shows unique expression patterns dependent on cell and tissue type, and differentiation stage. Thus, the INK4 proteins harbor redundant as well as non-overlapping properties, suggesting distinct regulatory modes, and diverse roles for the individual INK4 family members in cell cycle control, cellular differentiation, and multistep oncogenesis.

Cell cycle regulators in testicular cancer: loss of p18INK4C marks progression from carcinoma in situ to invasive germ cell tumours

Cell cycle regulators govern cellular proliferation, modulate differentiation and, when defective, contribute to oncogenesis. Here, we examined expression of cyclins A, B1 and E, and cyclin-dependent kinase (CDK) inhibitors p18INK4C (p18), p21WAF1/Cip1 (p21) and p27KiP1 (p27), in normal human adult testis (n = 5), and 53 testicular tumours, including 23 carcinomas in situ (CIS) and 30 germ cell tumours (GCTs). Immunohistochemical analysis revealed a correlation between proliferation and abundance of the cyclin proteins, and abundant p18 and the lack of p21 and p27 in normal spermatogenesis. Expression of p21 and/or p27 was induced in some differentiated structures seen in teratomas, and was recapitulated in cell culture, using human NTera2/D1 teratocarcinoma cells induced to differentiate into neurons. CIS lesions showed abundant p18, low cyclin E, and moderate p27, in contrast with most invasive seminomas and embryonal carcinomas with very low-to-negative p18, often elevated cyclin E, and, unexpectedly, sustained or increased p27. Our results suggest increased abundance of cyclin E, and particularly downmodulation or loss of p18INK4C as the features that correlate with progression from CIS to invasive germ cell tumours of the human testis.

Monoclonal antibody probes for p21WAF1/CIP1 and the INK4 family of cyclin-dependent kinase inhibitors

Inhibition of cyclin dependent kinases (cdk) by proteins of two families of cdk inhibitors (CKIs) represents one of the key modes of cell-cycle control. Although not fully understood at present, the functions of the individual members of the Cip/Kip and INK4 families of CKIs have been implicated in fundamental biological processes as diverse as cellular proliferation, responses to genotoxic stress, regulation of cellular differentiation, and senescence. In addition, the seven currently known CKIs qualify as either established or candidate tumor suppressors whose loss or inactivation contribute to molecular pathogenesis of a wide range of tumor types. In this study, we report the isolation and characterization of a panel of 10 mouse monoclonal antibodies (MAbs) that specifically recognize p21WAF1/CIP1 (p21) or the individual members of the INK4 family of CKIs: p15INK4b (p15), p16INK4a (p16), p18INK4c (p18), or p19INK4d (p19). These antibodies are proving to be invaluable molecular probes for analyses of protein abundance, subcellular localization, interacting cellular proteins, and ultimately the function(s) of these cell cycle regulators. Epitopes targeted by the antibodies were mapped by peptide enzyme-linked immunoadsorbent assay (ELISA), and performance of the MAbs assessed in a range of immunochemical techniques. Individual MAbs of our series recognize distinct pools of the respective CKIs, a feature reflected by their differential applicability in immunoblotting, immunoprecipitation, and immunostaining including immunohistochemistry on archival paraffin-embedded tissue sections. Together, these antibodies represent useful reagents to study CKIs in cells and tissues, a set of tools that should help elucidate the physiological roles played by the individual CKIs, and better understand the molecular mechanisms of loss or inactivation of these (candidate) tumor suppressors in human malignancies.

Immunohistochemical analysis of the D-type cyclin-dependent kinases Cdk4 and Cdk6, using a series of monoclonal antibodies

Cellular signal transduction cascades triggered by mitogenic or antiproliferative cues eventually converge on a biochemical mechanism centered around the retinoblastoma tumor suppressor (pRb), the so-called RB pathway that governs G1-phase progression and guards the commitment to enter S phase. pRb, together with its immediate upstream regulators, the D-type cyclins, their partner cyclin-dependent kinases Cdk4 and Cdk6, and the Cdk inhibitors, form a functional unit that is involved in major decisions about cellular fate, and whose components, including the proto-oncogenic cyclin D-dependent kinases, are commonly deregulated in many types of cancer. We report here the production and characterization of a series of 12 monoclonal antibodies (MAbs) that specifically recognize either Cdk4 or Cdk6. These antibodies are proving to be invaluable molecular probes for defining abundance, subcellular localization, binding partners, and ultimately the function(s) of these cell cycle-regulatory kinases. Localization of the target epitopes was mapped by peptide enzyme-linked immunoadsorbent assay (ELISA), and two antibodies recognizing sequences adjacent to N-terminus of Cdk4 can discriminate between the wild-type protein and the oncogenic, melanoma-associated R24C mutant of this kinase. Individual antibodies of our panel recognize distinct pools of Cdk4/6, a feature reflected by their differential applicability in immunoblotting, immunoprecipitation, kinase assays, and immunostaining including immunohistochemistry on archival paraffin-embedded tissue sections. Collectively, the antibodies described in this study provide the means for immunochemical analyses of the cyclin D-dependent kinases in human and animal cells, and represent useful molecular tools that should help better understand the biological roles of Cdk4 and Cdk6 in normal cell-cycle control, and their oncogenic activity in tumor cells.

D-type cyclins in adult human testis and testicular cancer: relation to cell type, proliferation, differentiation, and malignancy

D-type cyclins are proto-oncogenic components of the 'RB pathway', a G1/S regulatory mechanism centred around the retinoblastoma tumour suppressor (pRB) implicated in key cellular decisions that control cell proliferation, cell-cycle arrest, quiescence, and differentiation. This study focused on immunohistochemical and immunochemical analysis of human adult testis and 32 testicular tumours to examine the differential expression and abundance of cyclins D1, D2, and D3 in relation to cell type, proliferation, differentiation, and malignancy. In normal testis, the cell type-restricted expression patterns were dominated by high levels of cyclin D3 in quiescent Leydig cells and the lack of any D-type cyclin in the germ cells, the latter possibly representing the only example of normal mammalian cells proliferating in the absence of these cyclins. Most carcinoma-in-situ lesions appeared to gain expression of cyclin D2 but not D1 or D3, while the invasive testicular tumours showed variable positivity for cyclins D2 and D3, but rarely D1. An unexpected correlation with differentiation rather than proliferation was found particularly for cyclin D3 in teratomas, a conceptually significant observation confirmed by massive up-regulation of cyclin D3 in the human teratocarcinoma cell line NTera2/D1 induced to differentiate along the neuronal lineage. These results suggest a possible involvement of cyclin D2 in the early stages of testicular oncogenesis and the striking examples of proliferation-independent expression point to potential dual or multiple roles of the D-type cyclins, particularly of cyclin D3. These findings extend current concepts of the biology of the cyclin D subfamily, as well as of the biology and oncopathology of the human adult testis. Apart from practical implications for the assessment of proliferation and oncogenic aberrations in human tissues and tumours, this study may inspire further research into the emerging role of the cyclin D proteins in the establishment and/or maintenance of the differentiated phenotypes.

Activation of peroxisome proliferator-activated receptor gamma bypasses the function of the retinoblastoma protein in adipocyte differentiation

The retinoblastoma protein (pRB) is an important regulator of development, proliferation, and cellular differentiation. pRB was recently shown to play a pivotal role in adipocyte differentiation, to interact physically with adipogenic CCAAT/enhancer-binding proteins (C/EBPs), and to positively regulate transactivation by C/EBPbeta. We show that PPARgamma-mediated transactivation is pRB-independent, and that ligand-induced transactivation by PPARgamma1 present in RB+/+ and RB-/- mouse embryo fibroblasts is sufficient to bypass the differentiation block imposed by the absence of pRB. The differentiated RB-/- cells accumulate lipid and express adipocyte markers, including C/EBPalpha and PPARgamma2. Interestingly, adipose conversion of pRB-deficient cells occurs in the absence of compensatory up-regulations of the other pRB family members p107 and p130. RB+/+ as well as RB-/- cells efficiently exit from the cell cycle after completion of clonal expansion following stimulation with adipogenic inducers. We conclude that ligand-induced activation of endogenous PPARgamma1 in mouse embryo fibroblasts is sufficient to initiate a transcriptional cascade resulting in induction of PPARgamma2 and C/EBPalpha expression, withdrawal from the cell cycle, and terminal differentiation in the absence of a functional pRB.

Perspective: defects in cell cycle control and cancer

The past several years have witnessed a dramatic accumulation of experimental and clinical evidence supporting the notion that the cell cycle machinery is commonly targeted on oncogenesis. While numerous cell cycle regulators qualify as proto-oncogenes or tumour suppressors and their aberrations may provide direct proliferative advantage to cancer cells, defects in checkpoint mechanisms act more indirectly yet affect both tumour progression and response to anticancer therapy. In this review, the ways that cell cycle defects contribute to oncogenesis are briefly illustrated and the emerging benefits of the newly gained insights into the cell cycle clock for clinical oncology are critically considered. Given the many reviews on the subject, emphasis is put on concepts rather than comprehensive treatment of the selected topics, with particular attention given to controversial issues, unorthodox phenomena, and the challenge facing the 'cell cycle and cancer field' at the transition to the next millennium.

Breast development gives insights into breast disease

AIMS

Studies of developing human breasts are essential for understanding the organogenesis as well as molecular pathogenesis of benign and malignant breast diseases. In this study we have examined the distribution of TGF-alpha, TGF-beta 1, tenascin-C and collagen type IV with the aim of starting to build a picture of the profile of molecules that may be involved in the development of the human breast.

METHODS AND RESULTS

Ten fetal breasts (16 to 23 weeks of gestation) and 45 infant breasts, ranging in age from newborn to 2 years, were used in this study. Paraffin sections from these samples were immunostained with antibodies for these proteins and for Ki67 to elucidate the level of proliferative activity in different stages of breast development. TGF-alpha immunoreactivity was observed both in the stromal and the epithelial cells within fetal and infant breasts up to 25 days. TGF-beta 1 immunoreactivity was localized in the extracellular matrix. Tenascin-C was found around the neck of the developing breast bud and in the extracellular matrix of the infant with peaks in the newborn at 6-12 weeks. The immunoreactivity for type IV collagen was more intense in the region of the breast bud neck in the fetal breasts and reduced around the tips of lobular and terminal-end buds within the infant breasts.

CONCLUSIONS

The distribution of the growth factors and extracellular matrix proteins within the developing human breast indicates that they play a significant role in different cellular compartments during morphogenesis and provides insights into breast disease.

Cyclin D3: requirement for G1/S transition and high abundance in quiescent tissues suggest a dual role in proliferation and differentiation

The mammalian D-type cyclins D1, D2, and D3 activate the cyclin-dependent kinases CDK4 and CDK6 in G1 and thereby promote the cell's commitment to enter S phase. To elucidate the extent of functional overlap among the D-type cyclins, we have examined several aspects of the least characterized member of this subfamily of G cyclin proteins, cyclin D3. Microinjection of cyclin D3-neutralizing antibody inhibited G1/S transition in human (IMR-90) and rat (R12) diploid fibroblasts, indicating that analogous to cyclins D1 and D2, cyclin D3 is essential for timely progression through G1. In contrast to cyclins D1 and D2, cyclin D3 was (i) ubiquitously expressed among a panel of 70 human cultured cell types; (ii) strongly upregulated upon induction of HL-60 leukaemia cells to differentiate; and (iii) accumulated to high levels in a wide range of quiescent cell types in mouse and human differentiated tissues. Complementary analyses of human biopsies and mouse tissues at different stages of foetal and postnatal development revealed lineage-dependent transient or long-term accumulation of the cyclin D3 protein, correlating with initiation/establishment or maintenance of the mature phenotypes, respectively. Our data support the notion that the biological roles of the individual D-type cyclins are not fully redundant, and suggest a possible dual role for cyclin D3 in cell proliferation and induction and/or maintenance of terminal differentiation.

Aberrations of the G1- and G1/S-regulating genes in human cancer

Deregulated cell proliferation is the hallmark of cancer, and convergent data from the fields of cell-cycle research and molecular oncology have revealed the key role played by abnormalities of the cell-cycle control genes in multistep tumorigenesis. Along with the p53-mediated DNA damage checkpoint, the G1-governing pathway of D-type cyclins, their partner cyclin-dependent kinases (Cdk), Cdk inhibitors, and the retinoblastoma protein constitute a functional unit and prominent oncogenic target. We have learned a great deal about the molecular basis of G1 phase progression and G1/S transition, their proto-oncogenic defects, and potential clinical significance including diagnostic and prognostic applications and new approaches to gene therapy of cancer.

A requirement for cyclin D3-cyclin-dependent kinase (cdk)-4 assembly in the cyclic adenosine monophosphate-dependent proliferation of thyrocytes

In different systems, cyclic adenosine monophosphate (cAMP) either blocks or promotes cell cycle progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (TSH). The cAMP-dependent mitogenic pathway is unique as it is independent of mitogen-activated protein kinase activation and differs from growth factor-dependent pathways at the level of the expression of several protooncogenes/transcription factors. This study examined the involvement of D-type G1 cyclins and their associated cyclin-dependent kinase (cdk4) in the cAMP-dependent G1 phase progression of dog thyroid cells. Unlike epidermal growth factor (EGF)+serum and other cAMP-independent mitogens, TSH did not induce the accumulation of cyclins D1 and D2 and partially inhibited the basal expression of the most abundant cyclin D3. However, TSH stimulation enhanced the nuclear detection of cyclin D3. This effect correlated with G1 and S phase progression. It was found to reflect both the unmasking of an epitope of cyclin D3 close to its domain of interaction with cdk4, and the nuclear translocation of cyclin D3. TSH and EGF+serum also induced a previously undescribed nuclear translocation of cdk4, the assembly of precipitable cyclin D3-cdk4 complexes, and the Rb kinase activity of these complexes. Previously, cdk4 activity was found to be required in the cAMP-dependent mitogenic pathway of dog thyrocytes, as in growth factor pathways. Here, microinjections of a cyclin D3 antibody showed that cyclin D3 is essential in the TSH/ cAMP-dependent mitogenesis, but not in the pathway of growth factors that induce cyclins D1 and D2. The present study (a) provides the first example in a normal cell of a stimulation of G1 phase progression occurring independently of an enhanced accumulation of cyclins D, (b) identifies the activation of cyclin D3 and cdk4 through their enhanced assembly and/or nuclear translocation, as first convergence steps of the parallel cAMP-dependent and growth factor mitogenic pathways, and (c) strongly suggests that this new mechanism is essential in the cAMP-dependent mitogenesis, which provides the first direct demonstration of the requirement for cyclin D3 in a G1 phase progression.

The development of epithelial phenotypes in the human fetal and infant breast

In order to explain the molecular events that contribute to benign and malignant breast disease, it is essential to understand the cellular context in which these are occurring. This study describes a detailed analysis of the epithelial phenotypes in the human fetal and infant breast and provides a starting point for such consideration. Using methacarn-fixed, paraffin sections from ten fetal and 45 infant breast, immunostained with a panel of antibodies to cytoskeletal proteins and kappa-casein, it has been possible to define in detail the chronological evolution of the major cell types in the human breast from 16 weeks of intrauterine life to 2 years of age, in both sexes. Cells at the tips of the lobular buds and terminal end buds have a characteristic cytoskeletal protein profile, suggesting that they may have the capacity to generate both basal cells and luminal cells. Based on the expression of cytoskeletal proteins in the developing fetal and infant breast, a model system has been proposed for mammary epithelial differentiation.

The retinoblastoma protein pathway in cell cycle control and cancer

Recent discoveries in diverse fields of biomedical research have merged to reveal the molecular basis of cell cycle control and a critical role of subverting this homeostatic mechanism in cancer development. At the heart of these processes lies a late G1 checkpoint governed by the "RB pathway" whose molecular composition, functions, and cancer-associated defects are briefly evaluated in this review. This exciting new knowledge raises a plethora of conceptual issues in cell biology, with potential practical implications for biotechnology and medicine.

Cyclin D1 expression in mantle cell lymphoma is accompanied by downregulation of cyclin D3 and is not related to the proliferative activity

The cell cycle regulatory protein cyclin D1 is essential for G1-S phase transition in several epithelial and mesenchymal tissues but is apparently not essential in normal mature B cells. An overexpression of cyclin D1 is induced by the chromosomal translocation t(11;14)(q13;q32), which characterizes non-Hodgkin's lymphomas (NHLs) of mantle cell type. We studied 26 cases of mantle cell lymphoma (MCL) for the expression of cyclins D1 and D3. A total of 23 lymphomas showed a nuclear staining for cyclin D1, whereas reactive B cells of residual germinal centers were constantly negative. When compared with cyclin D3, an inverse staining pattern emerged. Whereas the B cells of residual germinal centers reacted strongly positive for cyclin D3, there was low or missing expression of cyclin D3 in MCL cells. In other B-cell lymphomas (n = 55), including chronic lymphocytic leukemia, low-grade lymphomas of mucosa-associated lymphatic tissue, follicular lymphomas, and diffuse large B-cell lymphomas, no cyclin D1 expression could be detected and 89% of these cases displayed cyclin D3 positivity. Lymphoma cell lines harboring the t(11;14) showed cyclin D1 protein but no or very low levels of cyclin D3; three other B-cell lines, a T-cell line, and peripheral blood lymphocytes strongly expressed cyclin D3 and reacted negatively for cyclin D1. We conclude that the chromosomal translocation t(11;14) leads to an abnormal protein expression of cyclin D1 in the tumor cells of MCL and induces a consecutive downregulation of cyclin D3. In contrast to other B-NHLs, cyclin D1 and D3 expression in MCL is not related to the growth fraction.

Potential role for concurrent abnormalities of the cyclin D1, p16CDKN2 and p15CDKN2B genes in certain B cell non-Hodgkin's lymphomas. Functional studies in a cell line (Granta 519)

Abnormalities of several cell-cycle regulatory genes including cyclin D1, p16CDKN2 and p15CDKN2B have been described in B cell non-Hodgkin's lymphoma (B-NHL). We describe a new B-NHL cell line (Granta 519), with concurrent abnormalities of the cyclin D1, pl6CDKN2 and pl5CDKN2B genes. An independent clinical case of mantle cell NHL (Mc-NHL) with concomitant overexpression of cyclin D1, and deletion of the p16CDKN2 gene was also identified, suggesting that this combination of oncogenic aberration is a pathophysiologic contribution to a subset of NHL cases. More in-depth functional studies of this concept were facilitated by the availability of the cell line Granta 519 which was derived from a case of high-grade NHL and has a mature B cell immunophenotype. Cytogenetic analysis identified translocation t(11;14)(q13;q32) and complex rearrangements involving chromosomes 9p22, 13p21, 17pl1, and 18q21. Molecular analysis identified overexpression of cyclin D1 mRNA and biallelic deletion of the p16CDKN2 and p15CDKN2B genes. To elucidate the effect of these genetic abnormalities on the G1 control of Granta 519 cells, the level and function of the major components of the cyclinD/retinoblastoma (RB) pathway were investigated. Cyclin D1 was dominant among the D-type cyclins, formed abundant complexes with cyclin-dependent kinase (Cdk) Cdk4 rather than Cdk6, and the immunoprecipitated cyclin D1/Cdk4 holoenzyme was active as a pRB kinase. Electroporation of wild-type pl6CDKN2 arrested the Granta 519 cells in G1, consistent with the p16CDKN2 loss as a biologically relevant event during multistep evolution of the tumor, and with the expression of functional pRB. Direct cooperation of these distinct abnormalities to cell-cycle, deregulation in NHL cells was suggested by G1 acceleration upon inducible overexpression of cyclin D1 in a control breast cancer cell line lacking p16CDKN2, an effect which could be prevented by ectopic expression of p16CDKN2. Taken together, these data suggest that concurrent overexpression of cyclin D1 and functional elimination of p16CDKN2 and p15CDKN2B may characterize certain cases of mantle cell NHL, and that cooperation of the abnormalities is likely to provide a growth advantage of the tumour cells through more efficient inactivation of the RB tumor suppressor. Further clinicopathologic studies of this possibility are warranted.

The retinoblastoma protein pathway and the restriction point

The emerging role of the retinoblastoma protein (pRb) as a major controller of the restriction point has been supported by recent discoveries, including pRb's ability to repress gene transcription by all three RNA polymerases, which suggests a link between DNA replication and cell growth. Convergent genetic and biochemical data provide new insights into the molecular events that are upstream of, at, and downstream of pRb phosphorylation, which is regulated by G1-phase cyclins and cyclin-dependent kinases (Cdks) and their inhibitors (CKIs). Major advances have also been made in our understanding of a key role of the pathway involving cyclin D, Cdks, CKIs, pRb and E2F both in commitment to traversing the cell cycle and in restraining oncogenesis.

Convergence of mitogenic signalling cascades from diverse classes of receptors at the cyclin D-cyclin-dependent kinase-pRb-controlled G1 checkpoint

The commitment of mammalian cells in late G1 to replicate the genome and divide in response to mitogenic growth factors operating via tyrosine kinase receptors depends on phosphorylation of the retinoblastoma protein (pRb), a process controlled by cyclin D-associated cyclin-dependent kinases (cdks) and their inhibitors. This study addressed the issue of whether also other mitogenic signalling cascades require activation of cyclin D-associated kinases or whether any mitogenic pathway can bypass the cyclin D-pRb checkpoint. We show that mitogenic signal transduction pathways from three classes of receptors, the membrane tyrosine kinase receptors activated by serum mitogens or epidermal growth factor, estrogen receptors triggered by estradiol, and the cyclic AMP-dependent signalling from G-protein-coupled thyrotropin receptors, all converge and strictly require the cyclin D-cdk activity to induce S phase in human MCF-7 cells and/or primary dog thyrocytes. Combined microinjection and biochemical approaches showed that whereas these three mitogenic cascades are sensitive to the p16 inhibitor of cdk4/6 and/or cyclin D1-neutralizing antibody and able to induce pRb kinase activity, their upstream biochemical routes are distinct as demonstrated by their differential sensitivity to lovastatin and requirements for mitogen-activated protein kinases whose sustained activation is seen only in the growth factor-dependent pathway. Taken together, these results support the candidacy of the cyclin D-cdk-pRb interplay for the convergence step of multiple signalling cascades and a mechanism contributing to the restriction point switch.

The p16-cyclin D/Cdk4-pRb pathway as a functional unit frequently altered in melanoma pathogenesis

The p16Ink4/CDKN2, D-type cyclins, their partners Cdk4/Cdk6, and pRb constitute a G1 regulatory pathway commonly targeted in tumorigenesis. Genetic, immunochemical, and functional cell cycle analyses showed abnormalities of this pathway in each of 22 human melanoma cell lines examined. Normal melanocytes and all melanoma lines expressed Cdk4, Cdk6, and cyclins D1 and D3. The tumor suppressors p16Ink4/CDKN2 and pRb were lost in 17 and 4 cases, respectively, due to various genetic mechanisms, including transcriptional block of p16 and nonsense mutations of RB1. Ectopic expression of p16 prevented S-phase entry of Rb+/p16- but not Rb-deficient melanoma lines. The SK29-MEL-1 cell line harboring an R24C mutation in Cdk4 expressed wild-type pRb and overabundant p16, the latter preventing endogenous Cdk6 but not Cdk4 from associating with cyclin D1. Microinjection of cyclin D1-neutralizing antibody arrested the SK29-MEL-1 cells in G1, whereas pl6 did not, indicating that the cyclin D1/Cdk4-R24C complex is required for G1 progression, and the resistance of the complex to p16 in vivo. These data strongly support the candidacy of Cdk4 as a novel proto-oncogene, provide further evidence for the p16-cyclin D/Cdk-pRb pathway as a functional unit, and suggest that deregulation of this checkpoint may represent a common step in the multistep progression of sporadic malignant melanomas.

Phenotype-related differences in the expression of D-type cyclins in human B cell-derived lines

Exposure of normal resting B lymphocytes to EBV in vitro leads to activation, subsequent immortalization, and the establishment of lymphoblastoid cell lines (LCLs). The endemic form of Burkitt's lymphoma (BL) is associated with EBV. EBV-positive BL lines maintain the original tumor phenotype (group I BL) initially and express only one EBV-encoded protein, EBV nuclear antigen (EBNA)-1. Most of them drift toward a LCL-like phenotype during in vitro culturing and express all nine EBV-encoded growth transformation-associated proteins (group III BL). Cyclin D2 and D3 have been found previously to differ in their mRNA expression in BL and LCL. Cyclin D2 expression has been attributed to EBV gene expression and to EBNA-2 and EBNA-5 in particular. We have studied cyclin D2/D3 expression in larger series of LCLs, BL lines, and freshly EBV-infected peripheral blood B lymphocytes, both at the mRNA and protein levels. The predominant cyclin D2 expression in the LCLs and group III BLs correlated with an activated B-cell phenotype. In contrast, only cyclin D3 was expressed in the group I BL lines. EBV-carrying group II BL lines that retained the BL-associated CD10 did not express cyclin D2. A collection of in vitro EBV-converted sublines of originally EBV-negative BLs that expressed a full set of the growth transformation-associated EBV proteins maintained their CD10 and cyclin D3 expression. Blood B lymphocytes expressed no D2 or D3 as judged by immunostaining. Cyclin D2 could be detected by immunostaining in a proportion of the activated blasts 40 h postinfection. Cyclin D3 that is expressed at a low level in the established LCLs was stained easily in B blasts at this time. The level of cyclin D3 at 72 h postinfection was two to three times higher than in the exponentially growing LCLs, as detected by immunoblotting. It was still 5-10 times lower than in group I BLs. Mitogen stimulation of primary B cells induced cyclin D3 at a similar level as in the LCLs. Our data are consistent with the notion of cell lineage-specific D-type cyclin expression. They also indicate that B cells may switch their D-type cyclin expression during differentiation.

Expression of CDK7/CAK in normal and tumor cells of diverse histogenesis, cell-cycle position and differentiation

The cyclin-dependent kinase 7 (CDK7) represents the 40-kDa catalytic subunit of the CDK-activating kinase, the enzyme responsible for activatory phosphorylation of multiple CDKs controlling G1, S and G2/M phases of the cell cycle. Here, we surveyed a wide range of normal and tumour cell types, in both cell culture and biopsy specimens, for abundance and subcellular localisation of the CDK7 protein. Immunoblotting and immunocytochemical analyses showed that CDK7 was (i) ubiquitously expressed in all cell types examined; (ii) exclusively nuclear; (iii) moderately elevated in tumour cells when compared with their normal cell counterparts; (iv) invariant throughout the cell cycle of normal lymphocytes, fibroblasts, breast epithelium and several cancer cell lines; and (v) clearly detectable even in quiescent cells, including highly differentiated cell types in situ. Our data are consistent with the emerging role for CDK7/CAK in multiple biological processes, possibly providing a link between cell-cycle control, transcriptional regulation and genomic integrity control.

Abundance and subcellular localisation of cyclin D3 in human tumours

The D-type cyclins are positive regulators of the G1 phase of the mammalian cell cycle. Cyclins D1 or D2 are over-expressed in several types of cancer, transform rodent cells in culture and therefore harbor hallmarks of cellular proto-oncogenes. In contrast, no data on expression of cyclin D3 in tissues and tumours are presently available. We have raised monoclonal antibodies (MAbs) specific for cyclin D3 and examined abundance and subcellular localisation of this G1 cyclin in a series of human cultured cell types and in 180 primary tumours of diverse histogenesis. Cyclin D3 localised predominantly in nuclei of normal and tumour cells both in culture and in situ, and a pronounced cell-to-cell variation of its abundance was reminiscent of cyclins D1 and D2. Immunohistochemical analysis of tumour and corresponding normal tissues showed strong aberrant accumulation of cyclin D3 in a subset (about 10%) of breast carcinomas, whereas only weak-to-moderate expression was found in colorectal, head and neck and uterine carcinomas, melanomas and soft tissue sarcomas. The specificity of the immunohistochemical data was confirmed by immunoblotting analysis of tissue and tumour lysates. Our results indicate that over-abundance of cyclin D3 is considerably less frequent than that of cyclin D1, yet we identify subsets of breast tumours, and potentially lymphomas, as candidate tumour types with elevated cyclin D3 expression.

Expression of integrin subunits in the human infant breast correlates with morphogenesis and differentiation

Integrins are widely expressed on normal tissues and their function is considered critical directly or indirectly with the control of cell growth and differentiation. Also, they are likely to play a crucial role in cell-matrix interactions during development. As the human breast develops after birth, it provides a rare opportunity in which to study human organogenesis. We have examined the distribution of integrins in the human infant breast with the aim of elucidating the possible role of these molecules in morphogenesis and differentiation. Necropsy breast specimens from six male and eight female infants, ranging in age from 1 day to 9 months, were used in this study. Cryostat sections were stained by the avidin-biotin complex technique, using a panel of monoclonal antibodies (MAbs) which recognize beta 1, alpha 2, alpha 6, beta 4, alpha v, and alpha v beta 3 integrin chains, which are candidate molecules for a role in mammory morphogenesis. MAbs to beta 1 (DH12) and alpha 2 (HAS3) showed positive membrane and cytoplasmic staining of basal cells and luminal epithelial cells. In addition, positive staining for the beta 1 integrin chain was found on fibroblasts. A MAb which recognizes the alpha 6 chain (MP4F10) showed positive staining of the basal cells and heterogeneous staining of the luminal epithelial cells, whilst beta 4 chain (439-9B) showed positive staining in the basement membrane domain of the basal cells with no staining of the luminal epithelial cells. There was a positive correlation between the intensity of expression and the structural development of the ductal system, with integrin expression reduced or absent in the end buds and lateral buds.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoblastoma-protein-dependent cell-cycle inhibition by the tumour suppressor p16

D-type cyclins, in association with the cyclin-dependent kinases Cdk4 or Cdk6, promote progression through the G1 phase of the cell cycle by phosphorylating the retinoblastoma protein (RB). The activities of Cdk4 and Cdk6 are constrained by inhibitors such as p16, the product of the CDKN2 gene on human chromosome 9p21 (refs 12-14). The frequent deletion or mutation of CDKN2 in tumour cells suggests that p16 acts as a tumour suppressor. We show that wild-type p16 arrests normal diploid cells in late G1, whereas a tumour-associated mutant of p16 does not. Significantly, the ability of p16 to induce cell-cycle arrest is lost in cells lacking functional RB, including primary fibroblasts from Rb-/- mouse embryos. Thus, loss of p16, overexpression of D-cyclins and loss of RB have similar effects on G1 progression, and may represent a common pathway to tumorigenesis.

Cyclin D2 is a moderately oscillating nucleoprotein required for G1 phase progression in specific cell types

To explore regulation and function of cyclin D2, a candidate cell cycle-regulatory proto-oncogene, we examined subcellular localisation, cell type- and cell cycle-dependent expression, and requirement of cyclin D2 protein for G1 progression, in a panel of 40 human normal and cancer cell types. Except for lymphoid cells and sarcoma cell lines, expression of cyclin D2 was considerably more restricted than that of cyclin D1, whereas both D-type cyclin proteins were low or undetectable in cells lacking functional retinoblastoma gene product. In G1 cells, the cyclin D2 protein was more resistant to extraction and localised predominantly to nuclei, whereas it became more soluble and distributed in both nuclei and cytoplasm from G1/S transition onwards. Centrifugal elutriation and multiparameter flow cytometry analyses of several cell types showed moderate cell cycle oscillation with maximum levels of the cyclin D2 protein reached in late G1. Microinjection and/or electroporation of antibodies to cyclin D2 during G1 arrested the cyclin D2-expressing lymphocytes, breast myoepithelium, and U-2-OS sarcoma cells in G1 phase, whereas cyclin D2-negative cell types were unaffected by such treatment. Consistent with the putative proto-oncogenic role of cyclin D2 in specific cell types, our data show that this G1 cyclin has properties closely resembling those of cyclin D1, including the essential positive role in regulation of G1.

Cyclin D1 is dispensable for G1 control in retinoblastoma gene-deficient cells independently of cdk4 activity

To elucidate the regulator-versus-target relationship in the cyclin D1/cdk4/retinoblastoma protein (pRB) pathway, we examined fibroblasts from RB-1 gene-deficient and RB-1 wild-type littermate mouse embryos (ME) and in human tumor cell lines that differed in the status of the RB-1 gene. The RB+/+ and RB-/- ME fibroblasts expressed similar protein levels of D-type cyclins, cdk4, and cdk6, showed analogous spectra and abundance of cellular proteins complexed with cdk4 and/or cyclins D1 and D2, and exhibited comparable associated kinase activities. Of the two human cell lines established from the same sarcoma biopsy, the RB-positive SKUT1B cells contained cdk4 that was mainly associated with D-type cyclins, contrary to a predominant cdk4-p16INK4 complex in the RB-deficient SKUT1A cells. Antibody-mediated neutralization of cyclin D1 arrested the RB-positive ME and SKUT1B cells in G1, whereas this cyclin appeared dispensable in the RB-deficient ME and SKUT1A cells. Lack of requirement for cyclin D1 therefore correlated with absence of functional pRB, regardless of whether active cyclin D1/cdk4 holoenzyme was present in the cells under study. Consistent with a potential role of cyclin D/cdk4 in phosphorylation of pRB, monoclonal anti-cyclin D1 antibodies supporting the associated kinase activity failed to significantly affect proliferation of RB-positive cells, whereas the antibody DCS-6, unable to coprecipitate cdk4, efficiently inhibited G1 progression and prevented pRB phosphorylation in vivo. These data provide evidence for an upstream control function of cyclin D1/cdk4, and a downstream role for pRB, in the order of events regulating transition through late G1 phase of the mammalian cell division cycle.

Aberrations of p16Ink4 and retinoblastoma tumour-suppressor genes occur in distinct sub-sets of human cancer cell lines

The p16Ink4/MTS1/CDKN2 is a cell-cycle regulatory inhibitor of cyclin-dependent kinase 4 (cdk4), and a candidate tumour suppressor whose gene on chromosome band 9p21 is frequently deleted or mutated in diverse types of cancer. Cdk4 in association with its D-type cyclin partners, together with p16Ink4, and the product of the retinoblastoma tumour-suppressor gene (pRB), appear to constitute a G1-phase-controlling pathway which can become de-regulated through oncogenic aberrations of any of the components. In an attempt to elucidate the underlying molecular mechanisms, we have now surveyed expression of p16Ink4, at the protein and the mRNA levels, in 21 human cell types expressing normal pRB, as compared with another series of 21 cell lines whose pRB is mutant and/or inactivated through sequestration by DNA tumour virus onco-proteins. In contrast to aberrant lack of p16 expression in the majority of RB-positive cell types, expression of apparently normal (as shown by electrophoretic mobility and/or the ability to form protein-protein complexes with cdk4 in vivo) p16 was uniformly preserved in the cancer cell lines whose RB function was compromised. These data indicate that p16 operates upstream of pRB along the same pathway in G1. The results are discussed in view of the nature of a selective growth advantage potentially gained by cells through de-regulation of this key cell-cycle control mechanism.

Cyclin D1 oncoprotein aberrantly accumulates in malignancies of diverse histogenesis

Cyclin D1 is a cell cycle regulator essential for G1 phase progression and a candidate proto-oncogene whose deregulated expression has been implicated in pathogenesis of several types of cancer. We have examined expression of cyclin D1 in 212 primary tumours of five histogenetically distinct types by immunohistochemistry and found strong aberrant accumulation of the protein in 21%, and a moderate overabundance in further 25% of cases. While the abnormalities were more frequent in carcinomas of the breast, i.e. the cancer type known for cyclin D1 gene amplification, aberrant expression was also seen in significant subsets of colorectal cancers, soft tissue sarcomas, uterine carcinomas and malignant melanomas. Comparison of distinct stages of tumour progression showed concordant cyclin D1 patterns in the in situ vs invasive breast carcinoma components (n = 37) and between primary and metastatic lesions (n = 51) of several tumour types. The specificity of the immunohistochemical data was supported by immunoblotting analysis of tissue and tumour lysates, and the tumour-specific over-expression was confirmed by computer-assisted image analysis. These observations suggest that alterations of cyclin D1 expression represent a common feature of malignancies of diverse histogenesis and indicate that both the spectrum of tumour types and the frequency of cyclin D1 aberrations significantly exceed previous estimations based on genetic analyses.