Distinct and nonoverlapping roles for pRB and cyclin D:cyclin-dependent kinases 4/6 activity in melanocyte survival

Diverse gene expression patterns in response to anticancer drugs between human and mouse cell lines revealed by a comparative transcriptomic analysis

The aim of the present study was to perform comparative genomics using gene expression profile datasets of mice and humans who had been treated with anticancer drugs, to determine the similarities and differences in the antitumor mechanisms in the two mammals. This involved data mining of antitumor gene expression regulation, and screening of genetic loci from experimental mouse models of antitumor targets, to provide a theoretical basis of drug design. Subsequently, 9 overlapping genes with opposite expression patterns were identified across mouse and human cell lines that were treated with a specific cyclin-dependent kinase 4/6 inhibitor, PD0332991. These genes included LIM homeobox 2, adenomedullin, bone marrow stromal cell antigen 1, caveolin 1, histone cluster 1 (HIST1) H2B family member C, HIST1 H3 family member F, low density lipoprotein-receptor related protein 11, prolyl 4-hydroxylase subunit α1 and torsin family 3 member A. In addition, the janus kinase-signal transducer and activator of transcription signaling pathway, Toll-like receptor signaling pathway, T cell receptor signaling pathway and the nucleotide-binding oligomerization domain-like receptor signaling pathway were identified as candidate pathways for explaining antitumor mechanisms.

CDK 4/6 inhibitor palbociclib (PD0332991) in Rb+ advanced breast cancer: phase II activity, safety, and predictive biomarker assessment

PURPOSE

The G1-S checkpoint of the cell cycle is frequently dysregulated in breast cancer. Palbociclib (PD0332991) is an oral inhibitor of CDK4/6. Based upon preclinical/phase I activity, we performed a phase II, single-arm trial of palbociclib in advanced breast cancer.

EXPERIMENTAL DESIGN

Eligible patients had histologically confirmed, metastatic breast cancer positive for retinoblastoma (Rb) protein and measureable disease. Palbociclib was given at 125 mg orally on days 1 to 21 of a 28-day cycle. Primary objectives were tumor response and tolerability. Secondary objectives included progression-free survival (PFS) and assessment of Rb expression/localization, KI-67, p16 loss, and CCND1 amplification.

RESULTS

Thirty-seven patients were enrolled; 84% hormone-receptor (HR)(+)/Her2(-), 5% HR(+)/Her2(+), and 11% HR(-)/Her2(-), with a median of 2 prior cytotoxic regimens. Two patients had partial response (PR) and 5 had stable disease ≥ 6 months for a clinical benefit rate (CBR = PR + 6moSD) of 19% overall, 21% in HR(+), and 29% in HR(+)/Her2(-) who had progressed through ≥2 prior lines of hormonal therapy. Median PFS overall was 3.7 months [95% confidence interval (CI), 1.9-5.1], but significantly longer for those with HR(+) versus HR(-) disease (P = 0.03) and those who had previously progressed through endocrine therapy for advanced disease (P = 0.02). Grade 3/4 toxicities included neutropenia (51%), anemia (5%), and thrombocytopenia (22%). Twenty-four percent had treatment interruption and 51% had dose reduction, all for cytopenias. No biomarker identified a sensitive tumor population.

CONCLUSIONS

Single-agent palbociclib is well tolerated and active in patients with endocrine-resistant, HR(+), Rb-positive breast cancer. Cytopenias were uncomplicated and easily managed with dose reduction.

Genomic DNA recombination with cell-penetrating peptide-tagged cre protein in mouse skeletal and cardiac muscle

The Cre-loxP recombination system has been used to promote DNA recombination both in vitro and in vivo. For in vivo delivery, Cre expression is commonly achieved through the use of tissue/cell type-specific promoters, viral infection, or drug inducible transcription and protein translocation to promote targeted DNA excision. The development of cell permeable (or penetrating) peptide tagged proteins has facilitated the delivery of Cre recombinase protein into cells in culture, organotypic slide culture, or in living animals. In this report, we generated bacterially expressed, his-tagged Cre protein with either a cardiac targeting peptide or an antennapedia peptide at the C-terminus and demonstrated efficient uptake and recombination in both cell culture and mice. To facilitate delivery to cardiac and skeletal muscle, we mixed proteins with pluronic F-127 hydrogel and delivered Cre protein into reporter Rosa26mTmG mouse skeletal muscle or Rosa26LacZ cardiac muscle via ultrasound guided injection. Activation of reporter gene expression indicated that these Cre proteins were enzymatically active. Recombination events were detected only in the vicinity of injection areas. In conclusion, we have developed a method to deliver enzymatically active Cre protein locally to skeletal muscle and cardiac muscle that may be adapted for use with other proteins.

GATA-3 promotes T-cell specification by repressing B-cell potential in pro-T cells in mice

Transcription factors orchestrate T-lineage differentiation in the thymus. One critical checkpoint involves Notch1 signaling that instructs T-cell commitment at the expense of the B-lineage program. While GATA-3 is required for T-cell specification, its mechanism of action is poorly understood. We show that GATA-3 works in concert with Notch1 to commit thymic progenitors to the T-cell lineage via 2 distinct pathways. First, GATA-3 orchestrates a transcriptional “repertoire” that is required for thymocyte maturation up to and beyond the pro-T-cell stage. Second, GATA-3 critically suppresses a latent B-cell potential in pro–T cells. As such, GATA-3 is essential to sealing in Notch-induced T-cell fate in early thymocyte precursors by promoting T-cell identity through the repression of alternative developmental options.

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.

p38 phosphorylates Rb on Ser567 by a novel, cell cycle-independent mechanism that triggers Rb-Hdm2 interaction and apoptosis

The retinoblastoma protein (Rb) inhibits both cell division and apoptosis, but the mechanism by which Rb alternatively regulates these divergent outcomes remains poorly understood. Cyclin dependent kinases (Cdks) promote cell division by phosphorylating and reversibly inactivating Rb by a hierarchical series of phosphorylation events and sequential conformational changes. The stress-regulated mitogen activated protein kinase (MAPK) p38 also phosphorylates Rb, but it does so in a cell cycle-independent manner that is associated with apoptosis rather than with cell division. Here, we show that p38 phosphorylates Rb by a novel mechanism that is distinct from that of Cdks. p38 bypasses the cell cycle-associated hierarchical phosphorylation and directly phosphorylates Rb on Ser567, which is not phosphorylated during the normal cell cycle. Phosphorylation by p38, but not Cdks, triggers an interaction between Rb and the human homologue of murine double minute 2 (Hdm2), leading to degradation of Rb, release of E2F1 and cell death. These findings provide a mechanistic explanation for how Rb regulates cell division and apoptosis through different kinases, and reveal how Hdm2 may functionally link the tumor suppressors Rb and p53.

The retinoblastoma protein modulates Tbx2 functional specificity

This study demonstrates that Tbx2 binds Rb1. The interaction with Rb1 increases Tbx2 DNA-binding activity and enhances the ability of Tbx2 to repress transcription. The results show that Tbx2 regulates the expression of genes involved in cell division and DNA replication and that Rb1 modulates Tbx2 target gene recognition and specificity.

Tbx2 is a member of a large family of transcription factors defined by homology to the T-box DNA-binding domain. Tbx2 plays a key role in embryonic development, and in cancer through its capacity to suppress senescence and promote invasiveness. Despite its importance, little is known of how Tbx2 is regulated or how it achieves target gene specificity. Here we show that Tbx2 specifically associates with active hypophosphorylated retinoblastoma protein (Rb1), a known regulator of many transcription factors involved in cell cycle progression and cellular differentiation, but not with the Rb1-related proteins p107 or p130. The interaction with Rb1 maps to a domain immediately carboxy-terminal to the T-box and enhances Tbx2 DNA binding and transcriptional repression. Microarray analysis of melanoma cells expressing inducible dominant-negative Tbx2, comprising the T-box and either an intact or mutated Rb1 interaction domain, shows that Tbx2 regulates the expression of many genes involved in cell cycle control and that a mutation which disrupts the Rb1-Tbx2 interaction also affects Tbx2 target gene selectivity. Taken together, the data show that Rb1 is an important determinant of Tbx2 functional specificity.

Melanocyte homeostasis in vivo tolerates Rb1 loss in a developmentally independent fashion

There has been uncertainty regarding the precise role that the pocket protein Rb1 plays in murine melanocyte homeostasis. It has been reported that the TAT-Cre mediated loss of exon 19 from a floxed Rb1 allele causes melanocyte apoptosis in vivo and in vitro. This is at variance with other findings showing, either directly or indirectly, that Rb1 loss in melanocytes has no noticeable effect in vivo, but in vitro leads to a semi-transformed phenotype. In this study, we show that Rb1-null melanocytes lacking exon 19 do not undergo apoptosis and survive both in vitro and in vivo, irrespective of the developmental stage at which Cre-mediated ablation of the exon occurs. Further, Rb1 loss has no serious long-term ramifications on melanocyte homeostasis in vivo, with Rb1-null melanocytes being detected in the skin after numerous hair cycles, inferring that the melanocyte stem cell population carrying the Cre-mediated deletion is maintained. Consequently, whilst Rb1 loss in the melanocyte is able to alter cellular behaviour in vitro, it appears inconsequential with respect to melanocyte homeostasis in the mouse skin.

Efficient CPP-mediated Cre protein delivery to developing and adult CNS tissues

Background

Understanding and manipulating gene function in physiological conditions is a major objective for both fundamental and applied research. In contrast to other experimental settings, which use either purely genetic or gene delivery (viral or non-viral) strategies, we report here a strategy based on direct protein delivery to central nervous system (CNS) tissues. We fused Cre recombinase with cell-penetrating peptides and analyzed the intracellular biological activity of the resulting chimerical proteins when delivered into cells endowed with Cre-mediated reporter gene expression.

Results

We show that active Cre enzymatic conjugates are readily internalized and exert their enzymatic activity in the nucleus of adherent cultured cells. We then evaluated this strategy in organotypic cultures of neural tissue explants derived from reporter mice carrying reporter "floxed" alleles. The efficacy of two protocols was compared on explants, either by direct addition of an overlying drop of protein conjugate or by implantation of conjugate-coated beads. In both cases, delivery of Cre recombinase resulted in genomic recombination that, with the bead protocol, was restricted to discrete areas of embryonic and adult neural tissues. Furthermore, delivery to adult brain tissue resulted in the transduction of mature postmitotic populations of neurons.

Conclusion

We provide tools for the spatially restricted genetic modification of cells in explant culture. This strategy allows to study lineage, migration, differentiation and death of neural cells. As a proof-of-concept applied to CNS tissue, direct delivery of Cre recombinase enabled the selective elimination of an interneuron subpopulation of the spinal cord, thereby providing a model to study early events of neurodegenerative processes. Thus our work opens new perspectives for both fundamental and applied cell targeting protocols using proteic cargoes which need to retain full bioactivity upon internalisation, as illustrated here with the oligomeric Cre recombinase.

Skin and hair: models for exploring organ regeneration

Skin is an excellent model to study the basic biology of organ regeneration and translational approaches to regenerative medicine. Because of the accessibility of the skin, a long history of regenerative approaches already exists. Identifying the commonalities between skin regeneration and the regeneration of other organs could provide major breakthroughs in regenerative medicine. The hair follicle represents a miniature organ with readily accessible stem cells, multiple cell lineages, and signaling centers. During the normal lifespan of a human, this miniature organ regenerates itself more than 10 times. The cells responsible for this remarkable process are called bulge stem cells. A plethora of molecular and genetic tools have been developed to follow their fate and to explore their ontogeny. Major advances have been made toward understanding the normal cell fate of bulge stem cells and their developmental plasticity. Recent studies suggest the epidermis and hair may have an untapped potential to form other organs. Understanding the mechanisms that regulate adult stem-cell proliferation is a major goal for regenerative medicine. In the hair follicle, pharmacologic agents, recombinant proteins, and artificial cell-permeable proteins have been developed to manipulate the proliferation of the quiescent bulge stem cells. These advances illustrate a potential roadmap for regenerative medicine using molecular tools developed for skin biology to promote organ regeneration by manipulating adult stem cells in situ.

NFATc1 balances quiescence and proliferation of skin stem cells

Quiescent adult stem cells reside in specialized niches where they become activated to proliferate and differentiate during tissue homeostasis and injury. How stem cell quiescence is governed is poorly understood. We report here that NFATc1 is preferentially expressed by hair follicle stem cells in their niche, where its expression is activated by BMP signaling upstream and it acts downstream to transcriptionally repress CDK4 and maintain stem cell quiescence. As stem cells become activated during hair growth, NFATc1 is downregulated, relieving CDK4 repression and activating proliferation. When calcineurin/NFATc1 signaling is suppressed, pharmacologically or via complete or conditional NFATc1 gene ablation, stem cells are activated prematurely, resulting in precocious follicular growth. Our findings may explain why patients receiving cyclosporine A for immunosuppressive therapy display excessive hair growth, and unveil a functional role for calcium-NFATc1-CDK4 circuitry in governing stem cell quiescence.

RBP2-H1/JARID1B is a transcriptional regulator with a tumor suppressive potential in melanoma cells

The RBP2-H1/JARID1B nuclear protein belongs to the ARID family of DNA-binding proteins and is a potential tumor suppressor that is lost during melanoma development. As we have recently shown, one physiological function of RBP2-H1/JARID1B is to exert cell cycle control via maintenance of active retinoblastoma protein. We now add new evidence that RBP2-H1/JARID1B can also directly regulate gene transcription in a reporter assay system, either alone or as part of a multimolecular complex together with the developmental transcription factors FOXG1b and PAX9. In melanoma cells, chromatin immunoprecipitation combined with promoter chip analysis (ChIP-on-chip) suggests a direct binding of re-expressed RBP2-H1/JARID1B to a multitude of human regulatory chromosomal elements (promoters, enhancers and introns). Among those, a set of 23 genes, including the melanoma relevant genes CDK6 and JAG-1 could be confirmed by cDNA microarray analyses to be differentially expressed after RBP2-H1/JARID1B re-expression. In contrast, in nonmelanoma HEK 293 cells, RBP2-H1/JARID1B overexpression only evokes a minor transcriptional response in cDNA microarray analyses. Because the transcriptional regulation in melanoma cells is accompanied by an inhibition of proliferation, an increase in caspase activity and a partial cell cycle arrest in G1/0, our data support an anti-tumorigenic role of RBP2-H1/JARID1B in melanocytic cells.

Conditional mutagenesis by cell-permeable proteins: potential, limitations and prospects

The combination of two powerful technologies, the Cre/loxP recombination system and the protein transduction technique, holds great promise for the advancement of biomedical and genome research by enabling precise and rapid control over mutation events. Protein transduction is a recently developed technology to deliver biologically active proteins directly into mammalian cells. It involves the generation of fusion proteins consisting of the cargo molecule to be delivered and a so-called protein transduction domain. Recently, the derivation of cell permeable variants of the DNA recombinase Cre has been reported. Cre is a site-specific recombinase that recognizes 34 base pair loxP sites and has been widely used to genetically engineer mammalian cells in vitro and in vivo. Recombinant cell-permeable Cre recombinase was found to efficiently induce recombination of loxP-modified alleles in various mammalian cell lines. Here we review recent advances in conditional expression and mutagenesis employing cell-permeable Cre proteins. Moreover, this review summarizes recent findings of studies aimed at deciphering the molecular mechanism of the cellular uptake of cell-permeable fusion proteins.

New technologies used in the study of human melanoma

The amount of information on tumor biology has expanded enormously, essentially due to the completion of the human genome sequencing and to the application of new technologies that represent an exciting breakthrough in molecular analysis. Often these data spring from experimental procedures, such as a serial analysis of gene expression (SAGE) and DNA microarrays, which cannot be defined as hypothesis-driven: it may appear to be a "brute force" approach through which no information can be directly generated concerning the specific functions of selected genes in a definite context. However, interesting results are fruitfully generated, and thus it is important to consider the enormous potential these new technologies possess and to learn how to apply this novel form of knowledge in the emerging field of molecular medicine. This review, after a limited outline regarding several classic aspects of human cutaneous melanoma biology, genetics, and clinical approaches, will focus on the proficient use of up-to-date technologies in the study of the neoplastic disease and on their capability to provide effective support to conventional approaches in melanoma diagnosis, prognosis, and treatment.

The retinoblastoma tumor-suppressor gene, the exception that proves the rule

The retinoblastoma tumor-suppressor gene (Rb1) is centrally important in cancer research. Mutational inactivation of Rb1 causes the pediatric cancer retinoblastoma, while deregulation of the pathway in which it functions is common in most types of human cancer. The Rb1-encoded protein (pRb) is well known as a general cell cycle regulator, and this activity is critical for pRb-mediated tumor suppression. The main focus of this review, however, is on more recent evidence demonstrating the existence of additional, cell type-specific pRb functions in cellular differentiation and survival. These additional functions are relevant to carcinogenesis suggesting that the net effect of Rb1 loss on the behavior of resulting tumors is highly dependent on biological context. The molecular mechanisms underlying pRb functions are based on the cellular proteins it interacts with and the functional consequences of those interactions. Better insight into pRb-mediated tumor suppression and clinical exploitation of pRb as a therapeutic target will require a global view of the complex, interdependent network of pocket protein complexes that function simultaneously within given tissues.

Cell cycle deregulation and loss of stem cell phenotype in the subventricular zone of TGF-beta adaptor elf-/- mouse brain

The mammalian forebrain subependyma contains neural stem cells and other proliferating progenitor cells. Recent studies have shown the importance of TGF-beta family members and their adaptor proteins in the inhibition of proliferation in the nervous system. Previously, we have demonstrated that TGF-beta induces phosphorylation and association of ELF (embryonic liver fodrin) with Smad3 and Smad4 resulting in nuclear translocation. Elf(-/-) mice manifest abnormal neuronal differentiation, with loss of neuroepithelial progenitor cell phenotype in the subventricular zone (SVZ) with dramatic marginal cell hyperplasia and loss of nestin expression. Here, we have analyzed the expression of cell cycle-associated proteins cdk4, mdm2, p21, and pRb family members in the brain of elf(-/-) mice to verify the role of elf in the regulation of neural precursor cells in the mammalian brain. Increased proliferation in SVZ cells of the mutant mice coincided with higher levels of cdk4 and mdm2 expression. A lesser degree of apoptosis was observed in the mutant mice compared to the wild-type control. Elf(-/-) embryos showed elevated levels of hyperphosphorylated forms of pRb, p130 and p107 and decreased level of p21 compared to the wild-type control. These results establish a critical role for elf in the development of a SVZ neuroepithelial stem cell phenotype and regulation of neuroepithelial cell proliferation, suggesting that a mutation in the elf locus renders the cells susceptible to a faster entry into S phase of cell cycle and resistance to senescence and apoptotic stimuli.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

A mutant allele of BARA/LIN-9 rescues the cdk4-/- phenotype by releasing the repression on E2F-regulated genes

It has been proposed that C. elegans LIN-9 functions downstream of CDK4 in a pathway that regulates cell proliferation. Here, we report that mammalian BARA/LIN-9 is a predominantly nuclear protein that inhibits cell proliferation. More importantly, we demonstrate that BARA/LIN-9 also acts downstream of cyclin D/CDK4 in mammalian cells since (i) its antiproliferative effect is partially blocked by coexpression of cyclin D1, and (ii) a mutant form that lacks the first 84 amino acids rescues several phenotypic alterations observed in mice null for cdk4. Interestingly, mutation of BARA/LIN-9 restores the expression of E2F target genes in CDK4 null MEFs, indicating that the wild-type protein plays a role in the expression of genes required for the G1/S transition.

Negative regulation of TSC1-TSC2 by mammalian D-type cyclins

The metazoan cell cycle is driven by the timely and composite activities of cyclin-dependent kinases (CDKs). Among these, cyclin D- and cyclin E-dependent kinases phosphorylate the pRb family proteins during G(1) phase of the cell cycle and thereby advance cells beyond the restriction point. Increasing evidence suggests that cyclin D-dependent kinases might affect events other than Rb pathway-mediated entry into S phase, such as accumulation of cell mass. However, little is known about cyclin D activity toward Rb-independent pathway(s) or non-pRb substrates. In this article, we show that the tumor suppressor TSC2 is a cyclin D binding protein. Coexpression of cyclin D1-CDK4/6 in cultured cells leads to increased phosphorylation and decreased detection of both TSC2 and TSC1, and promotes the phosphorylation of the mTOR substrates, 4E-BP1 and S6K1, two key effectors of cell growth that are negatively regulated by the TSC1-TSC2 complex. At the cellular level, ectopic expression of cyclin D1 restores the cell size decrease caused by TSC1-TSC2 expression. Intriguingly, down-regulation of TSC proteins was also observed by the expression of a mutant cyclin D1 that is unable to bind to CDK4/6, or by the coexpression of cyclin D1 with either an INK4 inhibitor or with catalytically inactive CDK6, indicating that cyclin D may regulate TSC1-TSC2 independently of CDK4/6. Together, these observations suggest that mammalian D-type cyclins participate in cell growth control through negative regulation of TSC1-TSC2 function.

Melanocytes in conditional Rb-/- mice are normal in vivo but exhibit proliferation and pigmentation defects in vitro

The function of the retinoblastoma tumour suppressor (Rb1), and the pocket protein family in general, has been implicated as an important focal point for deregulation in many of the molecular pathways mutated in melanoma. We have focused on the role of Rb1 in mouse melanocyte homeostasis using gene targeting and Cre/loxP mediated tissue-specific deletion. We show that constitutive Cre-mediated ablation of Rb1 exon 2 prevents the production of Rb1 and recapitulates the phenotype encountered in other Rb1 knockout mouse models. Mice with conditional melanocyte-specific ablation of Rb1 manifest overtly normal pigmentation and are bereft of melanocytic hyperproliferative defects or apoptosis-induced depigmentation. Histologically, these mice have melanocyte morphology and distribution comparable with control littermates. In contrast, Rb1-null melanocytes removed from their in vivo micro-environment and cultured in vitro display some of the characteristics associated with a transformed phenotype. They proliferate at a heightened rate when compared with control melanocytes and have a decreased requirement for mitogens. With progressive culture the cells depigment at relatively early passage and display a gross morphology which, whilst reminiscent of early passage melanocytes, is generally different to equivalent passage control cells. These results indicate that Rb1 is dispensable for in vivo melanocyte homeostasis when its ablation is targeted from the melanoblast stage onwards, however, when cultured in vitro, Rb1 loss increases melanocyte growth but the cells are not fully transformed.

Rb/E2F: a two-edged sword in the melanocytic system

Rb is a tumor suppressor that represses the expression of E2F regulated genes required for cell cycle progression. It is inactivated in melanomas and other cancer cells by phosphorylation catalyzed by persistent cyclin dependent kinase (CDK) activity. CDK activity is sustained in melanoma cells mostly by the elimination of the CDK inhibitor p16INK4A and by high levels of cyclins whose expression is maintained by stimuli emanating from activated cell surface receptors and/or mutated intracellular intermediates, such as N-Ras and B-Raf. However, Rb also suppresses the expression of apoptosis genes, and its presence protects normal melanocytes from cell death. Its high expression in human melanoma cells and tumors suggests a similar role in malignant cells as well. The differential release and suppression of E2F transcriptional activity is likely to depend on promoter-specific E2F/Rb interaction. Phosphorylated Rb is displaced from cell cycle genes but not from others. In addition, Rb gene repression is dependent on the nature of Rb-E2F interaction and the activity of the Rb-bound proteins recruited to the promoter. Deciphering the differences in Rb/E2F complex formation in normal and malignant melanocytes is likely to shed light on the mechanism by which Rb can exert tumor suppressing and promoting activities in this cellular system. The Rb/E2F pathway provides opportunities for efficient therapy at multiple levels. Novel drugs can reactivate Rb potential to suppress growth cycle promoting genes. In addition, the high E2F transcriptional activity in melanoma cells can be exploited to deliver cytotoxic molecules specifically to tumors, sparing the normal tissues.

Mitf cooperates with Rb1 and activates p21Cip1 expression to regulate cell cycle progression

The controls that enable melanoblasts and melanoma cells to proliferate are likely to be related, but so far no key regulator of cell cycle progression specific to the melanocyte lineage has been identified. The microphthalmia-associated transcription factor Mitf has a crucial but poorly defined role in melanoblast and melanocyte survival and in differentiation. Here we show that Mitf can act as a novel anti-proliferative transcription factor able to induce a G1 cell-cycle arrest that is dependent on Mitf-mediated activation of the p21(Cip1) (CDKN1A) cyclin-dependent kinase inhibitor gene. Moreover, cooperation between Mitf and the retinoblastoma protein Rb1 potentiates the ability of Mitf to activate transcription. The results indicate that Mitf-mediated activation of p21Cip1 expression and consequent hypophosphorylation of Rb1 will contribute to cell cycle exit and activation of the differentiation programme. The mutation of genes associated with melanoma, such as INK4a or BRAF that would affect either Mitf cooperation with Rb1 or Mitf stability respectively, would impair Mitf-mediated cell cycle control.

Overlapping roles of pocket proteins in the myocardium are unmasked by germ line deletion of p130 plus heart-specific deletion of Rb

The pocket protein family of tumor suppressors, and Rb specifically, have been implicated as controlling terminal differentiation in many tissues, including the heart. To establish the biological functions of Rb in the heart and overcome the early lethality caused by germ line deletion of Rb, we used a Cre/loxP system to create conditional, heart-specific Rb-deficient mice. Mice that are deficient in Rb exclusively in cardiac myocytes (CRbL/L) are born with the expected Mendelian distribution, and the adult mice displayed no change in heart size, myocyte cell cycle distribution, myocyte apoptosis, or mechanical function. Since both Rb and p130 are expressed in the adult myocardium, we created double-knockout mice (CRbL/L p130-/-) to determine it these proteins have a shared role in regulating cardiac myocyte cell cycle progression. Adult CRbL/L p130-/- mice demonstrated a threefold increase in the heart weight-to-body weight ratio and showed increased numbers of bromodeoxyuridine- and phosphorylated histone H3-positive nuclei, consistent with persistent myocyte cycling. Likewise, the combined deletion of Rb plus p130 up-regulated myocardial expression of Myc, E2F-1, and G1 cyclin-dependent kinase activities, synergistically. Thus, Rb and p130 have overlapping functional roles in vivo to suppress cell cycle activators, including Myc, and maintain quiescence in postnatal cardiac muscle.

Protein transduction technology

With the elucidation of the human genome, exhaustive analysis of genomic data related to gene transcription and the structure and function of translated protein products has progressed rapidly. Delivery of proteins and their functional domains or inhibitory peptides directly into the cell is ideal to use this protein information and analyze associated physiological functions. Protein transduction technology, which controls cell function via direct delivery of a desired protein into the cell, involves fusing the protein with a special peptide sequence consisting of 10-20 amino acids, referred to as the protein transduction domain. The recent discovery that the protein transduction domain can also be inserted into various macromolecules heightens expectations in terms of development of novel advanced experimental tools and clinical reagents.

MITF links differentiation with cell cycle arrest in melanocytes by transcriptional activation of INK4A

Cell cycle exit is required for proper differentiation in most cells and is critical for normal development, tissue homeostasis, and tumor suppression. However, the mechanisms that link cell cycle exit with differentiation remain poorly understood. Here, we show that the master melanocyte differentiation factor, microphthalmia transcription factor (MITF), regulates cell cycle exit by activating the cell cycle inhibitor INK4A, a tumor suppressor that frequently is mutated in melanomas. MITF binds the INK4A promoter, activates p16^Ink4a mRNA and protein expression, and induces retinoblastoma protein hypophosphorylation, thereby triggering cell cycle arrest. This activation of INK4A was required for efficient melanocyte differentiation. Interestingly, MITF was also required for maintaining INK4A expression in mature melanocytes, creating a selective pressure to escape growth inhibition by inactivating INK4A. These findings demonstrate that INK4A can be regulated by a differentiation factor, establish a mechanistic link between melanocyte differentiation and cell cycle exit, and potentially explain the tissue-specific tendency for INK4A mutations to occur in melanoma.