Lipoplatin formulation review article.

Patented platform technologies have been used for the liposomal encapsulation of cisplatin (Lipoplatin) into tumor-targeted 110 nm (in diameter) nanoparticles. The molecular mechanisms, preclinical and clinical data concerning lipoplatin, are reviewed here. Lipoplatin has been successfully administered in three randomized Phase II and III clinical trials. The clinical data mainly include non-small-cell lung cancer but also pancreatic, breast, and head and neck cancers. It is anticipated that lipoplatin will replace cisplatin as well as increase its potential applications. For the first time, a platinum drug has shown superiority to cisplatin, at least in non-squamous non-small-cell lung cancer as reported in a Phase III study which documented a simultaneous lowering of all of the side effects of cisplatin.

First safety and response results of a randomized phase III study with liposomal platin in the treatment of advanced squamous cell carcinoma of the head and neck (SCCHN).

BACKGROUND: Cisplatin is one of the most active chemotherapeutic agents used in the treatment of advanced squamous cell carcinoma of the head and neck (SCCHN). However, its clinical efficacy is limited by its renal and hematotoxicity profile. In a randomized, multicenter phase III trial, we replaced conventional cisplatin by a liposomal formulation of cisplatin (lipoplatin) and compared the safety and efficacy profiles of patients in the two treatment arms. PATIENTS AND METHODS: Main inclusion criteria were: histologically confirmed SCCHN, age between 18-75 years with sufficient renal function. Main endpoints for this interims analysis were hemato- and nephrotoxicity. First response data were collected. RESULTS: Forty-six patients were evaluable for outcome and toxicity. Grade III and IV hematotoxicity were more frequent in the cisplatin arm (31.7% vs. 12%), with grade IV leucopenia occurring in 22.2%. However, 16% of the patients in that treatment arm experienced grade III anemia compared to only 9.5% treated with the cisplatin regimen. A total 4% of the patients in the lipoplatin arm developed grade IV neuropathy, whereas in the cisplatin arm, 19% developed grade III neuropathy and none developed grade IV. The renal toxicity profile of both drugs also showed marked differences. In the cisplatin arm, 23.8% of patients suffered grade III toxicity. In contrast, no grade III or IV renal toxicity occurred in patients treated with lipoplatin. The efficacy results showed 38.8% objective partial remission in the cisplatin arm vs. 19% in the lipoplatin arm. However 64% of the patients achieved stable disease while being treated with lipoplatin/5-fluorouracil (5-FU), vs. 50% in the cisplatin/5-FU arm. CONCLUSION: Liposomal cisplatin seems to reduce both the renal and hematological toxicity to a clinically relevant extent as compared to conventional cisplatin. The clinical benefit rate is similar for both regimens.

Pharmacokinetics of liposomal cisplatin (lipoplatin) in combination with 5-FU in patients with advanced head and neck cancer: first results of a phase III study.

BACKGROUND: Lipoplatin, a novel liposomal formulation of cisplatin, is composed of cisplatin and liposomes based on dipalmityl phosphatidyl glycerol (DPPG), soy phosphatidyl choline (SPC-3), cholesterol and methoxypolyethylene glycol-distearoyl phosphatidylethanolamine (mPEG2000-DSPE). Liposomal encapsulation of cisplatin is designed to increase safety and tolerability by decreasing, e.g., nephrotoxicity through decreased exposure of organs to cisplatin, while effectively delivering the drug to the tumor. In an ongoing phase III trial comparing cisplatin to lipoplatin (both in combination with infusional high-dose 5-Fluoruracil) in advanced head and neck cancer (HNC), a sub-study to determine the pharmacokinetic profile of lipoplatin in comparison to conventional cisplatin was undertaken. MATERIALS AND METHODS: In total, twelve patients with advanced HNC received a combination chemotherapy with either lipoplatin/5-FU or cisplatin/5-FU. Plasma samples were analyzed for concentration of total platinum in patients from both arms. RESULTS: All twelve patients from the pharmacokinetic sub-study were male Caucasians at a mean age of 60 years. There was no difference in age or kidney function between the two treatment groups. The total body clearance for cisplatin was 1.25 L/(hxm2) for the liposomal formulation, compared to 0.62 L/(hxm2) for conventional cisplatin. The terminal half life was half as long for lipoplatin (10.98 h) as compared to cisplatin (24.5h). Even though the maximum observed concentration in the plasma (C(max) was greater for lipoplatin than for cisplatin, the area under the concentration time-curve (AUC) was less (6.5 microg/ml vs. 4.07 microg/ml and 66.85 microg/h/ml vs. 130.33 microg/h/ml, respectively). CONCLUSION: The pharmacokinetic profile of lipoplatin (in combination with 5-FU) suggests that the liposomal formulation results in a greater body clearance and shorter half life than conventional cisplatin, which confirms the clinical observation of decreased taxicity, especially renal deterioration.

Immunogene therapy of recurrent glioblastoma multiforme with a liposomally encapsulated replication-incompetent Semliki forest virus vector carrying the human interleukin-12 gene--a phase I/II clinical protocol.

Glioblastoma multiforme (GBM) is an incurable brain tumor resistant to standard treatment modalities such as surgery, radiation, and chemotherapy. Since recurrent GBM tends to develop predominantly within the infiltrative rim surrounding the primary tumor focus, novel therapy strategies need in addition to focal tumor destruction to target this somewhat diffuse area. This is a phase I/II clinical study in adult patients with recurrent GBM which is aimed at evaluating biological safety, maximum tolerated dose, and antitumor efficacy of a genetically modified replication-disabled Semliki forest virus vector (SFV) carrying the human interleukin 12 (IL-12) gene and encapsulated in cationic liposomes (LSFV-IL12). The vector will be administered in doses of 1 x 10(7)-1 x 10(9) infectious particles by continuous intratumoral infusion, thus exploiting the advantages of convection-enhanced drug delivery in the brain. The present protocol is also designed to investigate systemic and local immune response and to identify factors predicting tumor response to LSFV-IL12 therapy, such as volume of extracellular space of the tumor, volume of contrast enhancing lesion, and immune status of the patients. SFV, an insect alphavirus, infects mitotic and non-mitotic cells and triggers apoptosis in tumor cells within 48-72 h. Preclinical work with the LSFV-IL12 vector in breast and prostate cancer animal models demonstrated its biosafety and some antitumor efficacy. An ongoing phase I clinical study in patients with melanoma and renal cell carcinoma seems also to confirm the biosafety of intravenously administered vectors. This protocol will be the first study of SFV-IL12 therapy of human recurrent GBM.

High affinity MAR-DNA binding is a common property of murine and human mutant p53.

We recently reported that murine MethA mutant but not wild-type p53 specifically binds to MAR-DNA elements (MARs) with high affinity. Here we show that this DNA binding activity is exerted not only by MethA mutant p53 but also by other murine mutant p53 proteins isolated from the transformed murine BALB/c cell lines 3T3tx and T3T3 and differing in their conformational status. High affinity MAR-DNA binding was not restricted to the Xbal-IgE-MAR-DNA fragment from the murine immunoglobulin heavy chain gene enhancer locus [Cockerill et al. (1987): J Biol Chem 262:5394-5397] used in previous studies, as MethA p53 also specifically interacted with other A/T-rich bona fide MARs. Not only murine but also human mutant p53 proteins carrying the mutational hot spot amino acid exchanges 175Arg-->His, 273Arg-->Pro, or 273Arg-->His bound to the Xbal-IgE-MAR-DNA fragment. We therefore conclude that high affinity MAR-DNA binding is a property common to a variety of mutant p53 proteins.

Gene therapy of prostate cancer: p53, suicidal genes, and other targets.

Prostate tumor initiation and progression to malignancy may involve upregulation of the androgen receptor known to stimulate prostate cell proliferation; other etiologic mechanisms may include dysfunction of the apoptotic pathway but also deregulation in signal transduction and control of the cell cycle in prostate tissue; such abnormalities could arise from overexpression or mutations in a number of oncogenes or down-regulation by inactivating mutations, allelic loss, or other epigenetic mechanisms in tumor suppressor genes. The advantages and drawbacks of various delivery systems (retroviral, adenoviral, liposomes) used for human gene therapy are being considered. Several ex vivo and in vivo as well as cell culture studies are suggested for the therapy of the human prostate cancer using transfer and expression of genes that might be implicated in prostate carcinogenesis especially of the tumor suppressor p53. Expression of suicidal genes in prostate cancer cells using prostate-specific promoter and enhancer elements as well as targeting of the androgen receptor or the insulin-like growth factor genes with triplex technology in prostate cancer cells and their metastases, is expected to be of therapeutic value.

Nuclear import of DNA repair proteins.

DNA repair enzymes play a pivotal role in the maintenance of chromosome integrity and in the elimination of premutagenic lesions from DNA by patrolling the genome; nuclear import mechanisms are implicated in molecular carcinogenesis. We have attempted to predict cell trafficking and the nuclear importation of proteins involved in DNA repair by sequence analysis aimed at identifying karyophilic clusters (arginines, lysines, histidines) flanked by the helix breakers proline or glycine that could function as nuclear localization signals (NLSs). Most mammalian proteins that participate in DNA repair pathways seem to possess NLS peptides. Repair proteins with multiple nuclear signals are the ERCC6 helicase (eight signals), the XPC protein involved in the repair of the transcribed strand in active genes (eight strong and seven weak signals), and the Rep-3/Duc-1 mismatch repair protein (five strong one weak signal). We propose that it is unlikely to identify mutations on the genes encoding these proteins resulting in cytoplalsmic retention. However, a number of mammalian DNA repair proteins lack NLS clusters; these proteins include ERCC1, ERCC2 (XPD), mouse RAD51, and the HHR23B/p58 and HHR23A subunits of XPC. NLS-less S. cerevisiae proteins include both RAD51 and RAD52 that function in the recombination and in the repair of double-strand breaks as well as the RAD23 and HRR25 molecules. We propose that these proteins depend on their complexation with other proteins in the cytoplasm for their nuclear localization. The hMSH2 human mismatch repair protein linked to the hereditary nonpolyposis colon cancer gene, has a weak nuclear signal containing two histidines.

Nuclear localization signal peptides for the import of plasmid DNA in gene therapy (review).

Failure of cells to import specific proteins into nuclei can lead to carcinogenesis. Trafficking of nuclear proteins from the site of their synthesis in the cytoplasm to the sites of function in the nucleus through pore complexes is mediated by the presence of nuclear localization signals (NLSs) on proteins to be imported into nuclei. mRNA is exported through the same route as a complex with nuclear proteins. During gene therapy the foreign DNA needs to enter nuclei for its transcription in order to express the therapeutic protein; a pathway is proposed involving the complexation of plasmids and oligonucleotides with nascent nuclear proteins possessing NLSs as a prerequisite for their nuclear import. Covalent linkage of NLS peptides to oligonucleotides and plasmids or formation of complexes of plasmids with proteins possessing multiple NLS peptides is proposed to increase their import rates and the efficiency of gene expression. We suggest that cancer cells import more efficiently foreign DNA into nuclei compared with terminally differentiated cells because of their increased rates of proliferation and protein import and are easier targets for expression of foreign genes.

Histones, protamine, and polylysine but not poly(E:K) enhance transfection efficiency.

Liposomal gene delivery has a great potential for the treatment of cancer and other human diseases. In this work we have investigated the optimal conditions for liposome-mediated transfer of the luciferase gene to human erythroleukemia K562 cells. DDAB:DOPE liposomes were more efficient than lysyl-DOPE:DOPE (1:2) and DDAB:cholesterol for transfection. Total histones from bovine thymus, salmon sperm protamine, and polylysine at an optimal ratio of 0.5 mg protein/mg DNA enhanced up to 7-fold the transfection efficiency of luciferase plasmids; on the contrary, the synthetic polymer poly(E:K), containing glutamic acid and lysine residues in a random order at a ratio 1:4, diminished luciferase expression. Transfection was nearly zero at high histone:DNA ratio by reversal of the charge of the particle from negative to positive leading:to its inability to interact with cationic liposomes. The increase in luciferase gene expression by DNA-binding proteins might arise from an increased transfer across the cell membrane of the liposome-DNA-protein complex but also by an increase in nuclear import of the DNA-protein complex because of the presence of nuclear localization signals on the protein molecule used for DNA condensation.

Xeroderma pigmentosum and molecular cloning of DNA repair genes.

Human cells from patients suffering with xeroderma pigmentosum (XP) characterized by extreme sensitivity to UV light and a high incidence of skin tumors fall into seven complementation groups, XPA to XPG, and are lacking a functional helicase, endonuclease, or lesion-recognizing protein involved in the initial steps during nucleotide excision repair (NER); a number of proteins involved in DNA repair are termed XPA to XPG depending on which one is defective in a particular complementation group of XP and include: (i) proteins involved in the recognition of (6-4) photoproducts (XPE) and of a broad range of lesions such as pyrimidine dimers (XPA); (ii) proteins that are DNA helicases and integral parts of the general transcription factor TFIIH functioning in both transcription and repair (XPB, XPD); (iii) endonucleases that perform the two incisions, the XPG incising six nucleotides (nt) to the 3' side from a photodimer and the ERCC1-XPF protein complex incising 22 nt to the 5' side of the lesion; and (iv) single-strand DNA-binding proteins (XPC). The ERCC6 helicase is largely responsible for coupling transcription to repair whereas XPC seems to be responsible for the repair of the inactive parts of the genome as well as for the repair of the nontranscribed strand in active genes. p53 recognizes insertion/deletion mismatches as well as free ends of DNA produced by ionizing radiation to arrest the cell cycle. Most of the human DNA repair proteins have their counterparts in both budding and fission yeasts and some of them also in E. coli evoking an evolutionary conservation of DNA repair pathways. Accumulation of mutations within repair genes in single cells followed by their escape from the immune surveillance and in clonal expansion may greatly contribute to the appearance and development of human cancers.

Common structural features of replication origins in all life forms.

Origins of replication (ORIs) among prokaryotes, viruses, and multicellular organisms appear to possess simple tri-, tetra-, or higher dispersed repetitions of nucleotides, AT tracts, inverted repeats, one to four binding sites of an initiator protein, intrinsically curved DNA, DNase I-hypersensitive sites, a distinct pattern of DNA methylation, and binding sites for transcription factors. Eukaryotic ORIs are sequestered on the nuclear matrix; this attachment is supposed to facilitate execution of their activation/deactivation programs during development. Furthermore, ORIs fall into various classes with respect to their sequence complexity: those enriched in AT tracts, those with GA- and CT-rich tracts, a smaller class of GC-rich ORIs, and a major class composed of mixed motifs yet containing distinct AT and polypurine or GC stretches. Multimers of an initiator protein in prokaryotes and viruses that might have evolved into a multiprotein replication initiation complex in multicellular organisms bind to the core ORI, causing a structural distortion to the DNA which is transferred to the AT tract flanking the initiator protein site; single-stranded DNA-binding proteins then interact with the melted AT tract as well as with the DNA polymerase alpha-primase complex in animal viruses and mammalian cells, causing initiation in DNA replication. ORIs in mammalian cells seem to colocalize with matrix-attached regions and are proposed to become DNase I-hypersensitive during their activation.

Nuclear import of protein kinases and cyclins.

Karyophilic and acidic clusters were found in most nonmembrane serine/threonine protein kinases whose primary structure was examined. These karyophilic clusters might mediate the anchoring of the kinase molecules to transporter proteins for their regulated nuclear import and might constitute the nuclear localization signals (NLS) of the kinase molecules. In contrast to protein transcription factors that are exclusively nuclear possessing strong karyophilic peptides composed of at least four arginines (R) and lysines (K) within an hexapeptide flanked by proline and glycine helix-breakers, protein kinases often contain one histidine and three K+R residues; this is proposed to specify a weak NLS structure resulting in the nuclear import of a fraction of the total cytoplasmic kinase molecules as well as in their weak retention in the different ionic strength nuclear environment. Putative NLS peptides in protein kinases may also contain hydrophobic or bulky aromatic amino acids proposed to further diminish their capacity to act as strong NLS. Most kinases lacking karyophilic clusters (c-Mos, v-Mos, sea star MAP, and yeast KIN28, SRA1, SRA3, TPK1, TPK2) also lack acidic clusters, which is in contrast to most kinases containing both acidic and karyophilic peptides; this and the presence of R/K clusters in the transporter proteins supports a role of acidic clusters on kinases in nuclear import. Cyclins B lack karyophilic signals and are proposed to be imported into nuclei via their association with Cdc2.

DNA lesion-recognizing proteins and the p53 connection.

A great deal of the energy and time of a cell is invested in DNA repair activities. The first step in DNA repair pathways is recognition of the lesion on the DNA. The classical lesion-recognizing proteins interact with other repair proteins to form multiprotein complexes most notable of which are those that function in Nucleotide Excision Repair (NER). Proteins involved in lesion recognition include HMG1 and 2 recognizing cisplatin adducts but also maintaining active nucleosome structures and interacting with loops in cruciforms; HMG-box nuclear proteins; XPA and XPC lacking in xeroderma pigmentosum patients and involved in lesion recognition during NER; p53 recognizing strand breaks and insertion/deletion mismatches and causing arrest in the cell cycle; MSH2 mismatch repair protein identified as the human colon cancer gene product; and others including the transcription factor YB-1 that binds to depurinated DNA with a higher affinity compared with undamaged DNA. Other type of lesion-recognizing proteins are also repair enzymes like the O(6)-methylguanine-DNA methyltransferase and DNA glycosylases. Lesion recognition is an important process and might be the rate-limiting step in the overall repair process.

Cancer gene therapy and immunotherapy (review).

Gene therapy is a newly hatched field of biomedical research aimed at introducing therapeutically important genes into somatic cells of patients for the treatment of human disease. Whereas for inborn errors of metabolism transfer of a single gene can correct the disorder, cancer is a complex disease involving mutations in a number of protooncogenes and tumor suppressor genes as well as an imbalance and disarray in phosphorylation events and regulatory circuits of the cell cycle; transfer of the wild-type p53 or p21 tumor suppressor genes is a successful gene therapy approach leading to apoptotic death of cancer cells or in restrain of their chaotic growth. A different promising approach is transfer of the herpes simplex virus thymidine kinase (HSV-tk) gene (suicide gene) and systemic treatment with the prodrug ganciclovir which is converted by HSV-tk into a toxic drug killing dividing cells. Expression of suicide genes, p53, and other therapeutic genes preferentially in cancer cells can be achieved by regulatory elements from tumor-specific genes such as carcinoembryonic antigen, BRCA1, and PSA.

Gene therapy to human diseases.

The identification of defective genes associated with a number of human disorders (tyrosine hydroxylase for Parkinson's disease, aspartylglucosaminidase in lysosomal storage disease, CFTR in cystic fibrosis, and LDL receptor in familial hypercholesterolemia) has promoted the development of strategies aimed at transferring to the somatic cells of the patient or of animal models vectors carrying the corrected gene. The obstacles to overcome include targeting the specific cell type or organ (liver for Factors VIII and IX in hemophilia), enhancing entry to cells into non-lysosomal compartments, nuclear import, percentage of cells transduced with the therapeutic gene, sustained expression of the transgene in human tissues, and immunogenicity of the transduced cells expressing the recombinant or viral proteins. Improvements in each single of these steps are likely to enhance enormously the potential of gene transfer for the treatment of human diseases. A number of human diseases including HIV infections and hypertension are approached by somatic gene transfer. VEGF regulating vascular permeability, growth of endothelial cells and angiogenesis, and TGF-B implicated in wound healing and in stimulation in synthesis of extracellular matrix, are potential targets for restenosis, atherosclerosis, and cancer.

The 3' untranslated region of the human poly(ADP-ribose) polymerase gene is a nuclear matrix anchoring site.

The nuclear matrix displays the most dramatic changes among all cellular structures during carcinogenesis. Matrix attachment regions (MARs) organize chromatin into domains, harbor origins of replication and display a notable transcriptional enhancer activity. To understand the nature of MARs and their involvement in gene expression, replication, and carcinogenesis, we have cloned the MAR DNA fragments, of a size of 0.1-5.0 kb, isolated from human cells in culture. Over 150 clones have been sequenced. One MAR clone was identified as a stretch of 393 bp from the 3' untranslated region (3' UTR) of the human poly(ADP-ribose) polymerase (PARR) gene (100% homology). The 393 bp MAR fragment contains several repeats of TTGTTTTGT and related sequences (the TG boxes) and motifs with similarity to the binding site of the general yeast transcription factor GFI and to the ARS origins of replication in yeast. In addition, the 3' UTR of the PARP gene harbors MAR-type sequences found in other genes, kinked and curved DNA, two imperfect inverted repeats, and short alternating GA- and CT-rich motifs. The presence of TG-, GA-, and CT-rich motifs and of potential cruciforms is proposed to identify a novel type of MAR sequence. This report suggests that MAR sequences may reside in the 3' untranslated region of other genes and has important implications for a potential role of the nuclear matrix in transcription termination.

A unified model explaining the preferential repair of active over inactive genes and of the transcribed over the nontranscribed strand.

DNA lesions are removed more efficiently and at higher rates from active than from inactive genes and from the transcribed compared with the nontranscribed strand. A unifying model is presented explaining the heterogeneity in DNA repair activities through the genome in terms of chromatin structure, nuclear matrix anchorage, transcription factor binding, protein phosphorylation mechanisms, and the linkage of repair mechanisms to other nuclear functions including transcription and replication. Transcription factors, preferentially assembled into complexes on the regulatory regions of active but not of inactive genes are proposed to contribute to repair differences.

The non-uniform repair of active and inactive chromatin domains (review).

Substantial evidence demonstrates that DNA repair processes are not distributed homogeneously throughout the genome but that lesions are removed more efficiently and at higher rates from active than from inactive genes. Transcription-coupled repair (TCR) appears to be a sophisticated subpathway of nucleotide excision repair (NER) that targets the repair machinery to lesions present in the actively transcribed strand; TCR could explain the preferential repair of the transcribed over the nontranscribed strand in active genes and involves ERCC2 and ERCC3 helicases in human cells, integral components of TFIIH, as well as ERCCG. XPC protein is responsible for the repair of inactive regions. In S. cerevisiae RAD16 helicase as well as RAD9 and RAD24 proteins are involved in repairing nontranscribing regions whereas RAD3 and SSL2 helicases, homologs of the human ERCC2 and ERCC3, function for the repair of active regions. Unraveling the mechanisms that govern the heterogeneous repair rates among active and inactive parts of the genome is important in understanding mechanisms of carcinogenesis.

Liposome DNA delivery and uptake by cells (review).

Human gene therapy is a rapidly emerging field; therapeutic genes are engineered into adenovirus, retrovirus, or into plasmid-liposome complexes for their delivery into cells in culture or in vivo. Steps to improve for successful liposome-mediated gene delivery to somatic cells include persistence of the plasmid in blood circulation, port of entry and transport across the cell membrane, release from endosomal compartments into the cytoplasm, nuclear import by docking through the pore complexes of the nuclear envelope, expression driven by the appropriate promoter/enhancer control elements, and persistence of the plasmid in the nucleus for long periods. A number of strategies for enhancing the efficiency of uptake by the cells and release from endosomal compartments of liposome-plasmid or liposome-oligonucleotide complexes are reviewed here; emphasis is given to the direction of liposomes to caveolae vesicles.

Phosphorylation of transcription factors and control of the cell cycle.

Protein phosphorylation has evolved as the most versatile posttranslational modification widely used by cells. Signal transduction pathways mediated by activation of MAP kinases and protein kinase C trigger the exit of cells from the quiscence (Go-->G1 transition). Indeed, binding of growth factors at the cell surface triggers their receptors, usually possessing a tyrosine kinase on the cytoplasmic side, to phosphorylate other molecules passing on the information sequentially to GRB2 protein, to p21ras, to c-Raf-1, to MAP kinase kinase, to MAP kinase, to p90rsk, to transcription factors. Activated PKC, MAP kinase, and pp90src can translocate to the nucleus where they phosphorylate a number of protein transcription regulators in a cell cycle-dependent manner or in response to cell stimulation for exit from quiescence. The cell cycle is mainly regulated by p34cdc2 or otherwise called cdc2 in association with cyclins B at G2/M and by Cdk2 in association with cyclins A, D1, and E at G1/S checkpoints; phosphorylation of histone H1 and lamins by cdc2 triggers chromosome assembly and nuclear envelope breakdown, respectively, as a prelude to mitosis. Cdc2 activities functioning as a G2/M regulator are controlled by its phosphorylation and dephosphorylation at Ser/Thr residues. MAP kinases might be the missing link in the chain connecting the Go to G1 transition with the cell cycle regulation, whereas phosphorylation of replication protein factors, retinoblastoma, and p53 might link the G1 to S transition with the control of DNA synthesis. A number of transcription factors are known to stimulate DNA replication, including p53, c-Myc, AP-1, Oct-1, T-antigen; the DNA binding activities of all these proteins and their interaction with other transcription factors are controlled by phosphorylation. The nuclear import of several proteins including NF kappa B, Dorsal, glucocorticoid receptor, ISGF3, rNFIL-6, T antigen, and the kinases PKC, MAP, and p90rsk, are dependent on their phosphorylation at specific sites. Histone phosphorylation stimulated at discrete stages of the cell cycle or in response to cAMP or other stimuli might induce profound changes in chromatin organization.