Inhibitors of the ras oncogene as therapeutic targets

Polyisoprenylated Cysteinyl Amide Inhibitors Deplete K-Ras and Induce Caspase-dependent Apoptosis in Lung Cancer Cells

BACKGROUND

Non-small cell lung cancers (NSCLC) harboring mutation-induced dysregulation of Ras signaling present some of the most difficult-to-manage cases, since directly targeting the constitutively active mutant Ras proteins has not resulted in clinically useful drugs. Therefore, modulating Ras activity for targeted treatment of cancer remains an urgent healthcare need.

OBJECTIVE

In the current study, we investigated a novel class of compounds, the polyisoprenylated cysteinyl amide inhibitors (PCAIs), for their anticancer molecular mechanisms using the NSCLC cell panel with K-Ras and/or other mutant genes.

METHODS

The effect of the PCAIs on intracellular K-Ras levels, cell viability, apoptosis, spheroid and colony formation were determined.

RESULTS

Treatment of the lung cancer cells with the PCAIs, NSL-RD-035, NSL-BA-036, NSL-BA- 040 and NSL-BA-055 resulted in concentration-dependent cell death in both K-Ras mutant (A549, NCI-H460, and NCI-H1573), N-Ras mutant (NCI-H1299) and other (NCI-H661, NCI-H1975, NCIH1563) NSCLC cells. The PCAIs at 1.0 -10 μM induced the degeneration of 3D spheroid cultures, inhibited clonogenic cell growth and induced marked apoptosis via the extrinsic pathway. The most potent of the PCAIs, NSL-BA-055, at 5 μM induced a seven-fold increase in the activity of caspase- 3/7 and a 75% selective depletion of K-Ras protein levels relative to GAPDH in A549 cells that correlated with PCAIs-induced apoptosis. NSL-BA-040 and NSL-BA-055 also induced the phosphorylation of MAP kinase (ERK 1/2).

CONCLUSION

Taken together, PCAIs may be potentially useful as targeted therapies that suppress NSCLC progression through disruption of Ras-mediated growth signaling.

Targeting mutant KRAS for anticancer therapeutics: a review of novel small molecule modulators

The RAS proteins play a role in cell differentiation, proliferation, and survival. Aberrant RAS signaling has been found to play a role in 30% of all cancers. KRAS, a key member of the RAS protein family, is an attractive cancer target, as frequent point mutations in the KRAS gene render the protein constitutively active. A number of attempts have been made to target aberrant KRAS signaling by identifying small molecule compounds that (1) are synthetic lethal to mutant KRAS, (2) block KRAS/GEF interactions, (3) inhibit downstream KRAS effectors, or (4) inhibit the post-translational processing of RAS proteins. In addition, inhibition of novel targets outside the main KRAS signaling pathway, specifically the cell cycle related kinase PLK1, has been shown have an effect in cells that harbor mutant KRAS. Herein we review the use of various high-throughput screening assays utilized to identify new small-molecule compounds capable of targeting mutant KRAS-driven cancers.

Cutaneous metastases of internal tumors

Cutaneous metastases are relatively rare in clinical practice and their diagnosis requires a high index of suspicion because clinical findings can be subtle. These metastases reveal the presence of disseminated malignant disease and can lead to the diagnosis of unsuspected internal tumors or the spread or recurrence of an already diagnosed tumor. Early recognition of cutaneous metastases can facilitate prompt and accurate diagnosis resulting in early treatment; however, they are generally indicative of a poor prognosis. Some tumors have a predilection to metastasize to specific areas. Recognition of these patterns provides essential information that can guide the search for the underlying tumor.

Targeted therapies and predictive markers in epithelial malignancies of the gastrointestinal tract

CONTEXT

In recent years, there has been a tremendous amount of interest in the development of targeted therapies for the treatment of human cancers. Increased understanding of the specific molecular pathways and driver mutations critical to cancer cell growth have allowed the development of these advanced therapeutics. Among these, inhibitors of the epidermal growth factor receptor and HER2/neu pathways now play a major role in the management of gastrointestinal cancers in addition to other solid malignancies. In colorectal and gastric cancers, the use of epidermal growth factor receptor inhibitors and HER2/neu inhibitors has increased the available treatment options for patients with advanced disease.

OBJECTIVE

To focus on the current targeted therapies and predictors of response in malignancies of the gastrointestinal tract.

DATA SOURCES

Medical literature searchable on PubMed (US National Library of Medicine) as well as older studies revealed by the literature review were used as the source of data.

CONCLUSION

Gene testing of critical elements of the pathways targeted by these agents (such as KRAS mutational analysis in colorectal tumors and HER2/neu testing in gastric cancers) allows the ability to predict which patients will respond to these treatments. As the molecular profiling of tumors and our understanding of cancer genomics and epigenetic alterations continues to grow, it is expected that these personalized targeted therapies will form one of the mainstays of gastrointestinal cancer treatment.

Polyisoprenylation potentiates the inhibition of polyisoprenylated methylated protein methyl esterase and the cell degenerative effects of sulfonyl fluorides

The polyisoprenylation pathway incorporates a reversible step that metabolizes polyisoprenylated methylated proteins from the ester to the carboxylate form. Polyisoprenylated protein methyl transferase (PPMTase) catalyses the esterification whereas polyisoprenylated methylated protein methyl esterase (PMPMEase) hydrolyzes them. Significant changes in the balance between the two enzymes may alter polyisoprenylated protein function possibly resulting in disease. Previous studies show that PMPMEase is the serine hydrolase, Sus scrofa carboxylesterase. Its susceptibility to the nonspecific serine hydrolase inhibitor, phenylmethylsulfonyl fluoride (PMSF) paved the way for its use as a prototypical compound to design and synthesize a series of putative high affinity specific inhibitors of PMPMEase. Pseudo first-order kinetics revealed an over 680-fold increase in k(obs)/[I] values from PMSF (6 M(-1)-1s(-1)), S-phenyl (L-50, 180 M(-1)s(-1)), S-benzyl (L-51, 350 M(-1)s(-1)), S-trans, trans-farnesyl (L-28, 2000 M(-1)s(-1)), to S-trans-geranylated (L-23, 4100 M(-1)s(-1)) 2-thioethanesulfonyl fluorides. C10 S-alkyl substitution revealed a k(obs)/[I] value (1800 M(-1)s(-1)) that was 298 times greater than that for PMSF. The compounds induced the degeneration of human neuroblastoma SH-SY5Y cells with EC(50) values of 49, 130 and >1000 µM for L-28, L-23 and PMSF, respectively. The increased affinity with the polyisoprenyl derivatization is consistent with the observed substrate specificity and the reported hydrophobic nature of the active site. These results suggest that (1) PMPMEase is a key enzyme for polyisoprenylated protein metabolism, (2) regulation of its activity is essential for maintaining normal cell viability, (3) abnormal activities may be involved in degenerative diseases and cancers and (4) its specific inhibitors may be useful in combating cancers.

Regulation of polyisoprenylated methylated protein methyl esterase by polyunsaturated fatty acids and prostaglandins

Polyisoprenylation is a set of secondary modifications involving proteins whose aberrant activities are implicated in cancers and degenerative disorders. The last step of the pathway involves an ester-forming polyisoprenylated protein methyl transferase- and hydrolytic polyisoprenylated methylated protein methyl esterase (PMPMEase)-catalyzed reactions. Omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) have been linked with antitumorigeneis and tumorigenesis, respectively. PUFAs are structurally similar to the polyisoprenyl groups and may interfere with polyisoprenylated protein metabolism. It was hypothesized that PUFAs may be more potent inhibitors of PMPMEase than their more polar oxidative metabolites, the prostaglandins. As such, the relative effects of PUFAs and prostaglandins on PMPMEase could explain the association between cyclooxygenase-2 (COX-2) expression in tumors, the chemopreventive effects of the non-steroidal anti-inflammatory (NSAIDs) COX-2 inhibitors and PUFAs. PUFAs such as arachidonic (AA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids inhibited PMPMEase activity with Ki values of 0.12 to 3.7 μM. The most potent prostaglandin was 63-fold less potent than AA. The PUFAs were also more effective at inducing neuroblastoma cell death at physiologically equivalent concentrations. The lost PMPMEase activity in AA-treated degenerating cells was restored by incubating the lysates with COX-1 or COX-2. PUFAs may thus be physiological regulators of cell growth and could owe these effects to PMPMEase inhibition.

Farnesyl Transferase Inhibitors for Patients with Lung Cancer

The ras family of genes have been identified as potential targets for therapeutic intervention because of somatic mutations in different human cancers. They are mutated in non-small cell lung cancer (NSCLC) approximately 20% of the time. The enzyme farnesyl transferase is involved in posttranslational modification of the ras proteins by covalently linking a farnesyl group to the ras protein. This permits the ras protein to be translocated to the surface membrane, allowing the protein to be involved in signaling for increased proliferation and inhibition of apoptosis. The class of farnesyl transferase inhibitors is designed to block farnesylation and prevent the mature ras signaling and thus inhibit cell proliferation and facilitate apoptosis. Multiple agents that inhibit farnesylation have been developed, and two farnesyl transferase inhibitors have been tested in patients with lung cancer in three Phase II trials. R115777 has been studied in patients with NSCLC and in patients with relapsed small cell lung cancer (SCLC) after chemotherapy. There has been a single trial of L-778,123 in patients with untreated NSCLC. No objective tumor responses in patients with stage IIIB/IV NSCLC were seen in these studies. There were also no objective responses among the 22 patients with relapsed SCLC treated with R115777. The median survival for the 44 patients with NSCLC treated with R115777 was approximately 8 months, whereas it was 11 months for the 23 patients treated with L-778,123. R115777 and L-778,123 were well tolerated in these studies but showed no significant activity as single-agent therapy in relapsed SCLC or untreated NSLC.

Porcine Liver Carboxylesterase Requires Polyisoprenylation for High Affinity Binding to Cysteinyl Substrates

The polyisoprenylation pathway enzymes have been the focus of numerous studies to better understand the roles of polyisoprenylated proteins in eukaryotic cells and to identify novel targets against diseases such as cancer. The final step of the pathway is a reversible reaction catalyzed by isoprenyl carboxylmethyl transferase (icmt) whose products are then hydrolyzed by polyisoprenylated methylated protein methyl esterase (PMPMEase). Unlike the other pathway enzymes, the esterase has received little attention. We recently purified PMPMEase from porcine liver using an S-polyisoprenylated cysteine methyl ester substrate-dependent screening assay. However, no data is available showing its relative interaction with structurally diverse substrates. As such, its role as the putative endogenous PMPMEase has not been demonstrated. A series of substrates with S-alkyl substituents ranging from 2 to 20 carbons, including the two moieties found in polyisoprenylated proteins, were synthesized. Enzyme kinetics analysis revealed a 33-fold increase in affinity (K(M) values) from ethyl- (C-2, 505+/-63 microM), prenyl- (C-5, 294+/-25 microM), trans-geranyl- (C-10, 87+/-12 microM), trans, trans-farnesyl- (C-15, 29+/-2.2 microM) to all trans-geranylgeranyl- (C-20-, 15+/-2.7 microM) based analogs. Comparative molecular field analysis of the data yielded a cross-validated q(2) of 0.863+/-0.365 and a final R(2) of 0.995. Since the substrates with the S-trans, trans-farnesyl and S-all trans-geranylgeranyl moieties that occur in proteins show the highest affinity towards PMPMEase and are not hydrolyzed by the cholinesterases, the results suggest that polyisoprenylated proteins are the endogenous substrates of this esterase. The results suggest design strategies for high affinity and selective inhibitors of PMPMEase.

Advances in antisense oligonucleotide development for target identification, validation, and as novel therapeutics

Antisense oligonucleotides (As-ODNs) are single stranded, synthetically prepared strands of deoxynucleotide sequences, usually 18–21 nucleotides in length, complementary to the mRNA sequence of the target gene. As-ODNs are able to selectively bind cognate mRNA sequences by sequence-specific hybridization. This results in cleavage or disablement of the mRNA and, thus, inhibits the expression of the target gene. The specificity of the As approach is based on the probability that, in the human genome, any sequence longer than a minimal number of nucleotides (nt), 13 for RNA and 17 for DNA, normally occurs only once. The potential applications of As-ODNs are numerous because mRNA is ubiquitous and is more accessible to manipulation than DNA. With the publication of the human genome sequence, it has become theoretically possible to inhibit mRNA of almost any gene by As-ODNs, in order to get a better understanding of gene function, investigate its role in disease pathology and to study novel therapeutic targets for the diseases caused by dysregulated gene expression. The conceptual simplicity, the availability of gene sequence information from the human genome, the inexpensive availability of synthetic oligonucleotides and the possibility of rational drug design makes As-ODNs powerful tools for target identification, validation and therapeutic intervention. In this review we discuss the latest developments in antisense oligonucleotide design, delivery, pharmacokinetics and potential side effects, as well as its uses in target identification and validation, and finally focus on the current developments of antisense oligonucleotides in therapeutic intervention in various diseases.

Regulation of p21/ras protein expression by diallyl sulfide in DMBA induced neoplastic changes in mouse skin

Diallyl sulfide (DAS), a naturally occurring organosulfide, present in garlic, is known to possess pleiotropic biological effects. DAS is known to inhibit chemically induced tumors in a number of animal models. The chemopreventive properties of DAS seem to occur through a number of mechanisms, but its role on primary events on oncogenic activation is not well understood. In the present study, we demonstrated the modulatory effect of DAS on the expression of H-ras gene product, p21/ras protein as one of the mechanisms of its chemopreventive action in chemically induced mouse skin tumors. Our results showed that DAS administration leads to modulation of the DMBA-induced levels of p21/ras oncoprotein as early as 24h after the DMBA application, suggesting down-regulation of the p21/ras by DAS. Furthermore, the modulatory effects of DAS were also evident in DMBA-induced mouse skin tumors. DAS administration led to increase in the levels of cytosolic p21/ras and decrease in the levels of p21/ras in membrane fractions. DAS administration was also found to down regulate the DMBA-induced H-ras mRNA level in mouse skin tumors. The immunohistochemical staining of the skin/tumor showed 55.82 and 46.86% decrease in the area positive for p21/ras expression levels in DAS pre- and post-supplemented groups, respectively. Flow-cytometric analysis, further confirms our results as indicated by a shift in the mean fluorescence intensity (MFI) towards lower fluorescence in DAS administered groups in comparison to the DMBA treated group. Thus, one mechanism of the growth inhibitory properties of DAS is through the suppression of development of tumors that harbor ras mutations by inhibiting the membrane association of oncogenic p21/ras protein.

A live cell, image-based approach to understanding the enzymology and pharmacology of 2-bromopalmitate and palmitoylation

The addition of a lipid moiety to a protein increases its hydrophobicity and subsequently its attraction to lipophilic environments like membranes. Indeed most lipid-modified proteins are localized to membranes where they associate with multiprotein signaling complexes. Acylation and prenylation are the two common categories of lipidation. The enzymology and pharmacology of prenylation are well understood but relatively very little is known about palmitoylation, the most common form of acylation. One distinguishing characteristic of palmitoylation is that it is a dynamic modification. To understand more about how palmitoylation is regulated, we fused palmitoylation substrates to fluorescent proteins and reported their subcellular distribution and trafficking. We used automated high-throughput fluorescence microscopy and a specialized computer algorithm to image and measure the fraction of palmitoylation reporter on the plasma membrane versus the cytoplasm. Using this system we determined the residence half-life of palmitate on the dipalmitoyl substrate peptide from GAP43 as well as the EC(50) for 2-bromopalmitate, a common inhibitor of palmitoylation.

Activation of the extracellular signal regulated kinase (ERK) pathway in human melanoma

BACKGROUND

Several studies suggest that melanoma may be resistant to treatment because of resistance to apoptosis and that this may be the result of activation of the extracellular signal regulated kinase (ERK1/2) pathway.

AIMS

To test this hypothesis by examining the expression of ERK1/2 and its activated form in histological sections of melanoma and its relation to known prognostic features of the disease.

MATERIALS/METHODS

Immunohistochemistry with antibodies to ERK1/2 and phosphorylated ERK (p-ERK) was performed on formalin fixed sections from 42 primary melanomas, 38 metastases, and 20 naevi. Fourteen of the primary melanomas were in the radial and 28 in the vertical growth phase.

RESULTS

ERK1/2 was widely expressed (100%) in all the (pigmented) lesions studied. p-ERK1/2 expression was much lower in compound (32.4%) and dysplastic (54.5%) naevi than in primary melanoma (nodular 78.8%, superficial spreading 67%) and subcutaneous metastases (76.3%). p-ERK expression was much lower in lymph node metastases (48.5%), suggesting that the microenvironment may influence the activation of ERK. There was a (non-significant) trend for p-ERK expression to be higher in thick (>1.0 mm) versus thin (< or =1.0 mm) melanoma (p = 0.23). There was a trend for overall survival to be related to p-ERK expression in patients with melanoma over 1 mm in thickness.

CONCLUSIONS

Expression of activated ERK1/2 in melanocytic lesions appears to be related to malignant potential so that activation of ERK1/2 may be important in melanoma progression. These results provide important histological support for the proposal that inhibition of this signalling pathway may be useful in treatment of melanoma.

K-ras as a target for cancer therapy

The central role K-, H- and N-Ras play in regulating diverse cellular pathways important for cell growth, differentiation and survival is well established. Dysregulation of Ras proteins by activating mutations, overexpression or upstream activation is common in human tumors. Of the Ras proteins, K-ras is the most frequently mutated and is therefore an attractive target for cancer therapy. The complexity of K-ras signaling presents many opportunities for therapeutic targeting. A number of different approaches aimed at abrogating K-ras activity have been explored in clinical trials. Several of the therapeutic agents tested have demonstrated clinical activity, supporting ongoing development of K-ras targeted therapies. However, many of the agents currently being evaluated have multiple targets and their antitumor effects may not be due to K-Ras inhibition. To date, no selective, specific inhibitor of K-ras is available for routine clinical use. In this review, we will summarize the structure and function of K-ras with attention to its role in tumorigenesis and discuss the successes and failures of the various strategies designed to therapeutically target this important oncogene.

Intracellular signal transduction pathway proteins as targets for cancer therapy

Circulating cytokines, hormones, and growth factors control all aspects of cell proliferation, differentiation, angiogenesis, apoptosis, and senescence. These chemical signals are propagated from the cell surface to intracellular processes via sequential kinase signaling, arranged in modules that exhibit redundancy and cross talk. This signal transduction system comprising growth factors, transmembrane receptor proteins, and cytoplasmic secondary messengers is often exploited to optimize tumor growth and metastasis in malignancies. Thus, it represents an attractive target for cancer therapy. This review will summarize current knowledge of selected intracellular signaling networks and their role in cancer therapy. The focus will be on pathways for which inhibitory agents are currently undergoing clinical testing. Original data for inclusion in this review were identified through a MEDLINE search of the literature. All papers from 1966 through March 2005 were identified by the following search terms: "signal transduction," "intracellular signaling," "kinases," "proliferation," "growth factors," and "cancer therapy." All original research and review papers related to the role of intracellular signaling in oncogenesis and therapeutic interventions relating to abnormal cell signaling were identified. This search was supplemented by a manual search of the Proceedings of the Annual Meetings of the American Association for Cancer Research, American Society of Clinical Oncology, and the American Association for Cancer Research (AARC)--European Organisation for Research and Treatment of Cancer (EORTC)--National Cancer Institute (NCI) Symposium on New Anticancer Drugs.

Substituted azoloquinolines and -quinazolines as new potent farnesyl protein transferase inhibitors

A series of (4-chlorophenyl)-alpha-(1-methyl-1H-imidazol-5-yl)azoloquinolines and -quinazolines was prepared. These compounds displayed potent Farnesyl Protein Transferase inhibitory activity and tetrazolo[1,5-a]quinazolines are promising agents for oral in vivo inhibition.