Development of novel, highly potent inhibitors of V-RAF murine sarcoma viral oncogene homologue B1 (BRAF): increasing cellular potency through optimization of a distal heteroaromatic group.

We describe the design, synthesis, and optimization of a series of new inhibitors of V-RAF murine sarcoma viral oncogene homologue B1 (BRAF), a kinase whose mutant form (V600E) is implicated in several types of cancer, with a particularly high frequency in melanoma. Our previously described inhibitors with a tripartite A-B-C system (where A is a hinge binding pyrido[4,5-b]imidazolone system, B is an aryl spacer group, and C is a heteroaromatic group) were potent against purified (V600E)BRAF in vitro but were less potent in accompanying cellular assays. Substitution of different aromatic heterocycles for the phenyl based C-ring is evaluated herein as a potential means of improving the cellular potencies of these inhibitors. Substituted pyrazoles, particularly 3-tert-butyl-1-aryl-1H-pyrazoles, increase the cellular potencies without detrimental effects on the potency on isolated (V600E)BRAF. Thus, compounds have been synthesized that inhibit, with low nanomolar concentrations, (V600E)BRAF, its downstream signaling in cells [as measured by the reduction of the phosphorylation of extracellular regulated kinase (ERK)], and the proliferation of mutant BRAF-dependent cells. Concomitant benefits are good oral bioavailability and high plasma concentrations in vivo.

Catalytic and non-catalytic domains of the Fujinami sarcoma virus P130gag-fps protein-tyrosine kinase distinguished by the expression of v-fps polypeptides in Escherichia coli.

While protein-tyrosine kinases share a region of sequence identity corresponding to their kinase domains, the specific elements essential for catalysis, substrate binding and substrate specificity are largely undefined. The P130gag-fps transforming protein of Fujinami avian sarcoma virus is a cytoplasmic tyrosine kinase with a complex structure that includes a C-terminal kinase domain. To identify the precise N-terminal border of the v-fps catalytic region and to assess its interactions with non-catalytic domains, C-terminal v-fps polypeptide fragments of decreasing size were expressed in E. coli as trpE-v-fps hybrid proteins. All such polypeptides containing 263 or more residues derived from the C-terminus of P130gag-fps (i.e. residues 920-1182) were enzymatically active as tyrosine kinases. They autophosphorylated at physiological sites in vivo and phosphorylated exogenous substrates such as enolase and poly(glu,tyr) at tyrosine in vitro. Deletion of a further five amino acids from P130gag-fps residues 920-925 abolished all enzymatic activity. This deletion coincides with the predicted N-terminus of the v-fps ATP-binding site at residue 922. These data indicate that the N-terminal border of the ATP-binding site defines the start of the minimal v-fps tyrosine kinase catalytic domain, and show that this minimal domain is competent to bind substrates. More N-terminal non-catalytic sequences appear to functionally interact with the catalytic domain.

A cAMP-independent serine/threonine kinase activity is associated with the mos sequences of ts110 Moloney murine sarcoma virus-encoded P85gag-mos.

Two proteins, termed P85gag-mos and P58gag, are encoded by the temperature-sensitive transformation mutant, ts110 Moloney murine sarcoma virus (MuSV). Based on temperature-shift studies, P85gag-mos is believed to be important for the transforming potential of ts110 MuSV and has been found to be associated with a thermolabile kinase activity that phosphorylates both P85gag-mos and P58gag in immune complexes. Modifications of the original kinase assay conditions are reported here that have allowed a 30-fold increase in the specific activity of P85gag-mos phosphorylated in vitro. The in vitro P85gag-mos-phosphorylating activity was found to be unresponsive to 10 microM-cAMP or 10 microM-cGMP. Addition of 1 mM-pyrophosphate, a known phosphatase inhibitor, to the reaction mixture resulted in an increased yield of phosphorylated P85gag-mos and P58gag; the molar phosphate incorporation per mole of P85gag-mos increased from 0.032 to 0.9, whereas the specific activity of in vitro-phosphorylated P58gag increased 18-fold, from 0.013 to 0.234. pH curves of the in vitro kinase reaction further confirmed the presence of phosphatase activity; in the absence of pyrophosphate, a sharp optimum at pH 4 to 5 was observed, whereas it shifted broadly to pH 7.0 in the presence of pyrophosphate. Under the latter conditions, several experiments were performed in order to determine if the kinase was associated with either gag or mos sequences of P85gag-mos. Antisera directed against p15, p12 and p30 sequences of the gag protein region of P85gag-mos yielded immune complexes that allowed phosphorylation in vitro of P85gag-mos. No phosphorylating activity was detected in immune complexes containing MuSV-124-encoded P62gag. An anti-mos serum generated against a synthetic peptide representing the predicted v-mos amino acid residues 37 to 55 recognizes P85gag-mos and allowed phosphorylation of P85gag-mos in vitro in the absence of P58gag. Peptide mapping of both phosphorylated P85gag-mos and P58gag, by using a combination of Cleveland and Western/immunoperoxidase techniques, demonstrated that P85gag-mos became phosphorylated not only on gag sequences, but also at the N-terminal portion of v-mos. Phosphoamino acid analyses of P85gag-mos and P58gag phosphorylated in vitro under these modified conditions yielded predominantly phosphoserine and lesser amounts of phosphothreonine. Metabolically 32P-labelled P85gag-mos and P58gag were also found to contain phosphoserine and phosphothreonine. Based on these results, we conclude that a cAMP-independent, serine/threonine protein kinase activity is associated with the mos sequences of P85gag-mos.

An unusual oxygen-sensitive lactate dehydrogenase isozyme associated with Kirsten murine sarcoma virus in human serum.

An unusual isozyme of lactate dehydrogenase, lactate dehydrogenase associated with Kirsten murine sarcoma virus (LDHk) was found in the sera of many patients with malignant tumors, while the sera of healthy persons had little or no such activity. This isozyme was detectable only when assayed in a nitrogen atmosphere, and its activity showed little or no relationship to the total lactate dehydrogenase activity as measured by a standard clinical assay. The activity of serum LDHk appeared to be correlated with the presence of known metastases. Increased serum LDHk appeared in a wide variety of patients with cancer, although it appeared to be more common in certain types of cancer. Increased serum LDHk activity was also found in the sera of some patients with nonmalignant disease. The activity of serum LDHk may be useful to monitor recurrence or response to therapy in certain types of cancer.

High-level expression in Escherichia coli of enzymatically active Harvey murine sarcoma virus p21ras protein.

The gene for the Harvey murine sarcoma virus (Ha-MuSV) p21ras protein was fused to the amino-terminal portion of the bacteriophage lambda cII gene on the expression vector pJL6. The fusion was such that transcription was controlled by the well-regulated phage lambda pL promoter, and translation initiated in the cII gene continued in frame into the ras gene sequences that code for p21. When the pL promoter was derepressed, the Escherichia coli cells harboring the fusion plasmid synthesized 23,000-dalton protein, which represented more than 10 percent of the total cellular protein. This protein was chimeric and contained 14 residues, which were specified by the vector; these residues were followed by all of the amino acids that make up Ha-MuSV p21ras except for four residues at the amino-terminal end. The protein appears similar to Ha-MuSV p21ras in that it undergoes immunoprecipitation by monoclonal antibodies directed toward that protein, binds guanosine diphosphate, and is capable of autophosphorylation.

A synthetic peptide containing the autophosphorylation site of the transforming protein of Harvey sarcoma virus is phosphorylated by the EGF-stimulated tyrosine kinase.

The transforming proteins (p21) of Harvey and Kirsten sarcoma viruses threonine kinase activity, which phosphorylates threonine 59 of the p21 proteins themselves. A tridecapeptide: Arg-Arg-Leu56-Asp-Thr-Thr59-Gly-Gln-Glu-Tyr-Ser-Ala66 containing residues 56-66 of p21 is phosphorylated solely on tyrosine by the epidermal growth factor (EGF)-stimulated tyrosine kinase of A431 cell membranes. Km-Values of 240 and 80 microM and Vmax values of 1.7 and 0.1 nmol.min-1.mg-1 were obtained in the presence and absence of EGF, respectively.

LDHk, the lactate dehydrogenase associated with transformation by the Kirsten sarcoma virus: a re-evaluation.

A lactate dehydrogenase isozyme designated "LDHk" has recently been described in cells transformed by the Kirsten murine sarcoma virus. Several unusual properties were felt to distinguish this LDH isozyme from previously described LDH isozymes. These properties include cathodic electrophoretic migration in acrylamide gels using an imidazole/borate buffer system, inhibition of activity by oxygen and GTP, and stimulation of activity by cyanide. However, this report demonstrates that the muscle-type mammalian LDH isozyme, LDH 5, also exhibits these unusual properties when it is isolated and detected by the same methods previously used to demonstrate LDHk. The unusual properties attributed to LDHk are not intrinsic properties of the enzyme itself but rather result from the methods employed to demonstrate LDHk activity. Numerous similarities exist between LDH 5 and LDHk including electrophoretic mobility, substrate requirements, similar Michaelis constants for lactate, pyruvate, NAD, and NADH, and identical tissue distribution, indicating that LDHk represents mammalian LDH 5.

Comparison of complexes containing lysyl-tRNA synthetase from normal and virus-transformed cells.

We compared the lysyl-tRNA synthetases from normal (Balb/3T3) and murine sarcoma virus-transformed (KA31) mouse fibroblasts. In agreement with several other reports of mammalian systems, the lysyl-tRNA synthetases from these cells occurred in very large postmicrosomal complexes as determined by gel filtration on agarose columns. Arginyl-, isoleucyl-, methionyl-, phenylalanyl-, and tyrosyl-tRNA synthetases also occurred as part of a large complex or complexes. Activity of glycyl- or leucyl-tRNA synthetase was not detected in a complex. The specific activities of arginyl- and methionyl-tRNA synthetases were three- and five-fold higher, respectively, in a complex from KA31 as compared with a complex from Balb/3T3. In contrast, the specific activity of lysyl-tRNA synthetase from the Balb/3T3 complex was 50% higher than that of the KA31 complex. tRNALys obtained from the complexes of Balb/3T3 and KA31 was fractionated into isoacceptors on columns of RPC-5. The relative amounts of lysine isoacceptors in total preparations of tRNA from normal whole cells and in tRNA obtained from the normal enzyme complex were the same. However, two isoacceptors were present in greater amounts and two were present in lesser amounts in the KA31 enzyme complex as compared with lysine isoacceptors in a total preparation of tRNA from KA31 cells.

LDHk, a uniquely regulated cryptic lactate dehydrogenase associated with transformation by the Kirsten sarcoma virus.

A novel isozyme of lactate dehydrogenase is detected in various cells transformed by the Kirsten murine sarcoma virus (KiMSV). This isozyme, designated LDHk, is strongly inhibited by physiological concentrations of oxygen, in an apparently cooperative fashion. LDHk is inhibited by guanosine triphosphate and related compounds, in a noncompetitive fashion. LDHk is found with both 35,000- and 22,000-dalton subunits, although these probably cleave from a 57,000-dalton precursor. In studies utilizing a temperature-sensitive transforming gene mutant of the Kirsten sarcoma virus, we find in vivo expression of LDHk is also temperature-sensitive. In studies using either crude cell-free extracts or purified LDHk, we find the enzyme from cells infected with a temperature-sensitive transforming gene mutant of KiMSV is thermolabile relative to that from wild type KiMSV-infected cells.

Purified low-molecular-weight protein kinase from murine sarcoma virus particles catalyzes tyrosine phosphorylation endogenously but phosphorylates cellular proteins at serine.

The low-molecular-weight (LMW) protein kinase associated with high-titer murine sarcoma virions have been extensively purified by ammonium sulfate fractionation. Bio-Gel P-100 gel filtration, DEAE-cellulose and carboxymethyl cellulose chromatography. The purified enzyme migrates as a 16K polypeptide in polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The enzyme catalyzes phosphotransfer with ATP as a phosphate donor to various exogenously added proteins as acceptors; it requires Mg2+ and is independent of cyclic AMP. The enzyme preparation catalyzes a low level of phosphorylation in the absence of any exogenously added substrate and forms phosphotyrosine. However, in the presence of acceptor protein molecules including total soluble cytoplasmic proteins of murine sarcoma virus-transformed mouse cells, the phosphorylated end products contain predominantly phosphoserine. The virion-associated enzyme also shows a preference for phosphorylating certain polypeptides in the soluble cytoplasmic extracts of murine sarcoma virus-transformed cells.

Thermolabile protein kinase molecules in a temperature-sensitive murine sarcoma virus pseudotype.

Murine sarcoma virus-associated protein kinases that bind to actin have been purified by affinity chromatography on actin coupled to Sepharose. Heat inactivation studies showed the presence of thermolabile enzyme activity in pseudotypes containing a temperature-sensitivity mutant of murine sarcoma virus (MSV) but not in two independent wild-type MSV pseudotypes. Studies with Sephadex G-75 column fractions showed that a low molecular weight form, approximately 15,000, is the major thermolabile kinase in the temperature-sensitive MSV virions. Antibodies raised against the MSV-coded p60 protein, when added to the in vitro reaction mixtures, showed specific phosphorylation of the IgG heavy chain and a simultaneous reduction in the extent of phosvitin phosphorylation catalyzed by the various MSV pseudotype kinases. Thus a transforming retrovirus-coded enzyme activity that interacts directly with a major cytoskeletal protein and whose activity parallels the transforming ability of a conditional MSV mutant has now been identified.

A murine sarcoma virus-associated protein kinase: interaction with actin and microtubular protein.

A low molecular weight (LMW) protein phosphokinase enzyme that binds to actin has been isolated from murine sarcoma virions; this kinase activity is not present in nontransforming murine leukemia viruses. Sephadex G-75 gel filtration and affinity chromatography on actin-Sepharose conjugates allow a significant level of purification of this enzyme. The enzyme associates with microtubular proteins and inhibits the in vitro polymerization of microtubules. This study represents the first isolation of a sarcoma virus-associated protein that possesses the ability to interact directly with two major components of the cytoskeletal system.

Use of trypsin for rapid and efficient purification of murine sarcoma and leukemia virus.

A method is described for purification of MSV-MuLV from culture supernatant of chronically infected 78A,1 rat embryo cell line. This method involves direct polyethylene glycol-NaCl precipitation of the low speed supernatant of culture fluid followed by digestion of the pellet with trypsin. This procedure efficiently disrupts large aggregates which normally entrap most of the virus. Highly purified virus can be obtained in very good yield by a combination of sedimentation velocity and isopycnic centrifugation : yields up to 100 A280 units (17 mg of protein) of purified virus per liter of culture fluid can be observed. This procedure appears well suited for large scale isolation of virion associated enzymatic activities.

[Evidence of protein kinase activity in 2 murine oncornaviruses]. / Mise en évidence d'une activité protéine kinase dans 2 oncornavirus murins.

A protein kinase activity has been detected in two strains of murine Oncornaviruses, MSV/MLV and EFV. This activity phosphorylates not only endogenous viral proteins but also exogenous substrates (histones and phosvitin). The stimulation of enzyme activity by detergents along with the increase of specific activity in viruses treated with trypsin during purification suggest that the enzyme is located in the viral particle.

Poly rC-oligo dG specific template activity of mouse viral DNA polymerase detected by isoelectric focusing.

An acid fraction from polyacrylamide gel isoelectric focusing contained only DNA polymerase template activity for (rC)n-(dG)12-18 with little or no activity with (rA)n-(dT)12-18. A factor which stimulated (rC)n-(dG) 12-18 activity was also detected in low molecular weight fractions from G-100 Sephadex chromatography of the purified enzyme. Since (rC)n-(dG)12-18 acitivty is thought to be descriptive of viral DNA polymerase, this stimulation is proposed to be significant for the regulation and specificity of viral DNA synthesis.