Transactivation of the TPA-responsive element by the oncogenic C-erbB-2 protein is partly mediated by protein kinase C

Azatyrosine. Mechanism of action for conversion of transformed phenotype to normal

Azatyrosine [L-beta-(5-hydroxy-2-pyridyl)-alanine] has the unique property of converting ras- or c-erbB-2 transformed phenotype to normal. The administration of azatyrosine also inhibits tumor formation in transgenic mice harboring the normal human c-Ha-ras which is mutated during treatment with various chemical carcinogens. To elucidate the molecular mechanism, we investigated how azatyrosine functions and what are its major targets. Azatyrosine functions downstream of ras; azatyrosine does not alter either the level of GTP-bound Ras or the total amount of Ras. Instead, azatyrosine inhibits the activation of c-Raf-1 kinase by oncogenic c-ErbB-2, resulting in inactivation of AP1. It is interesting that azatyrosine also restores the expression of the rhoB gene, the product of which regulates the formation of actin stress fibers. Azatyrosine is incorporated into cellular proteins to replace tyrosine. Several experiments indicate that replacement of tyrosine is likely to be a cause for its conversion of transformed phenotype to normal. To prove this hypothesis, we are attempting to develop a mutant of tyrosyl-tRNA synthetase that, unlike wild type, can aminoacylate azatyrosine more efficiently than can tyrosine.

A novel zinc finger protein, Finb, is a transcriptional activator and localized in nuclear bodies

cDNAs encoding a novel DNA binding protein, termed Finb (finger protein in nuclear bodies) were molecularly cloned. The Finb protein consists of 1656 amino acids, containing 15 C2H2-type zinc fingers and three proline-rich regions. Finb efficiently activates transcription from the promoters of the metallothionein and thymidine kinase genes. The finb mRNA is ubiquitously expressed in various cell lines and tissues. The protein product is localized in nuclear bodies, whereas the N-terminally truncated protein spreads throughout the nucleus and cytoplasm. Further characterization of Finb would unravel an important role of nuclear body-involved transcriptional regulation.

Direct intratumoral gene transfer of the herpes simplex virus thymidine kinase gene with DNA-liposome complexes: growth inhibition of tumors and lack of localization in normal tissues

To constitute the site-specific expression of the herpes simplex virus thymidine-kinase (HSV-TK) gene in tumor cells, we have assessed the promoter function of the simian virus 40 (SV40) promoter and the 5'flanking region of c-erbB-2 gene using a luciferase-expressing reporter plasmid. After the transfection of the luciferase plasmid directed by the promoter region of c-erbB-2 gene, a large amount of luciferase activity was observed in c-erbB-2-expressing cells (Colo201, MCF-7, and HEC1-A), while none was detected in cells with no expression of c-erbB-2 protein (HRA and KF cells). On the other hand, a high level of luciferase activity was detected in all tumor cell lines tested, when the transfection was performed with SV40 promoter. The repeated transfection of the liposome-conjugated HSV-TK gene regulated by the SV40 promoter or by the promoter region of c-erbB-2 gene with cultivation in 100 micrograms/ml of aciclovir for 5 days in vitro resulted in growth inhibition for all four cell lines examined or for only c-erbB-2-expressing cells in the presence of SV40 promoter or c-erbB-2 promoter, respectively. Finally, direct injection of the DNA-liposome complex into established tumors in the presence of 50 mg/kg of aciclovir led to significant tumor volume reduction in all three tumors tested when SV40 promoter was employed. However, this anti-tumor effect was noted only in c-erbB-2-positive cells (Colo201 cells) upon intratumoral injection of HSV-TK gene regulated by c-erbB-2 promoter. In the case of intratumoral gene transfer, foreign DNA was detected in only one of seven mice by polymerase chain reaction (PCR) analysis performed 7 days following injection. When PCR analysis was carried out at 14 or 21 days following injection, no DNA signal was found at all. However, DNA was detected in several normal tissues at all three times tested in the case of intravenous injection. No abnormalities were seen in histologic examinations of normal tissues or in serum biochemical parameters following DNA liposome delivery. These results suggest that the direct gene transfer of HSV-TK gene regulated by tumor-specific transcriptional units may be one of the most clinically promising of the selective genetic strategies against cancer.

BE-23372M, a novel and specific inhibitor for epidermal growth factor receptor kinase

The fungal metabolite BE-23372M is a structurally novel protein kinase inhibitor. Its IC50 for epidermal growth factor (EGF) receptor kinase was 0.03 microM. IC50 values of BE-23372M for other protein tyrosine kinases, erbB-2, p43v-abl, insulin receptor kinase, and p60c-src were 0.42, 1.0, 3.3, and 4.5 microM, respectively, and the IC50 for protein kinase C, a serine/threonine kinase, was 4.1 microM. Cdc2 kinase, casein kinases I and II and cAMP-dependent protein kinase were not inhibited by 20 microM BE-23372M. A kinetic study showed that BE-23372M was competitive with respect to the substrate peptide and to ATP. Autophosphorylation of solubilized EGF receptor kinase was clearly inhibited by 0.1 microM BE-23372M. Autophosphorylation of EGF receptor in A431 cells was also inhibited. These results show that BE-23372M is a potent and specific EGF receptor kinase inhibitor. It should be a valuable tool for EGF receptor kinase research.

Therapeutic potential of protein kinase C inhibitors

The serine/threonine protein kinase, protein kinase C (PKC) is a family of closely related isoforms which are physiologically activated by diacylglycerol generated by the binding of a variety of agonists to their cellular receptors. Free fatty acids may also play a role in activating PKC. The enzyme apparently mediates a wide range of signal transduction processes in cells and, therefore, inhibitors directed selectively against PKC may have wide-ranging therapeutic potential. This review highlights the evidence that inappropriate activation of PKC occurs in a number of disease states. Such evidence, however, is often seriously flawed because it relies on the use of phorbol esters, which are potent and direct PKC activators but may not mimic the physiological triggering of the enzyme in cells, or on the use of non-selective protein kinase inhibitors such as H7 and staurosporine. A new generation of bis-indolylmaleimides, derived from the lead provided by staurosporine, shows a high degree of selectivity for PKC over closely related protein kinases and such agents may provide more appropriate tools to investigate the role of PKC in cellular processes.

Active c-erbB-2 induces short-term growth of FDC-P2 cells after IL-3 depletion

A retroviral expression vector carrying the c-erbB-2 gene was introduced into the FDC-P2 myeloid cell line, which is absolutely dependent on interleukin-3 (IL-3) for proliferation and survival. Since the c-erbB-2 protein appears to be the receptor of an as yet unidentified growth factor, epidermal growth factor receptor (EGFR) was used as a control of a ligand-dependent receptor. FDC-P2 cells expressing normal c-erbB-2 were unable to grow without IL-3 stimulation. The c-erbB-2 protein in these cells was under-phosphorylated on tyrosine residues in vivo. On the contrary, the active c-erbB-2 protein, in which Val-659 was replaced by Glu in the transmembrane domain, and EGF-stimulated EGFR showed significant levels of tyrosine phosphorylation in vivo. These active proteins could promote short-term growth of FDC-P2 cells without IL-3 stimulation, though not indefinitely. These findings suggested that immortalization of this factor-dependent cell line requires an additional oncogenic promoting process(es).