Tyrphostins that suppress the growth of human papilloma virus 16-immortalized human keratinocytes

Human papilloma virus 16 (HPV16) is considered to be the causative agent for cervical cancer, which ranks second to breast cancer in women's malignancies. In an attempt to develop drugs that inhibit the malignant transformation of HPV16-immortalized epithelial cells, we examined the effect of tyrphostins on such cells. We examined the effect of tyrphostins from four different families on the growth of HPV16-immortalized human keratinocytes (HF-1) cells. We found that they alter their cell cycle distribution, their morphology, and induce cell death by apoptosis. The effects of tyrphostins on HF-1 cells are different from their effects on normal keratinocytes. Growth suppression by AG555 and AG1478 is accompanied by 30% apoptosis in HF-1 cells, but this is not observed in normal keratinocytes. Tyrphostin treatment produces distinctive morphological changes in HF-1 cells and in normal keratinocytes; however, the culture organization of normal keratinocytes is less disrupted. These differential effects of the tyrphostins on HPV16-immortalized keratinocytes compared with their effects on normal keratinocytes suggests that these compounds are suitable candidates for the treatment of papilloma. Previous and present results indicate that group 1 tyrphostins, which inhibit Cdk2 activation, and group 2 tyrphostins, represented by AG1478, a potent epidermal growth factor receptor kinase inhibitor, induce cell cycle arrest; and, in the case of HF-1 cells, apoptosis and differentiation. Cells accumulate in the G(1) phase of the cell cycle at the expense of S and G(2) + M. These compounds block the growth of normal keratinocytes without inducing apoptosis or differentiation, causing them to accumulate in G(1). AG17, which belongs to group 4, exerts its antiproliferative effect mainly by increasing the fractions of cells in G(1) with a concomitant decrease in the fraction of cells in S and G(2) + M.

An excitatory scorpion toxin with a distinctive feature: an additional alpha helix at the C terminus and its implications for interaction with insect sodium channels

BACKGROUND

Scorpion neurotoxins, which bind and modulate sodium channels, have been divided into two groups, the alpha and beta toxins, according to their activities. The beta-toxin class includes the groups of excitatory and depressant toxins, which differ in their mode of action and are highly specific against insects. The three-dimensional structures of several alpha and beta toxins have been determined at high resolution, but no detailed 3D structure of an excitatory toxin has been presented so far.

RESULTS

The crystal structure of an anti-insect excitatory toxin from the scorpion Buthotus judaicus, Bj-xtrIT, has been determined at 2.1 A resolution and refined to an R factor of 0.209. The first 59 residues form a closely packed module, structurally similar to the conserved alpha and beta toxins ('long toxins') affecting sodium channels. The last 17 residues form a C-terminal extension not previously seen in scorpion toxins. It comprises a short alpha helix anchored to the N-terminal module by a disulfide bridge and is followed by a highly mobile stretch of seven residues, of which only four are seen in the electron-density map. This mobile peptide covers part of a conserved hydrophobic surface that is thought to be essential for interaction with the channel in several long toxins.

CONCLUSIONS

Replacement of the last seven residues by a single glycine abolishes the activity of Bj-xtrIT, strongly suggesting that these residues are intimately involved in the interaction with the channel. Taken together with the partial shielding of the conserved hydrophobic surface and the proximity of the C terminus to an adjacent surface rich in charged residues, it seems likely that the bioactive surface of Bj-xtrIT is formed by residues surrounding the C terminus. The 3D structure and a recently developed expression system for Bj-xtrIT pave the way for identifying the structural determinants involved in the bioactivity and anti-insect specificity of excitatory toxins.

Inhibition of Cdk2 activation by selected tyrphostins causes cell cycle arrest at late G1 and S phase

We have previously reported that certain tyrphostins which block EGF-R phosphorylation in cell-free systems fail to do so in intact cells. Nevertheless, we found that this family of tyrphostins inhibits both EGF- and calf serum-induced cell growth and DNA synthesis [Osherov, N.A., Gazit, C., Gilon, and Levitzki, A. (1993). Selective inhibition of the EGF and HER2/Neu receptors by Tyrphostins. J. Biol. Chem. 268, 11134-11142.] Now we show that these tyrphostins exert their inhibitory activity even when added at a time when the cells have already passed their restriction point and receptor activation is no longer necessary. AG555 and AG556 arrest 85% of the cells at late G1, whereas AG490 and AG494 cause cells to arrest at late G1 and during S phase. No arrest occurs during G2 or M phase. Further analysis revealed that these tyrphostins act by inhibiting the activation of the enzyme Cdk2 without affecting its levels or its intrinsic kinase activity. Furthermore, they do not alter the association of Cdk2 to cyclin E or cyclin A or to the inhibitory proteins p21 and p27. These compounds also have no effect on the activating phosphorylation of Cdk2 by Cdk2 activating kinase (CAK) and no effect on the catalytic domain of cdc25 phosphatase. These compounds lead to the accumulation of phosphorylated Cdk2 on tyrosine 15 which is most probably the cause for its inhibition leading to cell cycle arrest at G1/S. A structure-activity relationship study defines a very precise pharmacophore, suggesting a unique molecular target not yet identified and which is most probably involved in the regulation of the tyrosine-phosphorylated state of Cdk2. These compounds represent a new class of cell proliferation blockers whose target is Cdk2 activation.

Inhibitors of epidermal growth factor receptor kinase and of cyclin-dependent kinase 2 activation induce growth arrest, differentiation, and apoptosis of human papilloma virus 16-immortalized human keratinocytes

Human papilloma virus 16 (HPV 16) is associated with cervical cancer and is therefore considered a major health risk for women. Immortalization of keratinocytes induced by HPV infection is largely due to the binding of p53 and Rb by the the viral oncoproteins E6 and E7, respectively, and is driven to a large extent by a transforming growth factor alpha/amphiregulin epidermal growth factor receptor autocrine loop. In this study, we show that the growth of HPV 16-immortalized human keratinocytes can be blocked by a selective epidermal growth factor receptor kinase inhibitor, AG 1478, and by AG 555, a blocker of cyclin-dependent kinase 2 (Cdk2) activation. AG 1478 induces a massive increase in the Cdk2 protein inhibitors p27 and p21, whereas AG 555 appears to have a different mechanism of action, inhibiting the activation of Cdk2. Growth arrest induced by AG 1478 and AG 555 is accompanied by up to 20% of cells undergoing apoptosis. Following AG 1478 treatment but not AG 555 treatment, up to 50% of cells undergo terminal keratinocyte differentiation as determined by filaggrin expression and by the decline in the expression of cytokeratin 14. The growth-arresting properties of AG 1478 and AG 555 identifies them as possible lead antipapilloma agents.

The tryphostin AG17 induces apoptosis and inhibition of cdk2 activity in a lymphoma cell line that overexpresses bcl-2

Tyrphostins are low molecular weight compounds that specifically inhibit protein tyrosine kinases. We studied the effects of tyrphostins on OCI-Ly8, a cell line derived from a patient with immunoblastic lymphoma that carries the t(14;18) translocation and overexpresses the B-cell lymphoma/leukemia-2 gene (bcl-2). To test the possibility that tyrphostins induce apoptosis in these cells, overcoming the protection rendered by bcl-2, we screened 16 tyrphostins representing different families at a concentration of 0.5-50 microM. We found that AG17 was the most potent in this regard. Cell cycle analysis demonstrated that AG17 induces arrest at the G1 phase followed by apoptosis with general reduction of the intracellular level of tyrosine-phosphorylated proteins. To further elucidate the mechanism of action of AG17, we investigated its effect on some of the key proteins that regulate the cell cycle. Bcl-2 and cdk2 protein levels were not altered with AG17, whereas cdk2 kinase activity, as well as p21 and p16 protein levels, were reduced markedly. These results suggest that the target of AG17 is inactivation of cdk2. Because lymphoma cells with the t(14;18) translocation and bcl-2 overexpression are resistant to chemotherapy, novel drugs selectively able to induce apoptosis in these cells could offer a new approach to the treatment of lymphoma patients.

Ion binding and permeation at the GABA transporter GAT1

This study addresses the binding of ions and the permeation of substrates during function of the GABA transporter GAT1. GAT1 was expressed in Xenopus oocytes and studied electrophysiologically as well as with [3H]GABA flux; GAT1 was also expressed in mammalian cells and studied with [3H]GABA and [3H]tiagabine binding. Voltage jumps, Na+ and Cl- concentration jumps, and exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all produce capacitive charge movements. Occlusive interactions among these three types of perturbations show that they all measure the same population of charges. The concentration dependences of the charge movements reveal (1) that two Na+ ions interact with the transporter even in the absence of GABA, and (2) that Cl- facilitates the binding of Na+. Comparison between the charge movements and the transport-associated current shows that this initial Na(+)-transporter interaction limits the overall transport rate when [GABA] is saturating. However, two classes of manipulation--treatment with high-affinity uptake blockers and the W68L mutation-"lock" Na+ onto the transporter by slowing or preventing the subsequent events that release the substrates to the intracellular medium. The Na+ substitutes Li+ and Cs+ do not support charge movements, but they can permeate the transporter in an uncoupled manner. Our results (1) support the hypothesis that efficient removal of synaptic transmitter by the GABA transporter GAT1 depends on the previous binding of Na+ and Cl-, and (2) indicate the important role of the conserved putative transmembrane domain 1 in interactions with the permeant substrates.

Identification of tryptophan residues critical for the function and targeting of the gamma-aminobutyric acid transporter (subtype A)

The gamma-aminobutyric acid transporter is localized in nerve terminals. It catalyzes coupled electrogenic translocation of the neurotransmitter with two or three sodium ions and one chloride ion. The transporter contains 599 amino acids and 12 putative membrane spanning alpha-helices. It is the first described member of a neurotransmitter transporter superfamily. Using site-directed mutagenesis we have investigated the role of all 10 tryptophan residues predicted to reside in these helices. All 10 have been changed to serine as well as to leucine residues. Expression of mutant cDNAs in which the tryptophans, located in positions 68, 222, and 230, are replaced by either of these two amino acids reveals that they are severely impaired in gamma-aminobutyric acid transport. Mutants in which a phenylalanine or a tyrosine residue is introduced, at either position 68 or 230, are active. On the other hand, at the 222 position replacement of the tryptophan by the aromatic amino acids results in inactive transport. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of mutant transporters indicates that their expression levels are similar to those of the wild type. Reconstitution experiments, aimed to reveal the activity of transporter molecules not apparent in the plasma membrane, indicate that the lack of activity of the W230S transporter in intact cells is by and large due to its inefficient targeting to the plasma membrane. Tryptophan residues 68 and 222 appear to be required for the intrinsic activity of the transporter. Based on several observations, including one that tryptophan residue 222 is conserved in all amino acid transporter members of the superfamily, but not in those transporting biogenic amines, we hypothesize that the pi electrons of this tryptophan could be involved in the binding of the amino group of these neurotransmitters.

Culture of mature hippocampus slices for 4 days in a newly developed medium: preservation of transmitter release and leucine incorporation into protein

A procedure and a medium for sustaining mature hippocampus slices in vitro for 4 days are described. The ionic composition of the medium, in which the slices were incubated for 1 h of recovery following preparation, strongly affected their ability, 4 days later, to take up and to release D-[3H]aspartate and [14C]GABA. A medium deficient in Na+ and Ca2+ proved best for recovery of the fresh slices prior to transfer to culture medium. The newly developed CSF-like culture medium was the best among several media tested in maintaining the potential of the slices for uptake and for induced release of D-[3H]aspartate and [14C]GABA. Glutamine, present in most culture media, appeared to be particularly toxic. Relative to fresh slices, the slices after 4 days in culture maintained 118% and 97% of the uptake of D-[3H]aspartate and of [14C]GABA respectively. K(+)-induced release of D-[3H]aspartate and of [14C]GABA was 104% and 82% of the respective values in fresh slices. Under the optimal culture conditions worked out, the slices also regained a considerable capacity for incorporation of labelled leucine into protein, which was low in fresh slices.