Comparison of ability of protein kinase C inhibitors to arrest cell growth and to alter cellular protein kinase C localisation

Differential effects of staurosporine analogues on cell cycle, growth and viability in A549 cells

Staurosporine is a potent but non-specific kinase inhibitor. It has served as synthetic template for a variety of analogues, the indolocarbazoles, UCN-01 and CGP 41251, and the bisindolylmaleimides, Ro 31-8220 and GF 109203X, were investigated as growth inhibitors of human-derived A549 human lung adenocarcinoma cells. They were compared with respect to (1) effect on the cell cycle, (2) time dependency of growth arrest and (3) cytotoxic potency. Cells were exposed for 1, 2 and 4 days, or for 6, 12 and 24 h in the case of cycle-synchronised cells, to staurosporine analogues at concentrations at which they inhibited growth by 80% after 4 day exposure. Staurosporine and UCN-01 retarded cells in G0/1, and CGP 41251 appeared to inhibit cell growth without cell cycle specificity. Ro 31-8220 slowed progression of synchronised cells through the cycle; over a longer time period it induced a weak block in G2/M. GF 109203X induced potent G2/M arrest in synchronised cells. This was not so apparent in asynchronous cells, which by day 4 were slowed in G0/1 instead. Growth arrest induced by these inhibitors was more potent after incubation for 4 rather than 2 days. Incubation for 1 day followed by maintenance in drug-free medium for 3 days was sufficient to exert some cytostasis. The differences between cytotoxic and cytostatic concentrations, the former measured by release from cells of lactate dehydrogenase, were 15 000-fold for staurosporine, 300-fold for UCN-01, approximately 400-fold for CGP 41251, 25-fold for Ro 31-8220 and approximately 4-fold for GF 109203X. The results show that PKC-selective staurosporine analogues differ with respect to the mechanisms by which they interfere with the cell cycle. The necessity of long-term exposure for effective growth inhibition and the considerable margin between cytostatic and acute cytotoxic indolocarbazole concentrations are findings which might influence the planning and interpretation of clinical trials of these kinase inhibitors.

Comparison of staurosporine and four analogues: their effects on growth, rhodamine 123 retention and binding to P-glycoprotein in multidrug-resistant MCF-7/Adr cells

The potent kinase inhibitor staurosporine and its protein kinase C (PKC)-selective analogue CGP 41251 are known to sensitise cells with the multidrug resistance (MDR) phenotype mediated by P-glycoprotein (P-gp) to cytotoxic agents. Here four PKC-selective staurosporine cogeners, CGP 41251, UCN-01, RO 31 8220 and GF 109203X, were compared with staurosporine in terms of their MDR-reversing properties and their susceptibility towards P-gp-mediated drug efflux from MCF-7/Adr cells. Staurosporine was the most potent and the bisindolylmaleimides RO 31 8220 and GF 109203X the least potent cytostatic agents. When compared with MCF-7 wild-type cells, MCF-7/Adr cells were resistant towards the growth-arresting properties of RO 31 8220 and UCN-01, with resistance ratios of 12.6 and 7.0 respectively. This resistance could be substantially reduced by inclusion of the P-gp inhibitor reserpine. The ratios for GF 109203X, staurosporine and CGP 41251 were 1.2, 2.0 and 2.9 respectively, and they were hardly affected by reserpine. These results suggest that RO 31 8220 and UCN-01 are avidly transported by P-gp but that the other compounds are not. Staurosporine and CGP 41251 at 10 and 20 nM, respectively, decreased efflux of the P-gp probe rhodamine 123 (R123) from MCF-7/Adr cells, whereas RO 31 8220 and GF 109203X at 640 nM were inactive. CGP 41251 was the most effective and GF 109203X the least effective inhibitor of equilibrium binding of [3H]vinblastine to its specific binding sites, probably P-gp, in MCF-7/Adr cells. Overall, the results imply that for this class of compound the structural properties that determine susceptibility towards P-gp-mediated substrate transport are complex. Comparison with ability to inhibit PKC suggests that the kinase inhibitors affect P-gp directly and not via inhibition of PKC. Among these compounds CGP 41251 was a very potent MDR-reversing agent with high affinity for P-gp and least affected by P-gp-mediated resistance, rendering it an attractive drug candidate for clinical development.

Staurosporine induces a sequential program of mouse keratinocyte terminal differentiation through activation of PKC isozymes

Staurosporine (stsp) induces assembly of cornified envelopes in mouse keratinocyte cultures. To clarify whether this effect is the consequence of a coordinated differentiation program similar to that observed in epidermis, we assessed the expression of multiple differentiation-specific markers in stsp-treated keratinocytes. In medium containing 0.05 mM Ca2+, in which the basal cell phenotype is normally maintained, stsp induced dose-dependent increases in keratin 1, epidermal and keratinocyte transglutaminases, SPR-1, loricrin, and profilaggrin mRNA. Based on nuclear run-on analysis, stsp-mediated marker expression was found to be due at least in part to increased transcription. Since protein kinase C (PKC) activation is required for keratinocyte differentiation, we tested whether stsp influenced this signaling pathway. Stsp induced the translocation of multiple PKC isoforms from the cytosol to membrane and/or cytoskeletal fractions, inducing isozyme downregulation within 24 h. Moreover, AP-1 DNA binding activity was elevated in stsp-treated keratinocytes, consistent with the notion that this agent influences keratinocyte-specific gene expression via the PKC pathway. Stsp-mediated marker expression was inhibited by the PKC inhibitor GF 109203X. In cells pre-treated with bryostatin 1 to selectively down-modulate specific PKC isoforms, stsp-induced loricrin, filaggrin, and SPR-1 expression was suppressed when PKC alpha, epsilon, and/or delta were downregulated, suggesting that these isozymes may be necessary for marker expression in response to this agent. Thus, in addition to its effects on cornified envelope assembly, stsp induces a coordinate program of differentiation-specific keratinocyte gene expression that is mediated at least in part by the PKC signaling pathway.

Inhibition of protein kinase C-dependent protein phosphorylation correlates with increased polarity and locomotion in Walker 256 carcinosarcoma cells

Signal transduction pathways controlling tumor cell locomotion are not yet well understood. We have studied the role of protein kinase C (PKC)-dependent protein phosphorylation associated with changes in cell shape and locomotor activity of Walker carcinosarcoma cells in culture. We show that the inhibitory effect of phorbol-12-myristate-13-acetate (PMA), an activator of PKC, on cell polarity and locomotion can be suppressed by the PKC-selective inhibitor Ro 31-8220. PMA induces increased phosphorylation of at least 2 proteins, of 65 and 80 kDa, in intact Walker carcinosarcoma cells. These bands are enriched in cytosolic fractions isolated from cells incubated with 32PO4. Pre-incubation with Ro 31-8220 inhibits the PMA-induced phosphorylation of both bands in a concentration-dependent manner. This effect is very likely not due to inhibition of translocation of PKC to the membrane as Ro 31-8220 enhances, rather than inhibits, PMA-induced transfer of PKC beta(II) to the particulate fraction. We have carried out a quantitative analysis of phosphorylation of the 80-kDa band. Ro 31-8220 reverses both PMA-induced phosphorylation of this band and PMA-induced suppression of cell polarity in parallel. Increased phosphorylation of proteins via PKC may thus be a stop signal for locomoting Walker carcinosarcoma cells.

Modulators of signal transduction as cancer chemotherapeutic agents--novel mechanisms and toxicities

The recent explosion of knowledge of the molecular basis of regulation of growth and differentiation has engendered the search for novel anticancer drugs which act via interference with cellular signal transduction pathways. Among the agents which are under investigation are modulators of protein kinases, for example quercetin and bryostatin 1. Both compounds have undergone phase 1 clinical evaluation. The major dose-limiting toxicities of quercetin and bryostatin are nephrotoxicity and myalgia, respectively. Novel analogs of the microbial product staurosporine, a potent kinase inhibitor, possess intriguing specificities for different kinases.