Staurosporine increases toxicity of gemcitabine in non-small cell lung cancer cells: role of protein kinase C, deoxycytidine kinase and ribonucleotide reductase.

Gemcitabine, a deoxycytidine analog, active against non-small cell lung cancer, is phosphorylated by deoxycytidine kinase (dCK) to active nucleotides. Earlier, we found increased sensitivity to gemcitabine in P-glycoprotein (SW-2R160) and multidrug resistance-associated protein (SW-2R120), overexpressing variants of the human SW1573 non-small cell lung cancer cells. This was related to increased dCK activity. As protein kinase C (PKC) is higher in 2R120 and 2R160 cells and may control the dCK activity, we investigated whether gemcitabine sensitivity was affected by the protein kinase C inhibitor, staurosporine, which also modulates the cell cycle. Ten nmol/l staurosporine enhanced the sensitivity of SW1573, 2R120 and 2R160 cells 10-fold, 50-fold and 270-fold, respectively. Staurosporine increased dCK activity about two-fold and the activity of thymidine kinase 2, which may also activate gemcitabine. Staurosporine also directly increased dCK in cell free extracts. Staurosporine decreased expression of the free transcription factor E2F and of ribonucleotide reductase (RNR), a target for gemcitabine inhibition. In conclusion, staurosporine may potentiate gemcitabine by increasing dCK and decreasing E2F and RNR, which will lead to a more pronounced RNR inhibition.

Pretreatment deoxycytidine kinase levels predict in vivo gemcitabine sensitivity.

Deoxycytidine kinase (dCK) is essential for the phosphorylation of gemcitabine (2',2'-difluorodeoxycytidine), a deoxycytidine analogue active against various solid tumors. Cytidine deaminase (CDA) catalyzes the degradation of gemcitabine. We determined whether dCK and/or CDA levels would predict response to gemcitabine. Activities of dCK and CDA were measured in a panel of eight gemcitabine-sensitive and -resistant tumors of a different origin (pancreas, lung, colon, ovary, and head and neck) grown as s.c. tumors in mice. Sensitivity to gemcitabine was expressed as treated versus control (tumor volume treated mice/control mice). Gemcitabine was given on days 0, 3, 6, and 9 (q3dx4) at its maximum tolerated dose. In addition, we measured the mRNA expression and protein levels of dCK in seven human tumor xenografts. dCK activity (mean +/- SE) ranged from 3.3+/-0.3 to 18.4+/-1.2 nmol/h/mg protein. Sensitivity to gemcitabine, expressed as treated versus control, ranged from 0.98 to 0.02, and the activity of CDA varied from 2+/-2 to 411+/-4 nmol/h/mg protein. In contrast to CDA, dCK activity was clearly related to gemcitabine sensitivity (p = -0.93; P < 0.001). This indicates that dCK might be an important prognostic marker for gemcitabine sensitivity. Protein levels were significantly related to both dCK activity (r = 0.96; P < 0.001) and gemcitabine sensitivity (rho = -0.96; P < 0.001). dCK expression as determined by competitive template reverse transcriptase PCR was significantly related with the dCK activity (r = 0.88; P = 0.025) and protein levels (p = 0.80; P = 0.052) but not with gemcitabine sensitivity, suggesting a post-translational regulation of dCK. In conclusion, the clear correlation between dCK levels and gemcitabine sensitivity in various murine tumors and human tumor xenografts may be a prognostic parameter when considering gemcitabine therapy.

Sensitivity to ionizing radiation and chemotherapeutic agents in gemcitabine-resistant human tumor cell lines.

PURPOSE: To determine cross-resistance to anti-tumor treatments in 2',2'difluorodeoxycytidine (dFdC, gemcitabine)-resistant human tumor cells. METHODS AND MATERIALS: Human lung carcinoma cells SW-1573 (SWp) were made resistant to dFdC (SWg). Sensitivity to cisplatin (cDDP), paclitaxel, 5-fluorouracil (5-FU), methotrexate (MTX), cytarabine (ara-C), and dFdC was measured by a proliferation assay. Radiosensitivity and radioenhancement by dFdC of this cell panel and the human ovarian carcinoma cell line A2780 and its dFdC-resistant variant AG6000 were determined by clonogenic assay. Bivariate flowcytometry was performed to study cell cycle changes. RESULTS: In the SWg, a complete deoxycytidine kinase (dCK) deficiency was found on mRNA and protein level. This was accompanied by a 10-fold decrease in dCK activity which resulted in the >1000-fold resistance to dFdC. Sensitivity to other anti-tumor drugs was not altered, except for ara-C (>100-fold resistance). Radiosensitivity was not altered in the dFdC-resistant cell lines SWg and AG6000. High concentrations (50-100 microM dFdC) induced radioenhancement in the dFdC-resistant cell lines similar to the radioenhancement obtained at lower concentrations (10 nM dFdC) in the parental lines. An early S-phase arrest was found in all cell lines after dFdC treatment where radioenhancement was achieved. CONCLUSIONS: In the dFdC-resistant lung tumor cell line SWg, the deficiency in dCK is related to the resistance to dFdC and ara-C. No cross-resistance was observed to other anti-tumor drugs used for the treatment in lung cancer. Sensitivity to ionizing radiation was not altered in two different dFdC-resistant cell lines. Resistance to dFdC does not eliminate the ability of dFdC to sensitize cells to radiation.

3'-Azido-2',3'-dideoxythymidine induced deficiency of thymidine kinases 1, 2 and deoxycytidine kinase in H9 T-lymphoid cells.

Continuous cultivation of T-lymphoid H9 cells in the presence of 3'-azido-2',3'-dideoxythymidine (AZT) resulted in a cell variant cross-resistant to both thymidine and deoxycytidine analogs. Cytotoxic effects of AZT, 2',3'-didehydro-3'-deoxythymidine as well as different deoxycytidine analogs such as 2',3'-dideoxycytidine, 2',2'-difluoro-2'-deoxycytidine (dFdC) and 1-ss-D-arabinofuranosylcytosine (Ara-C) were strongly reduced in H9 cells continuously exposed to AZT when compared to parental cells (>8.3-, >6.6-, >9.1-, 5 x 10(4)-, 5 x 10(3)-fold, respectively). Moreover, anti-HIV-1 effects of AZT, d4T, ddC and 2',3'-dideoxy-3'-thiacytidine (3TC) were significantly diminished (>222-, >25-, >400-, >200-fold, respectively) in AZT-resistant H9 cells. Study of cellular mechanisms responsible for cross-resistance to pyrimidine analogs in AZT-resistant H9 cells revealed decreased mRNA levels of thymidine kinase 1 (TK1) and lack of deoxycytidine kinase (dCK) mRNA expression. The loss of dCK gene expression was confirmed by western blot analysis of dCK protein as well as dCK enzyme activity assay. Moreover, enzyme activity of TK1 and TK2 was reduced in AZT-resistant cells. In order to determine whether lack of dCK affected the formation of the active triphosphate of the deoxycytidine analog dFdC, dFdCTP accumulation and retention was measured in H9 parental and AZT-resistant cells after exposure to 1 and 10 microM dFdC. Parental H9 cells accumulated about 30 and 100 pmol dFdCTP/10(6) cells after 4hr, whereas in AZT-resistant cells no dFdCTP accumulation was detected. These results demonstrate that continuous treatment of H9 cells in the presence of AZT selected for a thymidine analog resistant cell variant with cross-resistance to deoxycytidine analogs, due to deficiency in TK1, TK2, and dCK.

Detection of an alternatively spliced form of deoxycytidine kinase mRNA in the 2'-2'-difluorodeoxycytidine (gemcitabine)-resistant human ovarian cancer cell line AG6000.

Gemcitabine (2'-2'-difluorodeoxycytidine (dFdC)) is a deoxycytidine analogue that is effective against solid tumors, including lung cancer and ovarian cancer. dFdC requires the phosphorylation by deoxycytidine kinase (dCK) as a primary step in its activation. Deficiency of dCK is associated with resistance against this compound both in vitro in cancer cell lines and in clinical practice in acute myeloid leukemia and solid tumors. The human ovarian cancer cell line AG6000 is 100,000-fold resistant against dFdC compared to its parent cell line A2780. This cell line proved to be dCK deficient in enzyme activity assays and by Western blot analysis, but by RT-PCR, a normal and a truncated dCK mRNA was found. Sequencing revealed that exon 3 was deleted from the dCK cDNA, resulting in a 74-aa-long open-reading frame due to the generation of a premature stop codon. No gross genomic alteration was observed at the dCK locus, suggesting the involvement of post-transcription mechanisms. Transient transfection experiments indicated that the truncated dCK transcripts are not translated to protein. To study the functional role of the truncated dCK transcripts, both A2780 cells and AG6000 cells were stably transfected with human and rat dCK. The results indicated that over-expression of full-length dCK genes in AG6000 failed to completely reverse the sensitivity to dFdC or other drugs.

Enhanced levels of deoxycytidine kinase and thymidine kinase 1 and 2 after pulsed low dose rate irradiation as an adaptive response to radiation.

After pulsed low dose rate irradiation the activities of deoxycytidine kinase and thymidine kinase 1 and 2 were increased 1.5-2-fold 6 h after treatment. Twenty-four hours after treatment the activities of these enzymes had returned to control levels. We presume that the increase of enzyme activities is part of an adaptive response to irradiation and that this increase could be an explanation for the increased survival in the initial part of the SW-1573 cell survival curve. The observation that not only S-phase specific thymidine kinase 1 but also mitochondrial thymidine kinase 2 increases, implies that both these enzymes play a role in an adaptive response of cells to irradiation.

Dietary magnesium, not calcium, prevents vascular calcification in a mouse model for pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by ectopic calcification of connective tissue in skin, Bruch's membrane of the eye, and walls of blood vessels. PXE is caused by mutations in the ABCC6 gene, but the exact etiology is still unknown. While observations on patients suggest that high calcium intake worsens the clinical symptoms, the patient organization PXE International has published the dietary advice to increase calcium intake in combination with increased magnesium intake. To obtain more data on this controversial issue, we examined the effect of dietary calcium and magnesium in the Abcc6(-/-) mouse, a PXE mouse model which mimics the clinical features of PXE. Abcc6(-/-) mice were placed on specific diets for 3, 7, and 12 months. Disease severity was measured by quantifying calcification of blood vessels in the kidney. Raising the calcium content in the diet from 0.5% to 2% did not change disease severity. In contrast, simultaneous increase of both calcium (from 0.5% to 2.0%) and magnesium (from 0.05% to 0.2%) slowed down the calcification significantly. Our present findings that increase in dietary magnesium reduces vascular calcification in a mouse model for PXE should stimulate further studies to establish a dietary intervention for PXE.

Disruption of Abcc6 in the mouse: novel insight in the pathogenesis of pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE) is a heritable disorder of connective tissue, affecting mainly skin, eye and the cardiovascular system. PXE is characterized by dystrophic mineralization of elastic fibres. The condition is caused by loss of function mutations in ABCC6. We generated Abcc6 deficient mice (Abcc6-/-) by conventional gene targeting. As shown by light and electron microscopy Abcc6-/- mice spontaneously developed calcification of elastic fibres in blood vessel walls and in Bruch's membrane in the eye. No clear abnormalities were seen in the dermal extracellular matrix. Calcification of blood vessels was most prominent in small arteries in the cortex of the kidney, but in old mice, it occurred also in other organs and in the aorta and vena cava. Newly developed monoclonal antibodies against mouse Abcc6 localized the protein to the basolateral membranes of hepatocytes and the basal membrane in renal proximal tubules, but failed to show the protein at the pathogenic sites. Abcc6-/- mice developed a 25% reduction in plasma HDL cholesterol and an increase in plasma creatinine levels, which may be due to impaired kidney function. No changes in serum mineral balance were found. We conclude that the phenotype of the Abcc6-/- mouse shares calcification of elastic fibres with human PXE pathology, which makes this model a useful tool to further investigate the aetiology of PXE. Our data support the hypothesis that PXE is in fact a systemic disease.