The effects of telomere shortening on cancer cells: a network model of proteomic and microRNA analysis.

Previously, we have shown that shortening of telomeres by telomerase inhibition sensitized cancer cells to cisplatinum, slowed their migration, increased DNA damage and impaired DNA repair. The mechanism behind these effects is not fully characterized. Its clarification could facilitate novel therapeutics development and may obviate the time consuming process of telomere shortening achieved by telomerase inhibition. Here we aimed to decipher the microRNA and proteomic profiling of cancer cells with shortened telomeres and identify the key mediators in telomere shortening-induced damage to those cells. Of 870 identified proteins, 98 were differentially expressed in shortened-telomere cells. 47 microRNAs were differentially expressed in these cells; some are implicated in growth arrest or act as oncogene repressors. The obtained data was used for a network construction, which provided us with nodal candidates that may mediate the shortened-telomere dependent features. These proteins' expression was experimentally validated, supporting their potential central role in this system.

Differential downregulation of telomerase activity by bortezomib in multiple myeloma cells-multiple regulatory pathways in vitro and ex vivo.

BACKGROUND: The importance of telomerase in multiple myeloma (MM) is well established; however, its response to bortezomib has not been addressed. METHODS: The effect of bortezomib on telomerase activity and cell proliferation was evaluated in four MM cell lines and in myeloma cells obtained from eight patients. The mechanism of telomerase regulation on epigenetic, transcriptional, and post-translational levels was further assessed in two selected cell lines: ARP-1 and CAG. Clinical data were correlated with the laboratory findings. RESULTS: Bortezomib downregulated telomerase activity and decreased proliferation in all cell lines and cells obtained from patients, albeit in two different patterns of kinetics. ARP-1 cells demonstrated higher and earlier sensitivity than CAG cells due to differential phosphorylation of hTERT by PKCα. Methylation of hTERT promoter was not affected. Transcription of hTERT was similarly inhibited in both lines by decreased binding of SP-1 and not of C-Myc and NFκB. The ex vivo results confirmed the in vitro findings and suggested existence of clinical relevance. CONCLUSION: Bortezomib downregulates telomerase activity in MM cells both transcriptionally and post-translationally. MM cells, both in vitro and in patients, exhibit different sensitivity to the drug due to different post-translational response. The effect of bortezomib on telomerase activity may correlate with resistance to bortezomib in patients, suggesting its potential utility as a pre-treatment assessment.

Combined antiproliferative activity of imatinib mesylate (STI-571) with radiation or cisplatin in vitro.

UNLABELLED: Little is known about the interaction of novel anticancer drugs with other treatment modalities. THE AIM of this study was to examine the effect of combining imatinib mesylate (STI-571) with radiation or cisplatin on the survival of two human solid tumor cell lines - SKNMC cells derived from Ewing sarcoma and breast cancer MCF-7 cells. METHODS: Cell proliferation was determined using the sulphorodamine B cytotoxicity assay. Cell cycle analysis was performed with flow cytometry. Apoptosis was determined using a commercial cell death ELISA plus kit. Phosphorylated AKT, which has been suggested to be involved in radiation resistance, was detected by Western blot analysis. RESULTS: Exposure of SKNMC cells to STI-571 resulted in a dose-dependent antiproliferative effect and a decrease in phosphorylated AKT expression. There was no evidence of apoptosis. The combination of STI-571 with radiation or cisplatin had an additive antiproliferative effect in SKNMC cells (60% reduction in cell number). A similar effect was observed in human MCF-7 breast cancer cells. CONCLUSION: STI-571 improves the outcome of cisplatin or irradiation treatment in vitro. AKT pathway may play a role in the additive effect of STI-571 and irradiation.

Effects of natural and novel synthetic jasmonates in experimental metastatic melanoma.

BACKGROUND AND PURPOSE: No current treatment reliably affects the course of metastatic melanoma. Consequently, novel approaches to the control of metastasis are actively sought. The overall goal of the present study was to identify new anti-metastatic agents active against melanoma cells. EXPERIMENTAL APPROACH: Two directions were taken: 1. To determine whether the natural plant hormone methyl jasmonate, which kills cancer cells selectively, can suppress the characteristic metastatic behavior of B16-F10 melanoma cells; 2. To synthesize and identify novel jasmonate derivatives with better cytotoxic and anti-metastatic activities than methyl jasmonate. KEY RESULTS: We found that methyl jasmonate suppressed B16-F10 cell motility and inhibited the development of experimental lung metastases of these cells. Furthermore, methyl jasmonate suppressed the motility of a sub-clone of these cells over-expressing P-glycoprotein and exhibiting multidrug resistance. The synthetic derivative Compound I (5,7,9,10-tetrabromo derivative of methyl jasmonate, the most active derivative) had greater cytotoxic potency (IC(50), 0.04 mM) than methyl jasmonate (IC(50), 2.6mM). Compound I prevented B16-F10 cell adhesion efficiently and inhibited the development of lung metastases at a much lower dose than methyl jasmonate. CONCLUSIONS AND IMPLICATIONS: Natural and synthetic jasmonates have anti-metastatic actions. Further development of these agents for the suppression of metastasis in melanoma and other types of cancer is warranted.

Imatinib mesylate (Gleevec) downregulates telomerase activity and inhibits proliferation in telomerase-expressing cell lines.

Imatinib mesylate (IM) is a tyrosine kinase inhibitor, which inhibits phosphorylation of downstream proteins involved in BCR-ABL signal transduction. It has proved beneficial in treating patients with chronic myeloid leukaemia (CML). In addition, IM demonstrates activity against malignant cells expressing c-kit and platelet-derived growth factor receptor (PDGF-R). The activity of IM in the blastic crisis of CML and against various myeloma cell lines suggests that this drug may also target other cellular components. In the light of the important role of telomerase in malignant transformation, we evaluated the effect of IM on telomerase activity (TA) and regulation in various malignant cell lines. Imatinib mesylate caused a dose-dependent inhibition of TA (up to 90% at a concentration of 15 microM IM) in c-kit-expressing SK-N-MC (Ewing sarcoma), SK-MEL-28 (melanoma), RPMI 8226 (myeloma), MCF-7 (breast cancer) and HSC 536/N (Fanconi anaemia) cells as well as in ba/F3 (murine pro-B cells), which do not express c-kit, BCR-ABL or PDGF-R. Imatinib mesylate did not affect the activity of other DNA polymerases. Inhibition of TA was associated with 50% inhibition of proliferation. The inhibition of proliferation was associated with a decrease in the S-phase of the cell cycle and an accumulation of cells in the G2/M phase. No apoptosis was observed. Inhibition of TA was caused mainly by post-translational modifications: dephosphorylation of AKT and, to a smaller extent, by early downregulation of hTERT (the catalytic subunit of the enzyme) transcription. Other steps of telomerase regulation were not affected by IM. This study demonstrates an additional cellular target of IM, not necessarily mediated via known tyrosine kinases, that causes inhibition of TA and cell proliferation.

Altered subcellular localization of p53 in estrogen-dependent and estrogen-independent breast cancer cells.

LCC2, an estradiol-independent tamoxifen (Tax)-resistant subline of MCF-7 human breast cancer cell line, is resistant relatively towards Tax and methotrexate (Mtx). The purpose of the present study is to evaluate the role of p53 in determining this resistance. While MCF-7 is sensitive to and undergoes apoptosis, as determined by propidium iodide stain, by Tax and Mtx, LCC2 is resistant to apoptosis induction by these agents. Both cell lines undergo apoptosis and are sensitive equally to doxorubicin (Adr). p53 cDNA of both sublines was evaluated by polymerase chain reaction (PCR) amplification and sequencing and was found to be of wild-type. p53 mRNA, as well as protein, are elevated markedly in LCC2 as compared to MCF-7 cells. p53 expression was increased by estradiol and Adr, not changed by Mtx, and decreased by Tax and estradiol-deprivation in both sublines. p53 modulation by the various agents, in both sublines, was evaluated by cytochemical staining and subcellular fractionation. This analysis showed that p53 is localized mainly in the nuclear fraction in MCF-7 cells, and in the cytoplasmatic fraction in LCC2 cells. Doxorubicin induced apoptosis in MCF-7 cells along with increase in its nuclear fraction. In contrast, LCC2 underwent apoptosis by Adr despite its cytoplasmatic sequestration. These experiments demonstrate that p53 is sequestered to cytoplasm in the estrogen-independent, Tax-resistant LCC2 cells. However, the differences in apoptotic rate between MCF-7 and LCC2 cells do not seem to be dependent on p53. The LCC2 cell line may serve as a useful model for the study of the mechanism of cytoplasmatic sequestration of wild type (wt) p53, its physiologic consequences, and its relation to estrogen-independence or Tax resistance of breast cancer cells.

Role of transforming growth factor beta in the growth inhibition of human breast cancer cells by basic fibroblast growth factor.

Recent studies from our laboratory have revealed that basic fibroblast growth factor (bFGF) selectively inhibits the proliferation of human MCF-7 breast cancer cells. It has also been shown to enhance cis-platinum-induced apoptosis, decrease levels of the anti-apoptotic gene product bcl-2, and increase levels of the cyclin-dependent protein kinase inhibitor p21/WAF1/Cip1. Transforming growth factor beta-1 (TGFbeta1), a cell growth regulator has been found to have an inhibitory effect on breast cancer cells. The aim of the present study was to evaluate the possible role of TGFbeta1 in the antiproliferative effects of bFGF in MCF-7 breast cancer cells. We found that exogenous, as well as endogenous (overexpressed) bFGF increased TGFbeta1 mRNA expression in the cells and enhanced the secretion of TGFbeta1 into culture medium. However, exogenous addition of TGFbeta1 neither led to a decrease in bcl-2 nor induced an increase in the levels of p21/WAF1/Cip1 and neutralizing antibodies to TGFbeta1, did not reverse bFGF-induced G1 arrest northe increase in p21/WAF1/Cip1 level. In contrast, antisense oligonucleotides to TGFbeta1 abrogated the antiproliferative effects and inhibited the induction of p21/WAF1/Cip1 by bFGF in MCF-7 cells. These data suggest that the anti-proliferative effects of bFGF in human MCF-7 breast cancer cells are mediated by endogenous TGFbeta1, while exogenous TGFbeta1 does not mimic all the effects of bFGF on these breast cancer cells. These findings provide an important basis for further investigations into the autocrine and paracrine processes that control the growth of breast cancer cells.

Differential sensitivity of MCF-7 and LCC2 cells, to multiple growth inhibitory agents: possible relation to high bcl-2/bax ratio?

Comparison of LCC2, the E(2)-independent, tamoxifen-resistant subline of the MCF-7 human breast cancer cell line with its parent line, disclosed that it is more resistant to growth inhibition and apoptosis induction by a variety of agents acting by diverse mechanisms. Thus, LCC2 cells can serve as a useful in-vitro model for the study of the molecular mechanisms of this resistance. It was found that bcl-2 protein and mRNA were elevated and that bax protein and mRNA were reduced in LCC2 compared with MCF-7 cells. Incubation of both lines in the presence of bcl-2 antisense caused growth inhibition and reduced bcl-2 protein levels only in MCF-7 cells, suggesting the involvement of bcl-2 in the regulation of normal growth of breast cancer cells. Increased bcl-2 expression in breast cancer cells may correlate with their resistance to growth inhibitory agents. Bcl-2 is a useful target for enhancing the effects of growth inhibitory agents.

Effects of psychotropic drugs on cell proliferation and differentiation.

Some of the psychotropic agents widely used for the amelioration of anxiety, depression, and psychosis also show an effect at the cellular proliferation level. Surprisingly little research, however, has been directed to the antitumoral potential of these drugs, alone or in combination with established cancer treatments. Our review of the literature to date has yielded some promising early findings. Ligands active at the benzodiazepine (BZ) receptors have been studied the most extensively and were found to have differential, concentration-dependent effects on the growth and proliferation of both normal and cancer cells. Of the phenothiazines tested, chlorpromazine (CPZ) and perphenazine (PPZ) had the most potent cytotoxic action on fibroblasts and glioma cells. Antiproliferative effects also were noted by these and other agents in leukemic and breast cancer cell lines. Additional psychotropic drugs studied include the atypical antipsychotics, antidepressants, and mood stabilizers, especially lithium. Most of the reported activities were observed in in vitro studies and were achieved at high pharmacological concentrations. Further in vivo studies in well-designed animal models are warranted to determine whether these well-tolerated, relatively inexpensive, and widely available drugs or their derivatives may be added in the future to the armamentarium of cancer pharmacotherapy.

Basic fibroblast growth factor potentiates cisplatinum-induced cytotoxicity in MCF-7 human breast cancer cells.

Basic fibroblast growth factor (bFGF) is a classical mitogen in fibroblasts and endothelial cells. Our previous studies have demonstrated that bFGF inhibits the growth of MCF-7 human breast cancer cells. The aim of the present study was to examine the effect of bFGF on cis-diamminedichloroplatinum(cisplatin)-induced cytotoxicity in MCF-7 breast cancer cells as compared to normal endothelial cells. MCF-7/NCF cells transduced with a vector expressing the bFGF gene and overexpressing its product, and MCF-7/N2 cells transduced with the backbone vector were incubated with a combination of bFGF and cisplatin for 5 days; results were compared with those obtained with bovine aortic endothelial cells. Cell proliferation was assessed with the sulforhodamine B colorimetric cytotoxicity assay. Apoptosis was quantitatively determined by flow-cytometric analysis for DNA damage and the apoptotic death assay for DNA fragmentation, and qualitatively by electron microscopy. Reverse transcriptase/polymerase chain reaction analysis and an enzyme immunoassay were used to determine the mRNA and protein level, respectively, of the anti-apoptotic bcl-2 gene product. We found that bFGF enhanced cisplatin-induced cytotoxicity in MCF-7 breast cancer sublines. bFGF enhanced proliferation of normal endothelial cells and did not increase cisplatin-induced cytotoxicity. This effect was accompanied by down-regulation of the anti-apoptotic protooncogene bcl-2 and the enhancement of cisplatin-induced apoptosis. We suggest that the improved understanding of the role of bFGF in the differential modulation of the response of breast cancer and normal endothelial cells to chemotherapy may enable active intervention to alter the therapeutic ratio favorably in breast cancer patients.

Antiproliferative and differentiating effects of benzodiazepine receptor ligands on B16 melanoma cells.

In this study, we evaluated the effect of several ligands active at the central-type and peripheral-type benzodiazepine receptor (BzR) (clonazepam, diazepam, PK11195 and Ro5-4864) on the growth and differentiation of B16 melanoma cells. All tested BzR ligands were able to suppress proliferation of the cells at the micromolar range and in a concentration-dependent manner. However, agents selectively active at the peripheral-type BzR (PK11195 and Ro5-4864) exhibited more potent antiproliferative activity. In addition, the BzR ligands were demonstrated to affect the cell cycle by reducing the percent of cells in the S phase and increasing the percent in the G2/M phase. BzR ligands induced cellular phenotypic alterations, which have been previously shown to be associated with melanoma cell differentiation. These alterations included: marked morphological changes, enhancement of melanogenesis, lipid accumulation and increase in the activity of gamma glutamyl transpeptidase. All BzR ligands induced a marked reduction in the concentration of UTP and most of them did the same in GTP and CTP, while ATP levels were not significantly altered. In summary, BzR ligands (clonazepam, diazepam, PK11195 and Ro5-4864) were found to exert antitumor effects in B16 melanoma cells. These findings encourage further studies of a possible therapeutic potential of BzR ligands in treatment of melanoma.

Comparison of growth rate of two B16 melanomas differing in metastatic potential in young versus middle-aged mice.

The rise of cancer frequency as a function of age is a well-established fact. The aspect of the host age-tumor progression relationship, namely the slower metastatic spread in aged patients, has been investigated to a lesser extent. In the present study, we examined whether host-age-dependent growth rate varies with metastatic capacity of the tumor. The parental B16 and the B16/Col/R, a highly metastatic variant, were employed. A more pronounced growth of both tumors in young as compared to middle-aged mice was found. However, the differential growth in middle-aged versus young mice was more evident in the highly metastatic variant. According to the tumor size data, a sixfold growth reduction in middle-aged mice was observed with B16/Col/R and an only twofold growth reduction was seen with the B16 melanoma. The data might eventually contribute to the finding of more appropriate treatment modalities for the middle-aged cancer patient.

Inhibition of B16 melanoma cell proliferation and alterations in p21 ras expression induced by interceptors of signal transduction pathways.

Anti-tumor effects of agents known to intervene with signal transduction pathways (ras and protein kinase c cascades) were examined in the B16 melanoma cell model. The compounds examined included: lovastatin, an inhibitor of HMG-CoA reductase, which interferes with membrane localization of p21 ras protein; H-7, a classic inhibitor of protein kinase C; and tiazofurin, a GTP depleting agent, that might affect the GTP/GDP ratio on p21ras. The three agents were found to inhibit the proliferation of B16 melanoma cells. Only tiazofurin, as expected, induced a significant decrease in GTP levels. Lovastatin and H-7 altered p21 subcellular localization. They reduced membrane expression of p21 ras, while increasing its expression in the cytosol. Following tiazofurin treatment a trend towards increased membranal p21 was observed. These results suggest that p21 is a target for the action of signal transduction inhibitors. However, the relationship between growth inhibition and altered p21 expression is not yet clear.

Metastatic potential and multidrug resistance correlation in the B16 melanoma system.

The question of whether metastatic potential and drug resistance are related phenotypes was addressed by comparing the biological behavior of the parental B16 melanoma and a multidrug resistant variant derived from it, the B16/Col/R. A more pronounced metastatic spread to lungs was observed in mice inoculated i.v. with the B16/Col/R variant than in those bearing the parental line. In addition, in the mice injected with the drug resistant melanoma, unusual tumor masses were observed. Large abdominal and spinal cord growths were seen with the MDR variant but not encountered in mice inoculated with the original B16 melanoma. We further attempted to test the capacity of the two cell types to perform several cellular functions relevant to the metastatic process. The B16/Col/R cells displayed a higher aggregability and cell motility than did the B16 cells. Adherence to endothelial cells was higher in the parental line than in the B16/Col/R, possibly supporting a more efficient extravasation of the variant cells. The drug resistant variant displayed a higher capacity to grow locally in kidney, spleen, cecum and peritoneum, as compared to the parental melanoma, indicating a higher ability of homing and growth in these potential target organs for metastasis. A correlation between metastatic potential and multidrug resistance appears therefore to exist in the system examined.

Early polycystic ovary-like syndrome in girls with central precocious puberty and exaggerated adrenal response.

Exaggerated adrenal response (ExAR), i.e. hypersecretion of both 17-hydroxypregnenolone (170HPreg) and 17-hydroxyprogesterone(17OHP) in response to adrenocorticotropic hormone (ACTH) stimulation, is frequently found in women with polycystic ovary (PCO) syndrome who had precocious adrenarche. In an earlier study we found an abnormal adrenal response in girls with idiopathic true central precocious puberty (CPP) at early stages of puberty. On follow-up it was noted that a significant number of girls with CPP develop PCO-like syndrome at a relatively young age. The aim of the present study was to determine if there is an association between ExAR and early PCO in girls with a history of CPP. Included were 49 girls with a history of CPP, 34 of whom were treated with gonadotropin-releasing hormone (GnRH) analog. All 49 were evaluated at full maturity, at ages 12.5-14 years, 0.5-4 years after menarche or resumption of menses. Of the 49 girls, 20 had at least 3/4 clinical signs of PCO (irregular menses, hirsutism, acne and obesity) and were defined as PCO-like+, whereas 29 did not fulfil the criteria and were considered PCO-like-. Girls with a definite enzyme deficiency were excluded from the study. All participants underwent a combined iv ACTH-GnRH test at early follicular phase. The PCO-like+ girls all revealed ExAR, i.e. an elevated stimulated 17OHPreg of 63.4 +/- 9.6 nmol/l (normal 28.6 +/- 9.2 nmol/l) and a normal stimulated 17OHPreg/17OHP ratio of 7.1 +/- 1.8 (normal 6.2 +/- 2.7), whereas all the PCO-like- had a normal adrenal response (30.0 +/- 8.7 and 5.3 +/- 2.0 nmol/l, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

High prevalence of abnormal adrenal response in girls with central precocious puberty at early pubertal stages.

Abnormal adrenal response is often observed in girls with precocious adrenarche (1). We studied the adrenal response in 112 girls with idiopathic true central precocious puberty (CPP) at early stages of puberty compared to that in 21 girls with normal puberty (controls). The aims of this study were to determine the prevalence of abnormal adrenal response at early stages of puberty, the possible correlation of abnormal adrenal response with pubertal signs at onset of puberty and with plasma androgen levels, and a possible association with the activity of the hypothalamic-pituitary-gonadal (HPG) axis. All participants underwent a combined i.v. adrenocorticotropic hormone (ACTH)-gonadotropin-releasing hormone (GnRH) test at Tanner stage 2-3: 62 of the CPP girls before and 50 during treatment with GnRH analog. The stimulated levels of 17-hydroxypregnenolone (17OHPreg) and the stimulated 17OHPreg/17-hydroxyprogesterone ratio were analyzed and compared to previously reported norms. The result revealed three patterns of adrenal response: normal (17OHPreg < or = 24 nmol/l and 17OHPreg/17OHP ratio < or = 7) in 50/112 (44.6%) CPP patients and 17/21 (80.9%) controls; exaggerated (17OHPreg > 24 nmol/l, 17OHPreg/17OHP ratio < or = 7) in 50/112 (44.6%) CPP patients and 3/21 (14.3%) controls; and non-classical 3 beta-hydroxysteroid dehydrogenase deficiency (17OHPreg > 24 nmol/l and 17OHPreg/17OHP ratio > 7) in 12/112 (10.8%) CPP patients and 1/21 (4.8%) controls. The clinical features at onset of puberty were comparable in all girls with the CPP in spite of the different adrenal response patterns.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of immune response as determinant of tumor progression in function of host age in the B16 melanoma.

Aging constitutes the major cause for the development of most neoplastic diseases. However, tumors in aged people present with a lower degree of aggressiveness than in young patients. The reasons for this paradoxical behavior are not clear. We attempted to verify whether the immune system has a role in the relation between host age, immune response and tumor progression. We compared the growth rate of B16 melanoma and a highly malignant variant, the B16/Col/R, in young and aged mice that have or have not undergone splenectomy. The following results were obtained: (1) Splenectomy stimulated growth in the parental melanoma in both young and aged mice, indicating a protective role of the spleen against this tumor at all ages; (2) Spleen ablation provoked inhibition of the highly-metastatic variant growth in young mice, suggesting a stimulatory role of the spleen in this case; (3) By contrast, in aged mice inoculated with the B16/Col/R variant, splenectomy enhanced tumor growth, indicating a defensive role of the spleen. Age favors a positive host response against the aggressive clone of the melanoma. Differential host response in young versus aged mice can explain, in this tumor system, the difference in tumor progression rate as a function of age.

Increased turnover of intrahepatic bile ducts induced by bromobenzene.

Intrahepatic bile duct epithelium consists of two kinetic compartments: a progenitor (P) and a functional (Q) compartment. Hitherto bromobenzene was known to poison only hepatocytes in the third acinus zone. The present experiment aims to demonstrate that bromobenzene affects also bile duct turnover. Thirty male adult rats received one intraperitoneal injection of bromobenzene and were sacrificed in groups of five at the following times: 1, 2, 3, 4, 7, and 14 days. They received [3H]thymidine 1 hr before sacrificing. Autoradiography was done. Bile ducts were evaluated in all portal tracts of the section. The number of epithelial cells in each duct cross section was counted and defined as bile duct class, which is roughly proportional to bile duct size. In each cross section the number of labeled cells was counted. Initially the labeling index was 0.76 +/- 0.3%. By day 3, it reached a peak of 4.1 +/- 1.1%, and then declined to its initial level. Following bromobenzene poisoning, hepatocyte and bile duct epithelia turn over in the same fashion. In both, labeling index and progenitor compartment size initially rise and return by the end of the first week to their initial level. We propose that bile duct epithelia and hepatocytes originate in one determined uncommitted stem cell that resides in the Herring duct. Bromobenzene-induced necrosis triggers proliferation of progenitors in both cell lineages, as well as in the stem cell itself.

Effects of differentiation-inducing agents on purine nucleotide metabolism in an ovarian cancer cell line.

The effects of the differentiation-inducing agents sodium butyrate (NaOBt), dimethylsulfoxide (DMSO) and mycophenolic acid (MA), on purine nucleotide metabolism, was studied in an ovarian carcinoma cell line (GZL-8). Exposure to these agents inhibited cell proliferation, but did not affect cell viability. Three hours following exposure, NaOBt and DMSO moderately decelerated purine synthesis de novo, but MA accelerated it three-fold, this being associated with a two-fold increase in the excretion of hypoxanthine and xanthine into the incubation medium. NaOBt and DMSO did not affect the cellular nucleotide content, but MA caused a 73% decrease in GTP content and about a 50% increase in the cellular content of UTP. The following alterations in cellular enzyme activity were observed 72 h following exposure: NaOBt decreased the activity of hypoxanthine-guanine phosphoribosyltransferase and increased the activity of IMP and of AMP 5'-nucleotidases, DMSO increased the activity of IMP 5'-nucleotidase, and MA increased the activity of the two nucleotidases. The results suggest that, in the carcinoma cell line studied, the differentiation process induced by NaOBt and DMSO may be associated with a general shift in the direction of purine metabolism from anabolism to catabolism, whereas that induced by MA is associated with a specific decrease in the production of GTP.

Novobiocin modulates colchicine sensitivity in parental and multidrug-resistant B16 melanoma cells.

The effect of the antibiotic agent novobiocin on the sensitivity of melanoma cells to colchicine and vinblastine was examined in drug-sensitive and drug-resistant B16 melanoma cells. A cell line COL/R was selected for colchicine resistance. The COL/R cell line (resistant to 80 ng/ml colchicine) was found to possess the multidrug-resistant (MDR) phenotype. The cells were shown to be cross-resistant to vinblastine and Adriamycin and to overexpress P glycoprotein. P glycoprotein activity was assessed by using the rhodamine 123 accumulation test. Rhodamine accumulation was markedly decreased in COL/R cells as compared to the parental B16 cells. Verapamil reversed drug resistance and increased rhodamine accumulation in COL/R cells. Novobiocin in combination with colchicine or vinblastine synergistically inhibited the proliferation of parental B16 cells. In COL/R cells, novobiocin markedly decreased colchicine resistance and increased rhodamine accumulation. These data show that novobiocin increases the sensitivity of both parental and MDR melanoma cells to microtubule-disrupting cytotoxic drugs.