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The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 6 2900-2907
Copyright © 2003 by The Endocrine Society

Antineoplastic Ribonucleases Selectively Kill Thyroid Carcinoma Cells Via Caspase-Mediated Induction of Apoptosis

Daniela Spalletti-Cernia, Rosanna Sorrentino, Sonia Di Gaetano, Angela Arciello, Corrado Garbi, Renata Piccoli, Giuseppe D’Alessio, Giancarlo Vecchio, Paolo Laccetti and Massimo Santoro

Istituto di Endocrinologia ed Oncologia Sperimentale del Consiglio Nazionale delle Ricerche (CNR), Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università di Napoli Federico II (D.S.-C., R.S., C.G., G.V., M.S.), 80131 Naples, Italy; Dipartimento di Chimica Biologica, Università di Napoli Federico II (S.D.G., A.A., R.P., G.D., P.L.), 80134 Naples, Italy; and Istituto di Biostrutture e Bioimmagini-CNR (S.D.G.), 80134 Naples, Italy

Address all correspondence and requests for reprints to: Dr. Massimo Santoro, Istituto di Endocrinologia ed Oncologia Sperimentale del CNR, Via S. Pansini 5, 80131 Naples, Italy. E-mail: masantor{at}unina.it.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Bovine seminal ribonuclease (BS-RNase), a natural dimeric homolog of bovine pancreatic RNase (RNase A), and HHP2-RNase, an engineered dimeric form of human pancreatic RNase (HP-RNase), are endowed with powerful antitumor effects. Here we show that BS- and HHP2-RNases, but not monomeric RNase A, induce apoptosis of human thyroid carcinoma cell lines. RNase-induced apoptosis was associated with activation of initiation caspase-8 and -9. This was followed by activation of executioner caspase-3, leading to the proteolytic cleavage of poly(ADP-ribose) polymerase. The caspase inhibitor Z-Val-Ala-Asp-(OMe)-fluoromethylketone protected thyroid cancer cells from BS-RNase-induced apoptosis. RNase-triggered apoptosis and caspase activation were accompanied by reduced phosphorylation of Akt/protein kinase B (PKB), a serine-threonine kinase that when phosphorylated is able to deliver survival signals to cancer cells. BS-RNase antitumor effects in nude mice were accompanied by caspase activation and apoptosis. Because of the high selectivity of apoptotic effects for malignant cells, BS- and HHP2-RNase are promising tools for the treatment of aggressive thyroid cancer.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
APPROXIMATELY 20,000 NEW cases of thyroid carcinoma are diagnosed each year in the United States, and 1,300 patients die of the disease. The biological behavior of thyroid carcinoma varies from indolent microcarcinomas to poorly differentiated carcinoma, which is associated with significant morbidity and mortality, and anaplastic carcinoma, one of the most aggressive human malignancies. At diagnosis, most anaplastic carcinoma patients have distant metastases, primarily in lung, bone, and liver. Doxorubicin, alone or combined with cisplatin, is the most widely used therapeutic agent, but it does not improve survival in anaplastic carcinoma patients (1, 2). Thus, although radioactive iodine is beneficial in cases of differentiated tumors, more effective chemotherapeutic agents are needed for aggressive thyroid cancers.

Bovine seminal ribonuclease (BS-RNase) from bull semen and Onconase (Onconase is a registered trademark of Alfacell, Inc., Bloomfield, NJ) from frog eggs (Rana pipiens) are selectively toxic for cancer cells (3). BS-RNase, the only dimeric RNase in the vertebrate superfamily (for a review, see Ref. 4), is not toxic for normal cells grown in vitro, nor does it exert appreciable toxic effects when administered in vivo to experimental animals (5), whereas it is very cytotoxic for a number of tumor cells of different origins (3). BS-RNase exerts remarkable antitumor effects in syngeneic and xenotransplant models (6), and antimetastatic activity (5). Onconase is currently being tested in phase II clinical trials (7). Conjugation of ribonucleases with polymers or antibodies results in increased stability and specificity. Onconase linked to a monoclonal antibody (anti-CD22) is a potent immunotoxin against B cell lymphoma (8).

The binding of BS-RNase to a component of the extracellular matrix (9, 10) or of Onconase to an unidentified cell surface receptor (11) is followed by endocytosis, translocation to the cytosol, degradation of RNA (rRNA or tRNA by BS-RNase and Onconase, respectively), and suppression of protein synthesis. The cytosolic ribonuclease inhibitor (cRI) tightly binds to internalized RNases, blocking their cytotoxic activity. Thus, only RNases that can evade cRI are capable of exerting a cytotoxic action (11, 12, 13, 14, 15). The resistance of BS-RNase to cRI inhibition depends on the dimeric structure of the enzyme (12, 13, 15). On the other hand, Onconase, the frog RNase, is inhibited by frog, but not mammalian, cRI.

We recently transformed human pancreatic RNase (HP-RNase), a monomeric noncytotoxic RNase, into a dimeric RNase (designated HHP-RNase). This was achieved by replacing four of its amino acid residues (Q28, K31, K32, and N34) with the corresponding residues (L28, C31, C32, and K34) from BS-RNase. HHP-RNase was selectively cytotoxic for malignant cell lines (16). A second generation dimeric mutant of HP-RNase, designated HHP2-RNase, obtained by introducing another mutation (E111G), was more cytotoxic than HHP-RNase (17). Given their human origin, HP-RNase dimeric variants are very promising anticancer agents because their effects are less likely to be curtailed by the host immune response.

Onconase and RC-RNase (a cytotoxic ribonuclease from Rana catesbeiana) promote apoptosis of HeLa (18) and MCF-7 breast cancer cells (19), respectively, and BS-RNase induces apoptosis of human neuroblastoma (20) and thyroid tumor cells (16, 21, 22). Two major apoptotic pathways, triggered by mitochondria and cell surface receptors, respectively, have been identified. The mitochondrial pathway is involved in the apoptotic response to DNA-damaging agents, for example, conventional cancer chemotherapeutics and radiation (for a review, see Ref. 23). Caspases, aspartate-specific cysteine proteases, are key effectors of apoptosis (for a review, see Ref. 24). The initiation caspase of mitochondria-driven apoptosis is caspase-9; the initiation caspase of the cell surface-triggered apoptosis is caspase-8. In turn, caspase-8 and -9 directly activate executioner caspase-3 (for a review, see Ref. 24).

We have studied BS-RNase- and HHP2-RNase-induced apoptosis in four thyroid carcinoma cell lines. Our results demonstrate that the two RNases kill thyroid carcinoma cells by engaging both the mitochondrial and the cell surface-initiated apoptotic machinery and suggest that Akt/protein kinase B (PKB) is a target of apoptotic RNase cell signaling.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
RNases

BS-RNase was purified from bovine seminal vesicles as reported previously (25). HHP2-RNase was generated and purified as previously described (17). RNase A was purchased from Sigma-Aldrich Corp. (St. Louis, MO).

Cell lines

NPA and ARO cells (26), a gift from J. A. Fagin, derive from human papillary and anaplastic thyroid carcinomas, respectively. CAL62 (27), a gift from J. Gioanni, and KAT4 (28), a gift from K. B. Ain, were established from human thyroid anaplastic carcinomas. All of these cell lines are tumorigenic when injected into nude mice (26, 27, 28). HDF are normal human diploid fibroblasts, and they were gifts from P. Dotto and C. Missero. All cell lines were grown in DMEM supplemented with 10% fetal calf serum (Life Technologies, Inc., Paisley, PA), penicillin (50 U/ml), and streptomycin (50 µg/ml; Sigma-Aldrich Corp.).

Analysis of apoptosis

For fluorescence microscopy analysis, 104 cells were plated on coverglasses in 24-well plates. Twenty-four hours after plating, RNases were added to a final concentration of 1.8 µM. Seventy-two hours later, adherent cells and those present in the medium were fixed for 20 min in 3.7% formaldehyde, washed in PBS, permeabilized for 5 min in 0.1% Triton X-100, and stained for 30 min with Hoechst 33258 (Sigma-Aldrich Corp.) at a concentration of 0.5 µg/ml in PBS. The stained cells were observed under an epifluorescent microscope (Axiovert 2, Carl Zeiss, New York, NY) interfaced with the image analyzer software KS300 (Carl Zeiss), and photographed.

For a time-dependent analysis of apoptotic DNA fragmentation, cells were plated in 96-well plates (1 x 103/well in 100 µl medium). After 24 h, each RNase was added to the culture medium (to a final concentration of 1.8 µM) for times ranging from 15 min to 72 h. Apoptosis was measured in quadruplicate samples using the Cell Death Detection ELISA Plus kit (Roche, Mannheim, Germany). This photometric sandwich enzyme immunoassay determines cytoplasmic histone-associated DNA fragments (mono- and oligonucleosomes) upon induction of cell death. Briefly, a mixture of biotin-labeled antihistone and peroxidase-conjugated anti-DNA antibodies was added to the cell lysates and placed in a streptavidin-coated microtiter plate. During incubation, the antihistone antibody binds to the histone component of the apoptotic nucleosomes and simultaneously captures the immunocomplex to the streptavidin-coated plates via its biotinylation. Meanwhile, the anti-DNA antibody reacts with the DNA component of the nucleosomes. The amount of nucleosomes was determined spectrophotometrically at 405 nm with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) as a substrate of peroxidase (Microplate reader, Bio-Rad Laboratories, Inc., Munich, Germany).

Determination of caspase activation

Cells were treated with RNases for various times. At the end of each treatment, caspases-3, -8, and -9 activities were tested with colorimetric protease assay kits (Alexis Biochemicals, San Diego, CA), in which the chromophore p-nitroanilide (pNA) is detected spectrophotometrically (at 400 or 405 nm) after cleavage from the labeled substrate. The caspase-3/CPP32, the Flice/caspase-8, and the caspase-9/Mch6 kits assay the activity of caspase-3, which recognizes the Asp-Glu-Val-Asp (DEVD) sequence; the activity of caspase-8, which recognizes the Ile-Glu-Thr-Asp (IETD) sequence; and the activity of caspase-9, which recognizes the Leu-Glu-His-Asp (LEHD) sequence, respectively. The increase in caspase activity was determined by comparing the absorbance of pNA from a treated sample vs. an untreated control. The results from three independent experiments were averaged for each time point.

Caspase inhibitors

Caspase inhibitors (Alexis Biochemicals, San Diego, CA) irreversibly bind to the caspase active sites because the peptide recognition sequence is linked to a fluoromethylketone (fmk). To increase cell permeability, their aspartic acid residues are esterified with methyl groups. Z-Val-Ala-Asp-(OMe)-fluoromethylketone (zVADfmk; 50 µM; added every 24 h) was used as a general upstream caspase inhibitor for the in vivo DNA fragmentation assay. Z-Ile-Glu-Thr-Asp(OMe)-fluoromethylketone, Z-Leu-Glu(OMe)-His-Asp(OMe)-fluoromethylketone, and Z-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone (to a final concentration of 10 µM) were used as caspase-8, -9, and -3 selective inhibitors, respectively.

Immunoblot

Protein lysates (100 µg) were electrophoresed through 7.5% sodium dodecyl sulfate-polyacrylamide gel, transferred to Immobilon membranes (Bio-Rad Laboratories, Inc.), and analyzed by immunoblot. Antipoly-(ADP-ribose) polymerase (anti-PARP) antibodies (1:2000) were purchased from Roche Molecular Biochemicals. Anti-Akt and antiphospho-Akt (Ser473), specific for the active Akt phosphorylated at serine 473, antibodies were obtained from Cell Signaling (Beverly, MA). Immunoreactive bands were visualized using the enhanced chemiluminescence detection reagents (Amersham Pharmacia Biotech, Arlington Heights, IL) according to the manufacturer’s protocol.

Tumorigenicity in nude mice

ARO cells (2 x 106/mouse) were inoculated sc into the right dorsal portion of 6-wk-old male BALB/c {nu}/{nu} mice (The Jackson Laboratory, Bar Harbor, ME). BS-RNase (20 µg/g body weight) or vehicle (PBS) was injected in the peritumor area starting 1 d after cell implantation for a total of six injections (at 72-h intervals). Tumors were harvested 14 d after cell implantation and weighed. Apoptotic rate and caspase-9 and -3 activity were measured as described above. No mouse showed signs of wasting or other signs of toxicity. Animals were maintained at the Dipartimento di Biologia e Patologia Cellulare e Molecolare Animal Facility. All manipulations were performed under isoflurane gas anesthesia and were conducted in accordance with Italian regulations for experimentation on animals.

Statistical analysis

A t test was used to evaluate the statistical significance of the results. Analyses were performed with the BMDP New System statistical package version 1.0 for Microsoft Corp. Windows (BMDP Statistical Software, Los Angeles, CA).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
BS-RNase and HHP2-RNase induce apoptosis of thyroid carcinoma cells

We previously demonstrated that dimeric RNases (16) are very cytotoxic for NPA, a highly tumorigenic human thyroid carcinoma cell line. To identify the mode of thyroid cell death, we treated NPA cells and HDF with BS-RNase and HHP2-RNase. Noncytotoxic RNase A and HP-RNase were used as controls. After treatment (1.8 µM for 72 h), cells were examined for the presence of apoptotic nuclear bodies by fluorescence microscopy after Hoechst 33258 staining. Cells were cytospun to analyze both adherent and detached cells; the experiments were performed in triplicate; at least 200 cells were counted for each assay. BS-RNase and HHP2-RNase treatment caused relevant apoptosis of NPA cells (25–30 ± 5%); apoptotic cells showed typical condensed chromatin and apoptotic bodies (Fig. 1AGo). In contrast, apoptosis was not detected (<2%) with RNase A (Fig. 1AGo) or HP-RNase (not shown). More importantly, apoptosis was not detected (<5%) in HDF cells treated with BS-RNase or HHP2-RNase (data not shown and see below).



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  		Figure 1. NPA cell apoptosis induced by cytotoxic RNases. A, NPA cells were exposed for 72 h to the indicated RNases or were left untreated (-). Cells were fixed and stained with Hoechst 33258 to reveal nuclear signs of apoptosis and photographed (x360). Insets, Apoptotic nuclei (x2400). B, NPA and HDF cells were treated with BS-RNase for the indicated times. Protein lysates were immunoblotted with anti-PARP and antitubulin for normalization. C, NPA and HDF cells were treated with the indicated RNases for times ranging from 15 min to 72 h. At the end of each treatment, DNA fragmentation was measured with the Cell Death Detection ELISA Plus kit. Values are the mean ± SD of quadruplicate determinations of three independent experiments.

 
PARP cleavage, a sensitive marker of caspase-mediated apoptosis, resulted in the disappearance of the 116-kDa full-size PARP band (p116PARP) monitored by Western blot. p116PARP levels decreased in NPA cells after 24 h of BS-RNase treatment (Fig. 1BGo); after 72 h, most of the full-size PARP had disappeared. Differently, BS-RNase treatment did not affect the amount of p116PARP in HDF cells (Fig. 1BGo).

We used an ELISA-based assay to quantitate apoptosis. As shown in Fig. 1CGo, apoptotic nucleosomes were detected after treatment of NPA cells with both cytotoxic ribonucleases; the greatest effects occurred at 24 h (BS-RNase) and 48 h (HHP2-RNase). As expected, apoptotic nucleosomes were not detected in HDF cells (Fig. 1CGo). To determine whether these findings also applied to other thyroid carcinoma cell lines, we measured apoptosis in highly tumorigenic anaplastic ARO, CAL62, and KAT4 cells after treatment for various times. These anaplastic cell lines were even more sensitive than NPA cells to the proapoptotic effects of HHP2-RNase (Fig. 2Go) and BS-RNase (not shown).



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  		Figure 2. DNA fragmentation of thyroid carcinoma cells treated with HHP2-RNase. The various cell lines were treated with HHP2-RNase for 0, 24, or 48 h. At the end of each treatment, DNA fragmentation was measured with the Cell Death Detection ELISA Plus kit. Values are expressed as the fold increase in apoptosis and are the mean ± SD of quadruplicate determinations of three independent experiments.

 
BS-RNase and HHP2-RNase trigger caspase cascades in thyroid cancer cells

To assess the role of caspases in RNase-induced apoptosis, we analyzed the kinetics of caspase activation in the four thyroid cancer cell lines using substrates specific for caspase-8, -9, and -3. In these assays the spectrophotometric detection of pNA chromophore, cleaved from the specific synthetic substrate, is a measure of caspase activity. Selective inhibitors of the three caspases served as a control of specificity. Caspase-8 activity increased significantly at 5 min and peaked at 30 min in NPA cells treated with BS-RNase (Fig. 3AGo). Its activity slowly declined thereafter, but was still significantly enhanced at 48 h. Similar kinetics were observed in HHP2-RNase-treated NPA cells (Fig. 3AGo). The extent of caspase-8 activation in the three anaplastic cell lines was, on the average, even greater than that in NPA cells (Fig. 3AGo). Caspase-9 activity began to increase at 6 h and peaked at 24 h in BS-RNase-treated NPA cells, whereas it peaked at 3–6 h after treatment with HHP2-RNase in all four cancer cell lines (Fig. 3BGo). Consistent with its downstream role, caspase-3 activity started to increase later than caspase-8 and -9 activities, with a peak at 24–48 h depending on the RNase tested and on the cell line (Fig. 3CGo). There was no caspase activation in HDF cells treated with either BS-RNase or HHP2-RNase. No apoptosis was detected in cells treated with RNase A.



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  		Figure 3. Caspase activation. Time course of caspase-8 (A), caspase-9 (B), and caspase-3 (C) substrate hydrolysis in cells exposed to RNases. Caspase activity was also measured in the presence of the caspase-8 inhibitor Z-Ile-Glu-Thr-Asp(OMe)-fluoromethylketone (zIETDfmk), the caspase-9 inhibitor Z-Leu-Glu(OMe)-His-Asp(OMe)-fluoromethylketone (zLEHDfmk), or the caspase-3 inhibitor Z-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone (zDEVDfmk) to prove specificity. Each point represents the mean ± SD of three determinations from three independent experiments.

 
A general caspase inhibitor protects thyroid cancer cells from BS-RNase-induced cell death

If RNase-induced apoptosis is a caspase-dependent process, DNA fragmentation should be partially or fully prevented in cells cotreated with RNases and caspase inhibitors. To address this question, we cotreated NPA cells with BS-RNase and zVADfmk (to a final concentration of 50 µM), a general upstream inhibitor of caspases. The inhibitor was replenished daily when the treatment exceeded 24 h. zVADfmk significantly decreased RNase-induced DNA fragmentation (Fig. 4Go). Similarly, HHP2-RNase-induced apoptosis of CAL62 cells was significantly decreased by zVADfmk treatment (not shown). However, apoptosis was reduced, but not abrogated, upon caspase inhibition.



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  		Figure 4. BS-RNase-mediated NPA cell apoptosis depends on caspase activation. Cells growing in 96-well plates were either untreated (time zero) or treated with BS-RNase for different times without ({square}) or with ({blacksquare}) zVADfmk. Apoptosis was measured as the mean ± SD of quadruplicate determinations of three independent experiments with the Cell Death Detection ELISA Plus kit.

 
Cytotoxic RNases decrease Akt/PKB phosphorylation

The serine/threonine protein kinase Akt/PKB (hereafter Akt) has emerged as a key factor in tumorigenesis (29). Akt is a downstream effector of phosphoinositide 3-kinase (PI3K). Amplification of PI3K and Akt family genes, loss of function mutations of the PTEN phosphatase (which negatively regulates the PI3K/Akt pathway), and gain of function mutations of upstream activators of Akt (e.g. oncogenic ras and receptor tyrosine kinases) are frequent features of human cancer, including thyroid carcinomas (30, 31, 32). Akt affects such important cancer cell functions as growth and survival. In particular, by phosphorylating Bad and caspase-9, Akt obstructs caspase activation and protects cancer cells from apoptosis (29). We examined Akt activation by immunoblot with antibodies specific for the activated form of Akt phosphorylated at serine 473 in NPA and Cal62 thyroid cancer cells treated with HHP2-RNase. RNase treatment caused a slow, but significant, decrease in Akt activation levels; at 24 h of treatment, Akt phosphorylation was approximately 3-fold lower than that in untreated cells, as measured by phosphorimaging (Fig. 5Go).



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  		Figure 5. RNase-mediated apoptosis is accompanied by reduced Akt phosphorylation. NPA and CAL62 cells were treated with HHP2-RNase for the indicated times. Protein lysates were immunoblotted with antiphospho-Akt (pAKT) and anti-Akt for normalization.

 
In vivo BS-RNase antitumor effects are associated with induction of apoptosis

We determined whether the proapoptotic effects of RNases could be detected in vivo in mice bearing tumors induced by thyroid neoplastic cells. Nude mice were inoculated with 2 x 106 ARO cells sc. Starting 24 h after cells injection, five mice were treated with six injections (at 72-h intervals) of BS-RNase (20 µg/g body weight) in the peritumoral area. Another group of five control mice was treated with PBS. Consistent with in vitro cytotoxic effects of BS-RNase, the average tumor weight of RNase-treated animals was 0.06 ± 0.01 g, whereas untreated tumors reached 0.4 ± 0.02 g 14 d after cell injection. Tumor tissues were harvested, and apoptotic rate and caspase activation were measured as described above. As shown in Fig. 6Go, apoptosis was significantly (P = 0.02) increased (>8-fold) in tumors treated with BS-RNase compared with vehicle-treated tumors. Moreover, the activity of downstream caspase-9 and -3 increased significantly (P = 0.02 and P = 0.009, respectively) in tumors treated with BS-RNase.



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  		Figure 6. In vivo induction of apoptosis by BS-RNase. Nude mice were inoculated with 2 x 106 ARO cells sc and treated with six injections of BS-RNase or PBS as a control. Tumor tissues from RNase-treated (+) and PBS-treated (-) mice were harvested, and triplicate determinations of apoptotic rate with the Cell Death Detection ELISA Plus kit (A) and caspase-9 (B) and -3 (C) activation were performed. The mean ± SD are shown.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Cytotoxic RNases are a nonmutagenic alternative to conventional DNA-damaging cancer therapy. We show that BS-RNase and HHP2-RNase readily kill thyroid carcinoma cells by apoptosis. No apoptosis was observed when the same cells were treated with monomeric RNases. Furthermore, no apoptosis was observed upon RNase treatment of normal human diploid fibroblasts, which confirms the selectivity of cytotoxic RNases for malignant cells. Importantly, induction of apoptosis was also observed when tumors induced by thyroid carcinoma cells in nude mice were treated in vivo with BS-RNase. The thyroid carcinoma cell lines studied lack functional p53 (26, 33) (Spalletti-Cernia, D., unpublished observations) and are thus resistant to chemotherapeutic drugs such as doxorubicin. Consequently, by inducing cell death regardless of p53 status, cytotoxic RNases have an advantage over current therapeutic protocols for advanced human thyroid carcinomas that frequently carry inactivated p53 (26).

Both BS-RNase and HHP2-RNase induced robust caspase activation in the carcinoma cells. RNase-mediated apoptosis was dependent on caspase activation, and cell treatment with zVADfmk significantly reduced DNA fragmentation. However, the constant presence of apoptotic cells despite zVADfmk probably indicates that other pathways, in addition to caspase activation, transduce the apoptotic signals of RNases. The kinetics of caspase activation after RNase treatment of thyroid cancer cells are consistent with a model in which caspase-8 activation is an upstream event, followed by activation of caspase-9 and then caspase-3. Caspase-8 mediates cytochrome c release from mitochondria by activating Bid (a proapoptotic Bcl-2 family member) that, in turn, triggers caspase-9 activation (34). Alternatively, caspase-9 may be activated independently of caspase-8. Whatever the case, the capability of BS- and HHP2-RNases of inducing two distinct caspase cascades probably accounts for their pronounced proapoptotic effect and may hinder the development of resistance of cancer cells.

Caspase-8 is typically activated by the engagement of cell surface proteins of the TNF receptor superfamily, such as Fas. Neoplastic thyroid cells are killed by caspase-8, whereas they are resistant to FasL (35). This is probably due to short-lived protein inhibitor(s) acting upstream from caspase-8, because treatment with protein synthesis inhibitors rescues sensitivity to Fas ligand (35). We did not detect Fas or Fas ligand overexpression after cell treatment with RNases (not shown). However, it is feasible that cytotoxic RNases act downstream from Fas in neoplastic thyroid cells by blocking the synthesis of the putative caspase-8 inhibitor(s). Our results differ from those of Iordanov and colleagues (19), who found activation of caspase-9, -3, and -7, but not of caspase-8, upon treatment of HeLa cells with Onconase. These discrepancies may be due to the different phenotypes of the cell lines and the different RNases used. The mode of RNase delivery may also determine which caspase cascades are activated. Indeed, although we simply added RNase to the culture medium, exploiting the natural capability of cancer cells to internalize cytotoxic RNases, Iordanov and colleagues (19) used lipofectin-mediated delivery of Onconase. Lipofectin treatment is very efficient because it skips one of the rate-limiting steps in RNase action, i.e. transport across the cell membrane. However, it might have perturbed the normal engagement of the RNase with the cell surface, thus modifying the caspase response.

The very rapid caspase-8 activation in thyroid cancer cells (starting as early as 5 min post-RNase and peaking at 30 min) suggests that either the cellular routing (membrane binding-endocytosis-cytosolic penetration-RNA hydrolysis-translation inhibition) of RNases is very rapid, or that RNases trigger intracellular signaling events soon after being engaged at the cell surface.

Cellular RNA degradation is probably a key step in the process of RNase-mediated cell death. Although the inhibition of protein synthesis per se could lead to apoptosis, evidence suggests that RNase-induced cell death does not entirely result from the inhibition of protein synthesis. Indeed, inhibitors of protein synthesis, such as cycloheximide, require much higher levels of translational inhibition, and they do so with slower kinetics compared with RNases. Thus, it is likely that apoptosis results not only from protein synthesis inhibition, but also from cell signaling events such as c-Jun N-terminal kinase activation (36) or Akt dephosphorylation (this paper). The therapeutic specificity of RNases remains enigmatic. Endocytosis or cellular routing rates of RNases may differ between malignant and nonmalignant cells. Alternatively, malignant cells might be more sensitive than normal cells to the toxic effects of RNA hydrolysis. Whatever the case, we show that there is a major difference between malignant and normal cells upstream from the cellular events leading to apical caspases (-8 and -9) activation. In addition, our data suggest that the down-modulation of Akt is at least one of the mechanisms leading to thyroid cell death.

Because they obstruct Akt, which is frequently activated in thyroid cancer, and because they kill thyroid cancer cells regardless of p53 status, BS-RNase and HHP2-RNase are promising novel chemotherapeutic agents for aggressive thyroid cancers.


   Acknowledgments
 
We are grateful to Jean Ann Gilder (Scientific Communication) for editing the text. We thank Paola Ferraro and Salvatore Sequino for animal manipulations and animal care.


   Footnotes
 
This work was supported by the Associazione Italiana per la Ricerca sul Cancro; the Italian Ministry of Instruction, University, and Research; the Italian Ministry of Health; and a fellowship from the European Community-European Social Fund-FSE (to D.S.C.).

D.S.-C. and R.S. are recipients of a fellowship from the BioGeM scar.l. (Biotecnologia e Genetica Molecolare nel Mezzogiorno d’Italia) Consortium.

Abbreviations: BS-, Bovine seminal; cRI, cytosolic ribonuclease inhibitor; fmk, fluoromethylketone; HDF, human diploid fibroblasts; HP-, human pancreatic; PARP, poly(ADP-ribose) polymerase; PI3K, phosphoinositide 3-kinase; pNA, p-nitroanilide; RNase, ribonuclease; zVADfmk, Z-Val-Ala-Asp-(OMe)-fluoromethylketone; Akt/PKB, protein kinase B.

Received March 11, 2002.

Accepted February 28, 2003.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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