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From the Istituto dei Tumori di Napoli,*
Naples, Italy;
the Divisione di Ostetricia e Ginecologia,
Facoltà di Medicina e Chirurgia di Catanzaro, Università
degli Studi di Catanzaro, Italy; Catanzaro, and the Dipartimento di
Medicina Sperimentale e Clinica,
Facoltà di Medicina e Chirurgia di Catanzaro, Università
degli Studi di Catanzaro, Catanzaro, Italy
| Abstract |
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| Introduction |
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,
ß, and
) based on their isoelectric point. The ß class comprises
structurally related, highly conserved acidic polypeptides. At first,
it was suggested that these proteins had some role in regulating the
immune response. However, more recent studies suggest that the
expression of thymosins is associated with growth and differentiation
of many cell types. In human tissues, thymosin ß-10 (TB10) is widely
distributed along with another component
of the class, the thymosin ß-4 (TB4).2
Both proteins
have been shown to bind to and sequester G-actin.3-4
TB10
expression is related to cell growth in proliferating tissues, and it
is associated with rat and human brain development.5-8
Previous reports indicated an enhanced TB10 expression in a variety of
human tumors such as renal, medullary thyroid carcinomas and
melanomas.9-11
Recently, we have shown that the TB10 gene
is highly expressed in human thyroid carcinoma cell lines and tissues
whereas its expression is almost undetectable in normal thyroid. The
expression was particularly abundant in the undifferentiated thyroid
carcinomas.12 To verify whether the TB10 gene induction is a general event in human neoplasias, we have analyzed TB10 mRNA levels in colon, breast, uterine, ovarian, germ-cell tumors, colon and esophageal carcinoma cell lines. TB10 overexpression was observed in all of the neoplastic tissues and cell lines. To further characterize the role of this protein as a progression marker, we have analyzed cell lines deriving from normal skin, and carcinogen-induced papillomas and carcinomas. A correlation was found between TB10 mRNA levels and the progression step of the carcinogenesis process. Immunohistochemical analysis on some carcinoma samples confirmed the data obtained at the RNA level. Therefore, TB10 detection may be considered as potential tool in the diagnosis of several human neoplasias.
| Materials and Methods |
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The human colorectal and esophageal carcinoma cell lines, used in this study, were a kind gift of Dr. H. Yamasaki, (International Agency for Cancer Research, Lyon, France). The murine normal and neoplastic skin-derived cell lines were a kind gift of Dr. A. Balmain, (Beatson Institute for Cancer Research, Glasgow, Scotland). C5N is a nontumorigenic immortalized keratinocyte cell line; P1 and P6 derive from papilloma benign lesions obtained from DMBA/TPA treated mice; A5 and Car B derive from murine spindle cell skin carcinoma; B9 derives from squamous skin carcinoma.13,14 All cell lines were cultured in DMEM containing 10% fetal calf serum (Life Technologies LTD, Paisley Scotland).
Breast, colon, and germ-cell tumor samples were collected at the Istituto Nazionale dei Tumori di Napoli, Naples, Italy. Ovarian and uterine carcinoma samples were collected at the Divisione di Ostetricia and Ginecologia of the Facoltà di Medicina e Chirurgia di Catanzaro, Catanzaro, Italy. They were immediately frozen in liquid nitrogen after the surgical removal.
RNA Extraction and Northern Blot Analyses
Total RNA was isolated from cultured cells by a modification of the guanidine thiocyanate method.15 For Northern blot analysis, 20 µg of RNA per sample were electrophoresed on 1.2% agarose gels containing 2.2 mol/L formaldehyde. After the electrophoresis, gels were blotted onto Hybond nylon membranes (Amersham, Little Chalfont, UK). As a TB10 probe, a Sau3A TB10 cDNA fragment 178 bp long was used. A specific TB4 probe was generated by polymerase chain reaction (PCR) amplification of a cDNA fragment based on the human sequence.16 A probe for the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was used to check for uniform RNA loading and integrity. Probes were radiolabeled by the random primer method of Feinberg and Vogelstein17 and used to hybridize Northern blots. Hybridization was carried out under stringent conditions (50% formamide, 42°C), followed by washing in 0.1 x standard saline citrate at 60°C. Filters were exposed to films for 12 to 120 hours. The sizes of the RNAs were determined by comparison with rRNA standards.
Immunohistochemistry
Paraffin sections (56 µm) were deparaffinized and placed in a solution of absolute methanol and 0.3% hydrogen peroxide for 30 minutes, then washed first in distilled water, then in phosphate-buffered saline (PBS), before immunoperoxidase staining. The slides were then incubated overnight at 4°C in a humidified chamber with the antibodies diluted 1:100 in PBS. The slides were subsequently incubated with biotinylated goat anti-rabbit IgG for 20 minutes (Vectastain ABC kits, Vector Laboratories, Burlingame, CA) and then with premixed reagent ABC (Vector) for 20 minutes. For immunostaining, the slides were incubated in diaminobenzidine (DAB-DAKO) solution containing 0.06 mmol/L DAB and 2 mmol/L hydrogen peroxide in 0.05% PBS, pH 7.6, for 5 minutes. After chromogen development, the slides were washed, hematoxylin-counterstained, dehydrated with alcohol and xylene, and mounted with coverslips using a permanent mounting medium (Permount). Micrographs were taken on Kodak Ektachrome film with a photo Zeiss system. The antibodies used in this study were raised against the synthetic peptide TIEQEKRSEIS specific for the TB10 protein18 and affinity-purified against the same synthetic peptide.
| Results |
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Seven colon carcinoma and seven esophageal cell lines were
analyzed for TB10 gene expression. The results are shown in Figure 1, A and B
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Subsequently, the expression of the TB10 gene was evaluated in 9 human
colon carcinoma samples by Northern blot (Figure 1C)
. The level of the
TB10-specific RNA transcript of 0.6 kb was found elevated in almost all
of the neoplastic samples (Figure 1C
, lanes 311) when compared with
those detected in the normal tissue surrounding the lesion (Figure 1C
,
lanes 1 and 2).
TB10 Gene Expression in Human Germ Cell Tumors
We analyzed TB10 gene expression in 32 human tumors of different
histological types (15 seminomas and 17 nonseminomas) by Northern blot
hybridization. The results are shown in Figure 2, A and B
. No expression of
TB10-specific transcript was detected in normal testis tissue (Figure 2A
, lane 1, and Figure 2B
, lane 1), whereas the neoplastic samples
showed very high TB10 mRNA levels. On average, the expression levels
observed in nonseminomas were higher than those detected in seminoma
samples. The TB10 transcript was 0.6 kb in size, as in all of the other
tissue examined. In the past, an additional band 0.75 kb long has been
observed in rat mature testis,18
but it was not detected
in normal and neoplastic human samples, suggesting a species-specific
pattern of TB10 gene expression in this tissue.
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Ten human breast carcinomas, 13 ovarian carcinomas, and 16
uterine tumors were also analyzed for TB10 and TB4 gene expression. The
results (apart from TB4 gene expression in ovarian carcinomas, which is
not shown) are shown in Figure 3
. All of
the tumor samples revealed a high expression of the TB10 and TB4 genes,
whereas very low levels were observed in normal breast, ovary, and
uterus. An additional band of approximately 2.5 kb was detected in 6
out of 10 breast carcinomas after hybridization with both probes. This
band has never been observed in other normal and neoplastic tissues.
Because it was detected with both the probes, we hypothesize
that it might be the product of an unknown, closely related,
ß-thymosin gene. Interestingly, a transcript of a similar size was
detected after serum stimulation of monkey kidney epithelial
cells.19
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To investigate the TB10 gene expression in the process of
carcinogenesis, we used the classical model of mouse skin carcinomas
induced by the combined action of dimethyl-benzanthracene (DMBA) and
12-O-tetradecanoyl-phorbol-13-acetate (TPA). Cell lines were
established from normal skin, benign papillomas, and malignant
carcinomas.14
The cell lines were as described in
Materials and Methods. As shown in Figure 4
, TB10 gene expression was detectable at
low levels in the normal keratinocytes (lane 1), whereas it was quite
abundant in three carcinoma cell lines (lanes 46). Two papilloma cell
lines showed intermediate levels (lanes 2 and 3).
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Immunohistochemical analysis can allow rapid and sensitive screening of histopathological tissues. Therefore, this technique was set up for the detection of TB10 protein using antibodies raised versus a peptide corresponding to the carboxyterminal region of the protein; this region is specific for each ß-thymosin family member.
Paraffin-embedded tissues from ovary, breast, and colon carcinomas and
the respective normal tissues were analyzed. A strong cytoplasmic
staining was observed in all of the neoplastic tissues. Conversely, no
or weak staining was observed in the normal tissues. The results are
summarized in Table 1
. Some
representative cases are shown in Figure 5
. Ovary (Figure 5, B and F)
, breast
(Figure 5D)
, and colon (Figure 5H)
carcinoma tissues showed
immunostaining localized to cytoplasm, whereas a very weak staining was
observed in normal breast, ovary, and colon tissues (Figure 5, C, E, and G
, respectively), with the exception of blood vessels, which showed
an intense TB10 positivity.
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| Discussion |
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The model of mouse skin carcinogenesis induced by chemical carcinogenesis was then used to verify the step of carcinogenesis at which the TB10 gene induction occurs. Very low TB10 expression was observed in normal cells, and an increased expression was already present at the preneoplastic papilloma stages. However, a further increase occurred at the carcinoma stages. Therefore, like the data that describe human melanoma tumors,11 the TB10 gene expression associates with the degree of tumor progression in the experimental model of skin carcinogenesis.
Subsequently, we set up an immunohistochemical technique for the detection of the TB10 protein on paraffin sections, using antibodies generated against a carboxy-terminal peptide fragment specific for the TB10 protein. All of the neoplastic samples analyzed, originating from breast, ovary, and colon carcinomas, showed an intense cytoplasmic staining, whereas no or a very weak staining was observed in normal tissues. Moreover, the reaction appeared specific for the neoplastic cells. Therefore, the analysis of a large series of tumor samples associated with the clinical data are required to assess the potential of TB10 protein detection as a diagnostic and, perhaps, a prognostic tool for human neoplasias.
As far as the role of TB10 overexpression in human carcinogenesis is concerned, it is known that the ß-thymosins bind and sequester G-actin,3,4 which plays a pivotal role in maintaining the cellular structure. The dynamic equilibrium between monomeric and filamentous actin is certainly perturbed in transformed cells, and, because motility and adhesion processes are important in tumor progression and metastasis formation, the finding of elevated TB10 expression in malignant tumors may be correlated with the pattern of actin polymerization. It is known that overexpression of exogenous TB10 causes disassembly of actin stress fibers,20 and, therefore, the changes observed in malignant growth may reflect the loss of filamentous actin with the corresponding increased size of G-actin pool.
Another recently discovered member of the ß-thymosin family, the thymosin ß-15 (TB15) gene, has been shown to be up-regulated in prostatic carcinoma cell lines that have metastasizing ability. This gene also inhibits actin polymerization and might thus increase cellular motility.21 At variance with prostate carcinomas, TB15 appears to be induced at the early stages of breast carcinomas. In fact, it has been detected in in situ ductal carcinoma.22 We have analyzed TB15 gene expression in our tumor samples; no detectable TB15 expression was observed by Northern blot using total RNA (data not shown). This suggests that the regulation of TB15 expression differs from that of TB4 and TB10.
In conclusion, the results of the present study prompt us to undertake further investigations to assess TB10 gene expression as a useful marker for diagnosis and prognosis of a large variety of human cancers.
| Footnotes |
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Supported in part by Ministero della Sanità, FSN1994, Rome, Italy.
Accepted for publication May 11, 1999.
| References |
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