Inhibition of experimental choroidal neovascularization by overexpression of tissue inhibitor of metalloproteinases-3 in retinal pigment epithelium cells

PURPOSE

To evaluate the feasibility of introducing exogenous tissue inhibitor of metalloproteinases-3 gene into the rat retinal pigment epithelium using hemagglutinating virus of Japan liposomes and to assess the effect of tissue inhibitor of metalloproteinases-3 overexpression in retinal pigment epithelium cells on the formation of experimental choroidal neovascularization.

METHODS

Hemagglutinating virus of Japan liposomes containing hemagglutin epitope-tagged tissue inhibitor of metalloproteinases-3 gene were injected into the subretinal space in rat eyes. Localization of oligonucleotides was evaluated by fluorescence microscopy. Exogenous tissue inhibitor of metalloproteinases-3 mRNA expression was assessed by reverse transcribed polymerase chain reaction. Exogenous tissue inhibitor of metalloproteinases-3 protein expression was visualized by immunostaining with monoclonal antibody 12CA5 against the hemagglutin epitope. Three days after transfection of tissue inhibitor of metalloproteinases-3 gene into retinal pigment epithelium cells, intense laser photocoagulation was performed and the incidence of choroidal neovascularization was assessed by fluorescein fundus angiography.

RESULTS

Exogenous tissue inhibitor of metalloproteinases-3 mRNA expression in the choroid and retina was detected on day 3. The efficiency of tissue inhibitor of metalloproteinases-3 gene transfection into retinal pigment epithelium cells was greatest on day 7 and decreased gradually thereafter. The incidence of choroidal neovascularization in tissue inhibitor of metalloproteinases-3 gene-transfected eyes was markedly decreased compared with controls.

CONCLUSIONS

This study shows that tissue inhibitor of metalloproteinases-3 gene can be transferred into rat retinal pigment epithelium using the hemagglutinating virus of Japan-liposome method and that tissue inhibitor of metalloproteinases-3 gene overexpression can inhibit development of experimental choroidal neovascularization. This method may represent a future treatment modality for human macular degeneration associated with choroidal neovascularization.

Gene therapy for attenuating cardiac allograft arteriopathy using ex vivo E2F decoy transfection by HVJ-AVE-liposome method in mice and nonhuman primates

Cardiac allograft arteriopathy, which limits the long-term survival of recipients, is characterized by diffuse intimal thickening composed of proliferative smooth muscle cells. The transcription factor E2F plays a pivotal role in the coordinated transcription of cell-cycle regulatory genes. To test the hypothesis that double-stranded DNA with specific affinity for E2F (E2F decoy) is effective in preventing intimal hyperplasia, we performed ex vivo single intraluminal delivery of E2F decoy into cardiac allografts of mice and Japanese monkeys using the hemagglutinating virus of Japan (HVJ) artificial viral envelope-liposome method. In murine models, antisense cyclin-dependent kinase 2 (cdk2) kinase oligodeoxynucleotide (ODN) and no transfers were performed to compare the effects. Severe intimal thickening was observed, and multiple cell-cycle regulatory genes were enhanced in untreated allografts. E2F decoy prevented neointimal formation and suppressed these genes for up to 8 weeks, whereas antisense cdk2 kinase ODN had limited effects. In primate models, E2F decoy dramatically prevented neointimal thickening and suppressed multiple cell-cycle regulatory genes, whereas intimal thickening developed in the nontransfected or mismatch decoy-transfected allografts. Gel mobility shift assay proved the specific effects of E2F decoy, and reverse transcriptase-polymerase chain reaction documented that neither complication nor dissemination of HVJ into other organs was observed. We demonstrate that ex vivo gene delivery to allografts is a potent strategy to modify allograft gene expression, resulting in prevention of graft arteriopathy without systemic adverse effects.

In vivo transfer of antisense oligonucleotide against urinary kininase blunts deoxycorticosterone acetate-salt hypertension in rats

We have previously reported that the renal kallikrein-kinin system suppressed the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Kinins were degraded in the kidney mainly by carboxypeptidase Y (CPY)-like kininase. Blockade of renal kinin degradation may reduce hypertension in the developmental stage. We constructed an antisense oligonucleotide against rat CPY homologue (5'-CAT-CTC-TGC-TTC-CTT-GTG-TC-3', AS) and its randomized control oligonucleotide (5'-TCC-TTC-CTG-CTT-GAG-TTC-CT-3', RC), and prepared an HVJ-liposome complex that prolongs and increases the effectiveness of the antisense oligonucleotide. Antisense oligonucleotide was transfected (25 nmole rat(-1), in terms of nucleotide) into the kidney from the renal artery. Blood pressure was measured through a catheter inserted into the abdominal aorta. Mean blood pressure (MBP) in DOCA-salt treated (for 2 weeks) Sprague Dawley strain rats was 130+/-3 mmHg (n=11), and was reduced significantly (P<0.05) more by AS transfection (122+/-4 mmHg, n=6) than by RC treatment (137+/-6 mmHg, n=5) 4 days after the transfection. This reduction in MBP was accompanied by increased urinary sodium excretion (AS, 8.4+/-1.5 mmole day(-1); RC, 4.6+/-0.5 mmole day(-1), P<0.05) and a reduction in urinary CPY-like kininase activity. Ebelactone B (5 mg kg(-1), twice a day, p.o.), an inhibitor for urinary CPY-like kininase, also reduced MBP and induced natriuresis to the same degree as AS. Lisinopril, an inhibitor for angiotensin converting enzyme (ACE) failed to reduce the elevated MBP. These results suggest that CPY-like kininase may have more contribution than ACE to degrade kinin in the kidney, and that knockdown of CPY-like kininase in the kidney may partly prevent rat DOCA-salt hypertension.

Ribozyme oligonucleotides against transforming growth factor-beta inhibited neointimal formation after vascular injury in rat model: potential application of ribozyme strategy to treat cardiovascular disease

BACKGROUND

Because the mechanisms of atherosclerosis or restenosis after angioplasty have been postulated to involve an increase in transforming growth factor (TGF)-beta, a selective decrease in TGF-beta may have therapeutic value. Thus, we used the ribozyme strategy to actively cleave the targeted gene to selectively inhibit TGF-beta expression.

METHODS AND RESULTS

We constructed ribozyme oligonucleotides (ONs) targeted to the sequence of the TGF-beta gene that shows 100% homology among the human, rat, and mouse species. The specificity of ribozyme against TGF-beta gene was confirmed by selective inhibition of TGF-beta mRNA in cultured vascular smooth muscle cells as well as balloon-injured blood vessels in vivo. Importantly, the marked decrease in TGF-beta resulted in significant inhibition of neointimal formation after vascular injury in a rat carotid artery model (P:<0.01), whereas DNA-based control ONs and mismatched ribozyme ONs did not have any inhibitory effect on neointimal formation. Inhibition of neointimal formation was accompanied by (1) a reduction in collagen synthesis and mRNA expression of collagen I and III and (2) a significant decrease in DNA synthesis as assessed by proliferating cell nuclear antigen staining. Moreover, we modified ribozyme ONs containing phosphorothioate DNA and RNA targeted to the TGF-beta gene. Of importance, modified ribozyme ONs showed a further reduction in TGF-beta expression.

CONCLUSIONS

Overall, this study provides the first evidence that selective blockade of TGF-beta resulted in inhibition of neointimal formation, accompanied by a reduction in collagen synthesis and DNA synthesis in a rat model. We anticipate that modification of ribozyme ON pharmacokinetics will facilitate the potential clinical utility of the ribozyme strategy.

The effects of DCG-IV and L-CCG-1 upon phencyclidine (PCP)-induced locomotion and behavioral changes in mice

The behavioral changes of mice induced by acute and repeated i.p. injection of phencyclidine (PCP) were observed by measuring locomotor activity and stereotyped behavior. Then, the effects of metabotropic glutamate receptor (mGluR) agonists, DCG-IV and L-CCG-1, on the above behavioral changes induced by PCP were found. The effects of DCG-IV were very strong and completely depressed the PCP-induced hyperlocomotion. The effects of L-CCG-1 were not so strong. Repeated injection of PCP for 20 days into mice induced lower locomotor activity than that in acutely injected mice. These behavioral changes may be related with the negative symptoms of schizophrenia. In order to examine some molecular mechanisms of PCP-induced behavioral changes, Northern blot analysis of total RNA from prefrontal cortical tissues of mice treated with PCP, DCG-IV, and L-CCG-1 was carried out.

Human parvovirus B19 capsid antigen in granulocytes in parvovirus-B19-induced pancytopenia after bone marrow transplantation

A patient with refractory anemia with an excess of blasts in transformation developed pancytopenia and a concurrent interstitial pneumonia 110 days after allogeneic bone marrow transplantation. Bone marrow examination showed 0.4% giant proerythroblasts and 86.2% granulocytes, some of them large with a bizarre configuration and the others of normal size. Serum folate level was found low, 0.6 ng/ml. Immunocytochemistry with a B19-specific monoclonal antibody MAB8292 revealed B19 capsid antigen only in erythroblasts and large, bizarre granulocytes, but not in granulocytes of normal size. In situ hybridization of bone marrow cells using digoxigenin-labeled DNA probes detecting parvovirus B19 also demonstrated positive signals in 8.5% of marrow cells. Parvovirus B19 DNA was isolated from the serum and the bronchoalveolar lavage fluid of this patient by the polymerase chain reaction. These findings suggest that neutropenia may be caused by an involvement with parvovirus B19 though a deficiency of folic acid may have in part contributed to the genesis of neutropenia in the patient. The relevance of parvovirus B19 to the interstitial pneumonia remains unclear.

Transcription factor decoy for NFkappaB inhibits TNF-alpha-induced cytokine and adhesion molecule expression in vivo

The expression of several cytokines and adhesion molecules is regulated by the transcription factor NFkappaB, which is activated by tumor necrosis factor alpha (TNF-alpha). In this study, we employed a mouse model of nephritis induced by TNF-alpha to examine whether inhibition of NFkappaB activity using transcription factor decoy oligonucleotides (ODN) blocks cytokine and adhesion molecule expression and attenuates the renal inflammatory response. First, we developed a method for delivering FITC-ODN in vivo into mouse kidney glomeruli by employing HVJ-liposome. Then, in order to study the feasibility of decoy strategy in vivo, the reporter gene chloramphenicol acetyltransferase (CAT) driven by three tandemly repeated NFkappaB binding sequences was transfected into the kidney. Intrapenetorial injection of TNF-alpha stimulated CAT expression in vivo, and the increase in CAT expression was completely abolished by NFkappaB decoy ODN, but not scrambled ODN. Therefore, we examined the effect of NFkappaB decoy ODN transfection on TNF-alpha-induced endogenous interleukin (IL)-1alpha, IL-1beta, IL-6, ICAM-1 and VCAM-1 gene expression as assessed by RT-PCR and Northern blotting. Our present data showed that NFkappaB decoy, but not scrambled, ODN abolished TNF-alpha induced gene expression in vivo, as well as glomerular inflammation as assessed by CD45 staining. Taken together, our results suggest the potential utility of NFkappaB decoy strategy for molecular therapy to glomerular inflammatory diseases.

[Treatment outcome in elderly patients with aggressive non-Hodgkin's lymphoma]

Seventy-one patients aged 61-84 years with previously untreated aggressive non-Hodgkin's lymphoma were treated with a doxorubicin-containing regimen and evaluated retrospectively. The patients comprised 49 men and 22 women with a median age of 68 years. The median observation period was 544 days. Histological examination revealed 17 cases of diffuse small cleaved, 11 cases of diffuse mixed, 40 cases of diffuse large, and 3 cases of immunoblastic lymphoma, classified according to the International Working Formulation. When the patients were divided according to the age-adjusted international index, group A (61-64 years; n = 21) comprised 5 low (L)-, 4 low-intermediate (LI)-, 7 high-intermediate (HI)-, and 5 high (H)-risk patients. The corresponding numbers in group B (> or = 65 years; n = 50) were 14, 12, 16, and 8, respectively. The overall three-year survival rate was 50%, being 78% in group A and 36% in group B (P = 0.02), and 77% for L + LI patients and 34% for HI + H patients (P = 0.003). The respective three-year survival rates for L + LI and HI + H patients were 100% and 67% in group A, and 68% and 16% in group B. HI + H patients in group B showed shorter survival than L + LI patients in group B (P = 0.002) and HI + H patients in group A (P = 0.03). The cause of death in most group B HI + H patients was lymphoma, although the dose intensity of doxorubicin, cyclophosphamide and vincristine did not differ significantly from that in the other groups. Thus, HI + H patients aged 65 and over had an essentially poor prognosis.

Transcription factor decoy for NFkappaB inhibits cytokine and adhesion molecule expressions in synovial cells derived from rheumatoid arthritis

OBJECTIVE

Numerous cytokines are expressed in lesions of synovial hyperplasia of patients with rheumatoid arthritis (RA), and their pathophysiological contributions have been the subject of speculation. These genes are regulated by the transcription factor NFkappaB which in turn is activated by tumour necrosis factor-alpha (TNF-alpha) and cytokines. In this study we examined the inhibition of the production of pro-inflammatory cytokines, adhesion molecule and matrix metalloproteinase (MMP) from synovial tissue of patients with RA by the introduction of synthetic double-stranded DNA with high affinity for the NFkappaB binding site.

METHOD

NFkappaB decoy oligonucleotides (ODN) were introduced with the aid of the haemagglutinating virus of Japan (HVJ)-liposome method into synovial tissue or synovial cells derived from patients with RA. The levels of interleukin-1beta (IL-1beta), IL-6, TNF-alpha, intercellular adhesion molecule-1 (ICAM-1) and MMP-1 were determined by means of enzyme-linked immunosorbent assay (ELISA) and Northern blotting analysis. A cell counting kit was used to study the effect of NFkappaB decoy ODN on synovial cell proliferation.

RESULTS

The production of these mediators was significantly inhibited by the introduction of NFkappaB decoy ODN compared with the effect of scrambled decoy ODN. Transfection of NFkappaB decoy ODN resulted in a significant inhibition of synovial cell proliferation as compared with that of scrambled decoy ODN.

CONCLUSION

The results demonstrated in this study suggest the potential usefulness of NFkappaB decoy ODN for gene therapy of inflammatory synovitis of RA.

Predictive factors for central nervous system involvement in non-Hodgkin's lymphoma: significance of very high serum LDH concentrations

Factors predictive for central nervous system (CNS) involvement at presentation were investigated in 152 patients with non-Hodgkin's lymphoma (NHL) except for lymphoblastic cell lymphoma and small noncleaved cell lymphoma. Twelve patients developed CNS involvement during their disease course. The incidence was 7.9% of all the patients studied and 17.0% of the patients with serum LDH concentration > or = two times the upper limit of normal (2N). By univariate analysis, stage IV disease (P = .023), a serum LDH concentration > or = 2 N (P = .009), and bone marrow involvement (P = .016) were risk factors for CNS involvement. Multivariate logistic regression analysis identified a serum LDH concentration > or = 2 N (P = .032) as an independent predictor for CNS involvement. All 12 patients who developed CNS involvement were among the 126 patients with diffuse lymphoma, whereas none of the 17 patients with follicular lymphoma developed CNS involvement, although the difference was not statistically significant. The median survival of the patients with CNS involvement was only 4.5 months. We conclude that a serum LDH concentration > or = 2N at presentation is a significant predictive factor for CNS involvement for NHL patients without lymphoblastic lymphoma and small noncleaved cell lymphoma. Therefore, we would suggest that CNS prophylaxis should be considered for patients with a serum LDH concentration > or = 2N at presentation and diffuse lymphoma once a complete remission is achieved.

Overexpression of CDC25B phosphatase as a novel marker of poor prognosis of human colorectal carcinoma

There is evidence to suggest that CDC25B phosphatase is an oncogenic protein. To elucidate the role of CDC25B in colorectal carcinoma, we examined the expression of CDC25B at the mRNA and protein levels. Reverse transcription-PCR assay indicated that CDC25B was overexpressed in tumor tissues relative to normal mucosa in 6 of 10 cases. Using immunohistochemistry, we identified high expression of CDC25B in 77 of 181 colorectal cases (43%). Univariate analysis showed that high expression was a significant predictor for poor prognosis compared with low expression (5-year survival rate; 59% versus 82%, respectively; P < 0.0001). Multivariate analysis indicated that CDC25B was an independent prognostic marker (risk ratio for death, 3.7; P < 0.0001) even after controlling for various factors such as lymph node metastasis, tumor size, degree of differentiation, and depth of invasion. Furthermore, the level of CDC25B expression clearly predicted the outcome of patients with Dukes' B and Dukes' C tumors. On the other hand, CDC25A mRNA was overexpressed in 9 of 10 colorectal cancer cases, and immunohistochemistry for CDC25A showed high expression in 52 of 111 cases (47%), but no significant correlation with prognosis. Our findings suggest that CDC25B is a novel independent prognostic marker of colorectal carcinoma and that it may be clinically useful for selecting patients who could benefit from adjuvant therapy.

Gene therapy as a potential treatment for restenosis and myocardial infarction

Cardiovascular disease is the leading cause of mortality and morbidity in developed countries. Most conventional therapy is inefficient and tends to treat the symptoms rather than the underlying causes of the disorder. Gene therapy offers a novel approach for the prevention and treatment of cardiovascular diseases. Gene transfer into somatic cells to interfere with the pathogenetic processes contributing to cardiovascular disease may provide such a novel approach for better prevention and treatment of cardiovascular disorders. The major development of gene transfer has importantly contributed to intense investigation of the potential of gene therapy in cardiovascular medicine. The amazing advances in molecular biology have provided a dramatic improvement in the technology that is necessary to transfer target genes into somatic cells. Recently, transfection of cis element double-stranded oligonucleotides (ODN), referred to as decoy ODN, has been reported to be a powerful tool in a new class of antigene strategies for gene therapy. In this article, we review the successful treatment of restenosis and myocardial infarction using the decoy strategy combined with an efficient gene transfer method.

Distinct transcriptional regulation and phylogenetic divergence of human LEFTY genes

BACKGROUND

Mouse lefty1 and lefty2 genes are expressed on the left side of developing embryos and are required for left-right determination. Here we have studied expression and transcriptional regulatory mechanisms of human LEFTY genes.

RESULTS

The human LEFTY locus comprises two functional genes (LEFTY1 and LEFTY2) and a putative pseudogene. LEFTY1 is expressed in colon crypts. However, whereas LEFTY1 mRNA is present in basal cells of the crypts, LEFTY1 protein is localized in the apical region, suggesting that this secreted protein undergoes long-range transport. Human LEFTY2 possesses a left side-specific enhancer (ASE) like mouse lefty2; however, the LEFTY2 ASE shows markedly higher activity in the floor plate than does the lefty2 ASE. In contrast to mouse lefty1, which is expressed predominantly in the floor plate under the control of a right side-specific silencer, human LEFTY1 is expressed mainly in left lateral plate mesoderm under the control of an ASE-like left side-specific enhancer. The presence of FAST-binding sites in the LEFTY1 enhancer (and their absence in lefty1) contributes to the difference.

CONCLUSION

These observations suggest that humans and mice have acquired distinct strategies during evolution for determining the asymmetric expression of LEFTY and lefty genes.

Identification of a 7-cM region of frequent allelic loss on chromosome band 16p13.3 that is specifically associated with anaplastic thyroid carcinoma

A total of 17 primary thyroid cancer specimens including seven anaplastic cancers, two papillary cancers adjacent to the anaplastic cancers, and eight papillary cancers were analyzed for loss of heterozygosity (LOH) on chromosome arm 16p. All tumors of anaplastic cancer showed LOHs at one or more loci, and a 7-cM region of the smallest deleted region was found on 16p13.3 between D16S423 and D16S406. This LOH was specifically found in the anaplastic cancer and not in the papillary thyroid cancer. Our present results suggest localization of the putative tumor suppressor gene on 16p13.3, which is likely to play an important role in the anaplastic transformation of thyroid cancer.

Antisense oligodeoxynucleotide inhibition of vascular angiotensin-converting enzyme expression attenuates neointimal formation: evidence for tissue angiotensin-converting enzyme function

It has been proposed that vascular angiotensin-converting enzyme (ACE) plays an important role in regulating vascular growth. Indeed, ACE inhibitors have been reported to prevent neointimal formation after vascular injury in a rat carotid artery model. However, classic pharmacological experiments cannot exclude the potential contributions of hemodynamics and the circulating renin-angiotensin system (RAS). In this study, we used antisense oligodeoxynucleotide (ODN) to obtain local blockade of vascular ACE expression without effects on systemic hemodynamics and circulating RAS. To increase the effectiveness of antisense action, we modified the hemagglutinating virus of Japan-liposome ODN delivery method by cotransfection with nuclear protein (high mobility group 1 [HMG-1]) and RNase H. In vitro experiments showed the enhanced efficacy of antisense ODN by cotransfection of HMG-1 and RNase H compared with ODN alone. In vivo transfection of antisense ACE ODNs into intact uninjured rat carotid artery resulted in a significant reduction of vascular ACE activity, and cotransfection of HMG-1 and RNase H showed further reduction. We examined the effects of local blockade of vascular ACE expression on neointimal formation after vascular injury. Transfection of antisense ACE ODNs resulted in the attenuation of neointimal formation, whereas sense and scrambled ODNs did not. Blood pressure, heart rate, and serum ACE activity were not affected by antisense treatment. The magnitude of vascular ACE inhibition correlated with the suppression of the neointimal size. Overall, this study demonstrates that local antisense ODN inhibition of vascular ACE expression attenuates neointimal formation independent of hemodynamics and circulating RAS. The results support the existence of a functional tissue angiotensin system in the rat vessel wall.

Transfection of antisense p53 tumor suppressor gene oligodeoxynucleotides into rat carotid artery results in abnormal growth of vascular smooth muscle cells

BACKGROUND

Although loss of activity of an antioncogene, the p53 tumor suppressor gene product, has been postulated in the pathogenesis of human restenosis, little is known about the role of p53 in the regulation of vascular smooth muscle cell (VSMC) growth. In this study, to clarify the role of p53 in the pathogenesis of restenosis, we examined transfection of antisense p53 oligodeoxynucleotides (ODN) into VSMC in vitro and rat carotid artery in vivo.

METHODS AND RESULTS

The specificity of antisense p53 ODN was confirmed by a significant decrease in p53 protein. Transfection of antisense p53 ODN into VSMC resulted in a significant increase in DNA synthesis and cell number as compared with sense and scrambled ODN (P<0.01). Importantly, transfection of antisense p53 ODN into rat intact carotid artery resulted in a significant increase in the ratio of neointima to medial area at 2 and 4 weeks after transfection, accompanied by a significant decrease in p53 protein (P<0.01). Moreover, cotransfection of wild-type p53 plasmid completely abolished neointimal formation induced by antisense p53 ODN. The sustained effect of a single antisense ODN administration was confirmed by the kinetics of ODN in the vessel wall with the use of FITC-labeled ODN.

CONCLUSIONS

Overall, the present study demonstrated that loss of p53 by antisense p53 ODN resulted in an abnormal VSMC growth in vitro and in vivo. These results demonstrated the potential contribution of p53 to the pathogenesis of restenosis.

Mammaglobin B gene as a novel marker for lymph node micrometastasis in patients with abdominal cancers

Mammaglobin B is a recently-isolated gene speculated to belong to the uteroglobin gene family and is overexpressed in primary breast cancers. We investigated mammaglobin B mRNA expression in various cancers of the digestive system. Given the absence of mammaglobin B expression in normal lymph nodes, we also assessed the usefulness of mammaglobin B as a marker for lymph node micrometastases in cancer patients. Mammaglobin B gene transcripts were frequently detected by reverse transcriptase-polymerase chain reaction (RT-PCR) assay in primary tumors of the esophagus (2/3), stomach (7/7), colon (15/15), pancreas (4/6), common bile duct (6/6), cholangioma (2/2) and gall bladder (1/1). Mammaglobin B overexpression was observed in three of 15 cases (20%) of colon cancer, suggesting its possible contribution to colon carcinogenesis. Down-regulated mammaglobin B expression was observed in hepatoma cells in comparison with corresponding non-cancerous livers (3/3). RT-PCR assay of mammaglobin B detected 14 of 15 histologically positive lymph nodes from patients with gastric cancer, colon cancer and cholangioma. Seven of 32 (22%), three of nine (33%), and three of seven (43%) histologically negative nodes from patients with gastric, colon and cholangiocellular carcinoma, respectively, were found to express mammaglobin B mRNA. Our results showed that expression of mammaglobin B was frequently detected in cancers originating in digestive organs, especially adenocarcinomas, and that mammaglobin B gene detected by RT-PCR may be a potentially useful molecular marker for lymph node micrometastases of various digestive organ cancers.

Strong inverse correlation between serum TPO level and platelet count in essential thrombocythemia

Serum thrombopoietin (TPO) levels in 50 essential thrombocythemia (ET) patients were measured using a highly sensitive sandwich ELISA. In nine cases, TPO levels were measured at two points with different platelet counts. ET patients showed significantly higher serum TPO levels (n = 59, 2.70 +/- 2.74 fmol/mL, P < 0.0001) than those of normal individuals (n = 29, 0.83 +/- 0.36 fmol/mL). Twenty-three previously untreated ET patients also showed significantly higher serum TPO levels (1.33 +/- 0.75 fmol/mL, P = 0.0066) than normal individuals. Extremely high serum TPO levels (5.46 +/- 3.68 fmol/mL) were observed in ET patients with normal platelet counts. Furthermore, a strong inverse correlation was found between serum TPO levels and platelet counts in ET patients (R = -0.729, P < 0. 0001). This inverse correlation also held for each of nine cases with two-point TPO measurements. In the clinical course of ET, megakaryocyte mass may parallel the platelet mass before and after chemotherapy. Although it is unknown whether overproduction of TPO exists or not in ET, total platelet and megakaryocyte mass, i.e., the total number of c-Mpl, may play a role to regulate serum TPO levels.