ABSTRACT
The inhibin field has been perplexed by the information that inhibin alpha is a tumour suppressor in mice yet is elevated in women with ovarian cancer. Furthermore, we have consistently observed a down-regulation or loss of inhibin alpha in prostate cancer patient samples and cell lines. However, our latest data have prompted us to re-evaluate the role of inhibin alpha in prostate and other cancers. Using the analogy of TGF-beta as a springboard for our hypothesis, we offer a unifying model whereby the previously conflicting observations in mice, men and women can be explained. We propose that initially inhibin alpha is tumour-suppressive and is expressed in benign and early-stage primary cancers. Tumour-suppressive inhibin alpha is then silenced as the tumour progresses but is reactivated as a pro-metastatic factor in advanced, aggressive cancers.
Subject(s)
Inhibins/metabolism , Ovarian Neoplasms/metabolism , Prostatic Neoplasms/metabolism , Transforming Growth Factor beta/metabolism , Animals , Carcinogens/metabolism , Disease Progression , Female , Genes, Tumor Suppressor/physiology , Humans , Inhibins/genetics , Male , Mice , Models, Biological , Neoplasm Proteins/metabolism , Neoplasms/metabolism , Severity of Illness Index , Transforming Growth Factor beta/geneticsABSTRACT
betaC-activin expression was assessed in rat tissues, using reverse transcription and real-time polymerase chain reaction, Western blotting and immunohistochemistry with a specific monoclonal antibody. betaC-activin mRNA was predominantly expressed in liver, but significant amounts were found in rat whole pituitary extracts (n = 5), and in three of five extracts of ovary, testis, and adrenal gland. Specific betaC-activin immunoreactivity was demonstrated in the cytoplasm of hepatocytes, neurosecretory cell terminals in posterior pituitary, ovarian primordial follicles, theca interna, large luteal cells and rete ovarii, spermatogonia, pachytene spermatocytes and Leydig cells of the testis, uterine endometrium, oviduct epithelium and zona glomerulosa of the adrenal. The observation of stage-specific expression in gonadal cells suggests this activin subunit has specific roles, different from those of other activin/inhibin subunits. Small amounts of mRNA in the presence of significant betaC-activin protein highlights the importance of examining betaC-activin expression at both the mRNA and protein level.
Subject(s)
Adrenal Glands/metabolism , Inhibin-beta Subunits/metabolism , Liver/metabolism , Ovary/metabolism , Testis/metabolism , Animals , Blotting, Western , Female , Immunoenzyme Techniques , Inhibin-beta Subunits/genetics , Male , RNA, Messenger/genetics , RNA, Messenger/metabolism , Rats , Rats, Sprague-Dawley , Reverse Transcriptase Polymerase Chain ReactionABSTRACT
Activins are formed by dimerization of beta-subunits and, as members of the TGF-beta superfamily, have diverse roles as potent growth and differentiation factors. As the biological function of the activin C homodimer (betaC-betaC) is unknown, we sought to compare activin A (betaA-betaA), B (betaB-betaB), and C homodimer bioactivities and to investigate the consequences of activin betaC-subunit overexpression in prostate tumor cells. Exogenous activin A and B homodimers inhibited cell growth and activated activin-responsive promoters. In contrast, the activin C homodimer was unable to elicit these responses. We previously showed that the activin betaC-subunit heterodimerized with activin betaA in vitro to form activin AC. Therefore, we hypothesize that the activin betaC-subunit regulates the levels of bioactive activin A by the formation of activin AC heterodimers. To test this hypothesis, we measured activin AC heterodimer production using a novel specific two-site ELISA that we developed for this purpose. In the PC3 human prostate tumor cell line, activin betaC-subunit overexpression increased activin AC heterodimer levels, concomitantly reduced activin A levels, and decreased activin signaling. Overall, these data are consistent with a role for the activin betaC-subunit as a regulatory mechanism to reduce activin A secretion via intracellular heterodimerization.
Subject(s)
Activins/metabolism , Inhibin-beta Subunits/physiology , Prostate/metabolism , Animals , CHO Cells , Cell Division/drug effects , Cell Line , Cricetinae , Dimerization , Enzyme-Linked Immunosorbent Assay , Humans , Inhibin-beta Subunits/genetics , Inhibin-beta Subunits/metabolism , Inhibin-beta Subunits/pharmacology , Male , Promoter Regions, Genetic/drug effects , Promoter Regions, Genetic/physiology , Sensitivity and Specificity , Tumor Cells, CulturedABSTRACT
Amounts of betaA-activin, betaC-activin, activin receptor subunits ActRIIA and ActRIIB mRNA, and betaA- and betaC-activin subunit protein immunoreactivity were investigated in male Lewis rats, either untreated or after 5 or 10 weeks of CCl(4) treatment to induce cirrhosis. Apoptosis was assessed histologically and with an in situ cell death detection kit (TUNEL). Reverse transcription and polymerase chain reaction were used to evaluate mRNA levels. Activin betaA- and betaC-subunit immunoreactivity was studied by immunohistochemistry using specific monoclonal antibodies. Hepatocellular apoptosis (P<0.001), increased betaA- and betaC-activin mRNAs (three- to fourfold; P<0.01) and increased betaA- and betaC-activin tissue immunoreactivity were evident, whereas ActRIIA mRNA concentrations fell (30%; P<0.01) after 5 weeks of CCl(4) treatment. The mRNA concentrations at 10 weeks were not significantly different from controls, despite extensive hepatic nodule formation. We conclude that the increased activin subunit expression is associated with apoptosis, rather than hepatic fibrosis and nodule formation.
