Inhibin and activin are members of the TGF beta superfamily of growth and differentiation factors. They were first identified as gonadal-derived regulators of pituitary FSH and were subsequently assigned multiple actions in a wide range of tissues. More recently, the inhibin alpha subunit was considered as a tumor suppressor based on functional studies employing transgenic mouse models. This review evaluates the functional and molecular evidence that the inhibin alpha subunit is a tumor suppressor in endocrine cancers. The evaluation highlights the discrepant results from the human and mouse studies, as well as the differences between endocrine tumor types. In addition, we examine the evidence that the activin-signaling pathway is tumor suppressive and identify organ-specific differences in the actions and putative roles of this pathway in endocrine tumors. In summary, there is a considerable body of evidence to support the role of inhibins and activins in endocrine-related tumors. Future studies will define the mechanisms by which inhibins and activins contribute to the process of initiation, promotion, or progression of endocrine-related cancers.

Tissue-specific aromatase production is significant in breast cancer and osteoporosis. Prostatic aromatase expression has been equivocal, and any local actions of estrogens are considered secondary to centrally mediated androgen suppression. We examine local aromatase expression and estrogen biosynthesis in the human prostate. Pure samples of stroma and epithelia from biopsy tissues were isolated by laser capture microdissection. Aromatase protein was detected by Western blot analysis, mRNA by RT-PCR, and enzyme activity by tritiated water assay, whereas promoter use was examined by real-time PCR. In nonmalignant prostate tissues, aromatase mRNA expression was absent from epithelium, but did localize to stroma. Presence of protein was confirmed, and expression was driven by promoter PII. Aromatase was expressed and active in LNCaP, PC3, and DU145 cells in addition to microdissected epithelial tumor cells; benign prostate epithelial cells showed no expression or activity. Promoter use in LNCaP and microdissected tumor cells was via PII, whereas PC3 and DU145 cells used promoter I.4. This study demonstrates local estrogen biosynthesis in prostate-induced aromatase gene expression in malignancy and potential alteration of aromatase promoter use with disease progression. These data provide a basis for continued investigation of local estrogen production and its potential role in prostate disease.

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.

Mesenchymal stem cells (MSCs) are multipotent cells that have the potential to differentiate into various mesenchymal lineages in vitro and in vivo. Due to their availability from tissues such as bone marrow, synovium, fat, and muscle, and their highly proliferative capacity, MSCs have evoked interest as a potential cell source for repair and regeneration of various types of tissues. Characterization by the expression of a panel of surface markers and the ability of MSCs to undergo multilineage differentiation is the benchmark for identifying this stem cell population. In this chapter, the protocols for the differentiation of MSC to chondrogenic, osteogenic, and adipogenic lineages and histological and immunostaining protocols for confirming trilineage differentiation of the MSC cells are described.

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