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Thyrotropin-independent mutant clones from FRTL5 rat thyroid cells: hormonal control mechanisms in differentiated
Abstract:
Mutant cells varying in the pathways of their responses to hormonal stimulation are useful in defining the subcellular steps in the mechanisms of hormone action. FRTL5, a strain of normal and differentiated cells originally derived from adult rat thyroids, which depends on TSH for growth in vitro, was used to produce five TSH-independent mutants, after chemical mutagenesis and selection in medium lacking TSH. Their characterization and comparison with wild type cells demonstrated full retention of differentiated thyroid function markers such as thyroglobulin production and active iodide transport, and a slower growth rate. Characterization of cAMP metabolism in mutants revealed levels of basal cAMP and adenylate cyclase and phosphodiesterase activities similar to those of wild type cells kept in a nonproliferative state in medium lacking TSH. Adenylate cyclase responsiveness to very low doses of TSH (10(-12) M) was fully retained in all mutant clones, but the TSH-dependent cAMP elevation, although comparable to that reported in wild type cells, was not followed by significant growth stimulation in mutants. These findings demonstrate that the persistence of functional TSH receptors in these cells and that of growth regulation in them is independent of cAMP elevation.
Insights
Mutant rat thyroid cells (FRTL5) became independent of thyroid-stimulating hormone (TSH) for growth. These cells retained TSH receptors and differentiated functions, showing growth regulation is independent of cAMP elevation.
Area of Science:
- Endocrinology
- Cell Biology
- Molecular Biology
Background:
- Mutant cell lines are crucial for dissecting complex cellular signaling pathways.
- Thyroid-stimulating hormone (TSH) regulates thyroid cell growth and function through specific cellular mechanisms.
- Understanding hormone action requires identifying key molecular steps and regulatory points.
Purpose of the Study:
- To generate and characterize TSH-independent FRTL5 cell mutants.
- To investigate the role of cAMP signaling in TSH-mediated growth regulation.
- To determine if functional TSH receptors and growth regulation are linked to cAMP elevation.
Main Methods:
- Chemical mutagenesis of FRTL5 rat thyroid cells.
- Selection of TSH-independent mutant clones in vitro.
- Assessment of differentiated thyroid function markers (thyroglobulin, iodide transport).
- Analysis of cAMP metabolism, including basal cAMP levels and adenylate cyclase/phosphodiesterase activities.
- Evaluation of TSH-induced cAMP elevation and subsequent growth stimulation in mutant cells.
Main Results:
- Five TSH-independent FRTL5 cell mutants were successfully generated.
- Mutants retained differentiated thyroid functions and showed slower growth rates.
- Basal cAMP levels and adenylate cyclase/phosphodiesterase activities were similar to wild-type cells.
- All mutants exhibited retained adenylate cyclase responsiveness to low-dose TSH.
- TSH-induced cAMP elevation in mutants did not lead to significant growth stimulation.
Conclusions:
- FRTL5 cell mutants demonstrate that functional TSH receptors can persist independently of TSH-driven growth.
- Growth regulation in these cells is uncoupled from cAMP elevation, suggesting alternative signaling pathways.
- These findings highlight the complex, potentially divergent, mechanisms of TSH action on thyroid cell proliferation and differentiation.