# Effect of Short-Term and Long-Term Non-Physiological T3 Concentrations on Cardiac Stromal Cells: From Cellular Response to In Vivo Adaptation

## TL;DR

This study shows how abnormal thyroid hormone levels affect heart cells, causing short-term changes in cell growth and metabolism that eventually return to normal.

## Contribution

The study reveals the acute and transient effects of non-physiological T3 levels on cardiac stromal cells and their alignment with in vivo metabolic responses.

## Key findings

- Non-physiological T3 levels cause acute increases in cardiac stromal cell proliferation and ECM-related gene expression.
- Cardiac glucose uptake increases in acute hypothyroidism and decreases in acute hyperthyroidism, normalizing with chronic exposure.
- Changes in gene expression and metabolism are transient and return to baseline after prolonged exposure to altered T3 levels.

## Abstract

Background/Objectives: Epidemiological and clinical studies have linked both hypothyroidism and hyperthyroidism to adverse cardiac outcomes, including heart failure and myocardial fibrosis. Triiodothyronine (T3), a biologically active thyroid hormone, is important for cardiovascular homeostasis. While the effects of physiological and non-physiological T3 levels on cardiomyocytes have been extensively investigated, the impact of hypothyroidism and hyperthyroidism on cardiac stromal cells (CSCs), which constitute the majority of the cells in the heart, remains understudied. Given CSCs’ essential role in extracellular matrix (ECM) remodeling and paracrine signaling, understanding their response to altered T3 states is necessary to fully elucidate the thyroid hormone-induced cardiac responses. Methods: Cardiac stromal cells were isolated from human atrial appendages and cultured under hypothyroid (0 nM T3), euthyroid (2.5 nM T3), and hyperthyroid (25 nM T3) conditions for 24 (short term) and 120 h (long term). The cells were harvested after 24 h of treatment using trypsin and automatically counted, and their ECM-related gene and growth factor expression levels were assessed using quantitative RT-PCR. Cardiac glucose uptake in hypothyroid, euthyroid, and hyperthyroid mice was monitored using [18F]-FDG PET/CT at acute (7 days) and chronic (42 days) time points. Results: Both hypo- and hyperthyroidism significantly increased the number of CSCs harvested after 24 h. There were acute alterations in the expression of the ECM-related genes COL1A1, COL3A1, TIMP3 (p < 0.05), and TIMP1 (p < 0.01). Similarly, growth factors such as PDGF-A (p < 0.001), TGF-b, and IGF1 (p < 0.05) were transiently upregulated under non-physiological T3 conditions, especially hypothyroidism. Most of these alterations were attenuated or reversed at the 120 h time point. In vivo PET imaging revealed significant increases in cardiac glucose uptake under acute hypothyroidism (p < 0.05) and decreases under acute hyperthyroidism (p < 0.05). However, these metabolic shifts normalized with chronic exposure, paralleling the transient nature of the gene expression changes observed in vitro. Conclusions: Non-physiological T3 concentrations induce proliferation and changes in ECM-related and growth factor gene expression in CSCs. Most of these changes are acute and return to normal levels after chronic exposure. These transient cellular responses correlate closely with the cardiac metabolic response patterns to acute and chronic hypothyroidism and hyperthyroidism.