Thyroidectomy induces thyroglobulin formation by parotid salivary glands in rats

Highlights

• Thyroidectomized rats had significantly higher TSH and lower T4 serum levels.

• The PGs of these rats depicted a newly expressed thyroglobulin protein.

• TG was localized in the interstitium, intralobular ductal cells, and within their lumen.

• This functional change accompanied by structural changes in PGs.

• PG of the thyroxin-supplemented depicted vanishment of TG.

Abstract

Structural changes in parotid gland (PG) were previously reported following ablation of thyroid gland. However, the functional alterations (especially for proteins) have not been elucidated yet. Herein, we investigated the effect of rat thyroidectomy on PG structure and protein content and studied the ability of thyroxin-supplementation to alleviate the associated structural and functional changes. Male young adult 4-month old albino rats (n = 48) were allocated equally into 4 groups (control, sham-operated, thyroidectomized, and thyroxin-supplemented). PGs were examined histologically, and their proteins expression and localization were analyzed using western blot (WB) and immunohistochemistry (IHC), respectively at 3 w and 5 w post-surgery. Functionally, PGs of thyroidectomized rats formed a newly expressed 300 KDa protein, which was confirmed to be thyroglobulin (TG) by WB and IHC, with higher expression at 5 w. TG was localized in the interstitium, within capillaries, in the cytoplasm of the intralobular ductal cells, in the secretory products within the ductal lumen, and in the cytoplasm of individual small cells at the periphery of the acini. This functional change accompanied by structural changes in PGs (presence of dark and light acinar cells, TG-like colloid material, and high periductal vasculature). Noteworthy, PG of the thyroxin-supplemented depicted vanishment of TG. From these data, it could be concluded that thyroidectomy could alter the morphology and function of the parotid that induce a thyroid-like reprogramming of the parotid to secrete TG and thyroxin supplementation could alleviate this effect.

Introduction

Parotid gland (PG) is a serous salivary gland which secretes proteins via two different pathways: the regulated pathway (unidirectional release of proteins upon receiving stimulus) and the constitutive (endocrine) pathway (continuous and non-directional release of proteins dependant on their translation rate) (Ball et al., 1993). Presence of salivary growth factors in rat serum after submandibular gland (SMG) and PG ductal inoculation confirmed the endocrine function of these glands (Castle and Castle, 1998). Unlike other salivary glands, PG has high density of subepithelial capillary network with fenestrated endothelium which could signify the unique PG microvasculature and its putative endocrine function (Nilsson and Fagman, 2017). For these criteria, PG was selected as a model for our experiment.

PG and thyroid glands have close embryological site in the vicinity of the first pharyngeal arch (Tartaglia et al., 2014), suggesting a physiological relation between the two glands. Previous studies reported some structural and functional changes in salivary glands following loss of function experiments (by ablation or chemicals) and gain experiments for parathyroid and thyroid glands. Adriaansen et al. (2011) found normalization of serum calcium and high levels of acinar expression of the parathyroid protein in PG of parathyroidectomized rats. Additionally, methimazole-induced hypothyroidism resulted in PG structural changes (glandular fibrosis and pyknotic acinar nuclei), suggesting that thyroid hormones were essential for PG normal function (Hayat et al., 2010). However, this previous study induced hypothyroidism chemically, rather than by thyroidectomy which is more effective. Moreover, Gorr et al. (2005) reported that thyroid hormones supplementations induced formation of granular convoluted tubules (GCT)-like structure, which secretes various GCT-specific polypeptides in PG (Miyaji et al., 2008).

In general, these previous thyroid and parathyroid gain and loss of function studies did not detail the functional changes (particularly at protein level) in PG and to the best of our knowledge, no previous study analyzed whether PG can secrete thyroglobulin protein following hypothyroidism. We hypothesized that thyroidectomy could alter the morphology and function of the PG that induce a thyroid-like reprogramming of the parotid to secrete thyroglobulin. Therefore, this study was conducted to investigate the effect of rat thyroidectomy on PG histological structure and protein content and studied the ability of thyroxin-supplementation to alleviate the associated structural and functional changes.