Thyroid Proliferation

1.2.1 Development of the Thyroid

The thyroid gland has a dual embryonic origin, with the most abundant thyroid follicular cells, arising from the embryonic endoderm (thyroid anlage), which emerges as a visible bud. The thyroid develops from the anterior foregut endoderm in which progenitor cells expressing four critical transcription factors, NKX2.1, PAX-8, FOXE-1, and Hhex, assemble to form the thyroid bud [21]. Thyroid C cells (which secrete calcitonin) arise from the ultimobranchial bodies (originating from pharyngeal pouch of the embryo). Thyroid cell

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specification occurs in parallel to morphological and biochemical changes that make these cells clearly different from their neighbouring cells. Later the thyroid lobes expand and the gland obtains its definitive form, with a narrow isthmus connecting the two lateral lobes. Fully differentiated follicular cells express different thyroid-specific transcription factors. The beginning of thyroid development is on the floor of the primitive pharynx, which is followed by a migration to reach its final position at the pharynx, where the two lobes expand and the follicles formed (reviewed in [3]).

After migration, thyroid cells express the essential proteins to start hormone biosynthesis. Expression of TG, TPO and TSHR starts first [22], then NIS, Duox1 & Duox2 [23, 24]. The biosynthesis of T4 starts before T3 [25]. Maturation of the hypothalamic–pituitary–thyroid system is also required for the hormone to be synthesised [26]. In addition to their function at the early embryonic stage, NKX2.1, PAX-8, FOXE-1, and Hhex have been shown to be playing an important role in the expression of these essential proteins of the gland. FOXE-1 and PAX-8 have binding domains in the promoter regions of the TG, TPO, TSHR, and NIS genes, however NKX2.1 and PAX-8 have overlapping binding domains in TG and TPO genes promoters. In addition Hhex has been shown to play a fundamental role not only in the formation of the thyroid but also in the functional differentiation of the gland [21, 27-29]. Although these factors are present in other tissues, they are only expressed together in the thyroid. NKX2.1 and FOXE-1 are required for differentiation of both follicular and C cells of the thyroid, while PAX-8 is important only in differentiation of follicular cells. On the other hand lack of Hhex causes absence of the above three factors while absence of NKX2.1 and

PAX-8 causes absence of Hhex [3, 29].

Mutations in FOXE-1, NKX2.1 or PAX-8 have been shown to be the cause of severe congenital hypothyroidism (CH) in humans, while germ line deletions of these transcription factors in animal models have provided important information on thyroid dysgenesis as a genetic disease [30-32]. A recent study [33] has reported that a transient overexpression of NKX2.1 and PAX-8 is sufficient to direct mouse embryonic stem-cell differentiation into thyroid follicular cells that are capable for iodide organification in vitro. The cells have shown an ability to rescue thyroid hormone deficits in athyroid animals. That has opened a new field for treating hypothyroid patients using stem-cell technologies.

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In addition insulin like growth factor-1 (IGF-1) and epidermal growth factor (EGF) can promote thyroid cell proliferation in culture [34, 35]. They are expressed during embryonic life and could be the primary regulators of thyroid growth at that time [36, 37], (will describe more in ‎1.1.2).

1.2.2 Physiological regulation of Thyroid growth

Human thyroid cells divide about five times in adulthood, which reveals that there is a slow constant turnover (division and death) of these cells. Adult thyroids maintain their size with a slow cell turnover, but keep the capacity to grow by cell hypertrophy and proliferation when stimulated. During adulthood TSH enhances thyroid growth at several stages as TSH signalling has been shown to be the best growth stimulus for adult thyroid cells. However, this signalling is not a global regulator of thyroid function during the embryonic stage [25, 27].

On the other hand, some non-thyroidal factors act as thyroid proliferators, such as IGF-1 [38], transforming growth factor ß (TGF-ß)[39], and EGF [40]. Increased expression of these factors has been reported in proliferating thyroid tissue. Although IGF-1 plays a role in regulating growth in children generally and has anabolic effects in adults, it may also effect thyroid function and growth [41]. Cooperation between TSH, the major goitrogen, and IGF-1 to regulate thyroid growth has been suggested, as studies showed both to be required for cell growth [42]. Moreover, enlarged thyroid and reduced TSH levels were also reported when IGF-1 and IGF-1 receptor (IGF-1R) were overexpressed in thyroid [43]. TGF-ß on the other hand, is a type of cytokine which has a role in the cell cycle (will be described in ‎1.6.2) and acts as an anti-proliferative factor [44]. In contrast, TGF-ß has been shown to down regulate NIS in BRAFV600E mutant rat thyroid cells and its overexpression was observed in aggressive forms of human PTC in which absence of NIS was reported [45]. The growth factor, EGF binds to its receptor and stimulates cell growth, differentiation, proliferation and survival [46]. It has been reported to be increased in gene expression arrays of PTC tumours and shown to be a MAPK pathway activator [47].

Thyroid gland was shown to be more active in children and adolescents than in young adults, while elderly thyroid is less active[48]. Several studies [48-50]on T4 and T3 levels during adulthood have pointed out that T4 secretion is reduced by age, which affects T3 levels,

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because most circulating T3 derives from the deiodination of T4. However some studies have shown decrease in T3 but still in the normal range. Others have not found a decrease in T3 even in very old individuals [51]. The fact that the mean calculated ratios of serum rT3, T4, free T3 and free T4 progressively increased with age suggest that the relative rate of T4 conversion to rT3 increases with age during childhood and adolescence. In the thyroids of both adults and children, TSH plays the main role in thyrocyte growth (reviewed in [3]).