**2.5. Paracrine role played by C cells**

56 Thyroid Hormone

less intense CGRP-positive C cells. (x400).

**CGRP (***Calcitonin Gene-Related* 

**CCP-II (***CT Carboxyl terminal* 

**PTHrP (***Parathyroid Hormone-*

**CART (***Cocaine- and Amphetamine-*

**NERP (***NeuroEndocrine Regulatory* 

**Figure 4.** Immunohistochemical demonstration of chromogranin A (A), calcitonin (B) and CGRP (C) in consecutive sections of the rat thyroid gland. All C cells express the different markers, being slightly

**Calcitonin (CT)** Pig, dog Bussolati and Pearse, 1967

**Somatostatin** Mammals Kameda et al., 1982 ([33] **Katacalcin-I/CCP-I/PDN-21** Human Ali-Rachedi et al., 1983 [31]

*Peptide***)** Mammals Sabate et al., 1985 [30, 40];

**Serotonin** Sheep Barasch et al., 1987 [41] **GRP (***Gastrin-Releasing Peptide***)** Human Sunday et al., 1988 [32] **Helodermin-like peptide** Human, rat, pig, dog, etc. Grunditz et al., 1989 [35] **CCK/ C-terminal gastrin** Rat Arias et al., 1989 [42] **TRH (***Thyroid Releasing Hormone***)** Rat Gkonos et al., 1989 [36] **Neuromedin U** Rat Domin et al., 1990 [34]

*Peptide***)** Human Cohen et al., 1992 [43]

*related Protein***)** Sheep Okada et al., 1995 [44] **Melatonin** Rat Kvetnoy, 1999 [45] **Neuropéptide Y** Human Scopsi et al., 1990 [46] **Ghrelin** Rat, human Raghay et al., 2006 [38] **Pneumadin** Rat Kosowicz et al., 2003 [47]

*Regulated Transcript***)** Pig Wierup et al., 2004 [37] **Motilin** Human Xu et al., 2008 [48]

*Peptide***)** Human Matsuo et al., 2010 [49]

**Table 2.** Regulatory factors identified in the cytoplasm of C cells, according to a chronological order.

[39]

Kameda, 1987 [30, 40]

**Regulatory factors Species Authors**

Apart from their role in calcium homeostasis, C cells are also probably involved in the intrathyroidal regulation of follicular cells. This hypothesis is supported by different features, such as their characteristic 'parafollicular' position, their predominance in the central region of the thyroid lobe – the so-called "C-cell region" [19]– where the most-active follicles of the gland predominate [20], and their implication in the secretion of many different regulatory factors [1, 46, 56]. Some of these regulatory peptides display an inhibiting action on thyroid hormone secretion, such as calcitonin, calcitonin gene-related peptide (CGRP) and somatostatin [1], whereas others act as local stimulators of thyroid hormone synthesis, such as gastrin-releasing peptide (GRP), helodermin, and serotonin [35, 57, 58]. For any of those peptides, a requirement to exert an effective paracrine regulation of follicular cells is the presence of their specific receptors in this endocrine cell population. At this point, the existence of some of those receptors on follicular cells, such as somatostatin or serotonin receptors, has already been described [58, 59]. Furthermore, we have recently demonstrated the expression of TRH-receptor on follicular cells [60].

There is additional evidence that C cells and follicular cells somehow interact functionally. Thus, C cells, in the normal-appearing thyroid tissue adjacent to follicular tumours, have been reported to display reactive changes. These changes may include the development of a C cell hyperplasia [61] or the presence of an increased percentage of immunopositive C cells for GRP [32] or somatostatin [62], being the latter a potent inhibitor of TSH-enhanced mitotic activity and T3 and T4 synthesis by follicular cells. Moreover, we have found hyperplastic changes in the C-cell population of rat thyroid glands in a model of non-hypercalcemic hypothyroidism induced by propylthiouracil administration [63]. As we have recently demonstrated the presence of TSH receptors on C cells [64], we can infer that those hyperplastic changes may be due to the corresponding increased TSH-serum levels. Nevertheless, the influence of local secretion of growth factors by follicular cells themselves, or by the surrounding tissue could not be discarded.

Paracrine Regulation of Thyroid-Hormone Synthesis by C Cells 59

Calcitonin is synthesized as a 136 amino acid precursor which is processed by proteolytic cleavage and by amidation of the carboxy terminal proline residue before being secreted. The gene transcript encoding CT also encodes calcitonin gene related peptide (CGRP), another peptide in the CT family of peptides. The CT/CGRP gene was one of the first described examples of tissue-specific alternative RNA processing, it has six exons, of which the first four are spliced together to generate calcitonin mRNA in C cells, which encodes calcitonin and katacalcin (CCP), while the transcripts from CT/CGRP gene in neurons are processed into CGRP mRNA containing exons 1-3, 5 and 6 [30, 73] (see figure 5). In thyroid C cells 95% of the transcripts encode calcitonin, while more than 98% of the transcripts

**Figure 5.** Schematic representation of alternative RNA processing of CT/CGRP gene. Transcription of the CT/CGRP gene produces a pre-mRNA, which has two polyadenylation sites. From the primary transcript two different mRNAs are produced, one coding for the calcitonin (CT)/katacalcin (CCP) precursor and the other coding for the CGRP precursor. The blue boxes indicate sequences

complementary to exons, the lines between them are the sequences complementary to introns and the

The alternative product of the CT/CGRP gene, a neuropeptide referred to as CGRP, is a 37 amino acid peptide widely expressed in nerves in both the central and peripheral nervous systems and in several neuroendocrine tissues [74]. CT and CGRP have some homologies in the amino terminal region, but are almost entirely different in the rest of the molecule [74]. CGRP is a potent vasodilator, relaxant of mesangial cells and has functional roles in

CT interacts with a member of seven-transmembrane-domain G-protein-coupled receptor superfamily cloned in 1991 [76]. High-affinity CT receptors (CTRs) were characterized on isolated osteoclasts, the physiological CT target cells. Moreover, calcitonin receptors have been identified in a variety of tissues, such as brain, testis, spermatozoa, kidney, skeletal

encode CGRP in the nervous system [69].

black boxes indicate the noncoding sequences of RNA.

response to painful stimuli [75].

The main secretagogue of C cells is calcium, which is concordant with the classical role of calcitonin as a plasma calcium- and bone metabolism-related hormone. Nevertheless, C-cell activity appears to be subjected to a more complex regulation, for which different regulatory peptides, either with inhibitory (somatostatin) or stimulatory effect (glucagon, pentagastrin), have being reported [65]. Furthermore, Suzuki et al. in 1998 [66] reported that C cells expressed the thyroid transcription factor 1 (TTF-1), which is typically expressed by follicular cells and known for its critical role in thyroid-specific gene expression. According to these authors, TTF-1 is calcium-modulated and coordinately regulates genes involved in calcium homeostasis in C cells. All these data constitute new evidence of interdependence between the two endocrine cell populations of the thyroid gland.
