Degradation of calcitonin by homologous thyroid acid proteinase

Crinophagy in Thyroid Follicular and Parafollicular Cells of Male Obese Zucker Rat

Comparison between lean (Fa/?) and obese (fa/fa) young adult male Zucker rat thyroids reveals that obese rats display larger clusters of parafollicular cells than the lean ones with a lesser blood supply. Fa/? thyroid typically shows single or "twin" C cells in follicles; fa/fa parafollicular cells appear with three functional aspects. Crinophagy is found in the fa/fa C cells amassing numerous aberrant calcitonin-containing vesicles among which lysosomes build these autophagic bodies by capturing vesicle contents, other organelles and, fusing with each other, increase their size. Other C cells contain many secretory vesicles but show few or no crinophagic structures. Another parafollicular cell type is revealed with scant organelles and highly contrasted secretory vesicles, different from calcitonin. Hypercalcemia of fa/fa rats corresponds to increased C cells population with accrued calcitonin production but a low calcitonin plasma level - verified by others - is likely caused by crinophagy of the altered vesicles. In addition, the T thyrocytes of fa/fa rats exhibit crinophagy bodies; this can confirm their hypothyroidism. Possibly, the known leptin mutation along with other unknown paracrine secretions alter both T and C thyrocytes' functions of the fa/fa rats, allowing high intracellular calcium and lower pH favoring autophagocytosis. Other longitudinal, interdisciplinary studies should further clarify the complex paracrine interactions existing between these endocrine structures because this animal model could be useful to understand human defects, such as the metabolic syndrome that involves obesity, cardiovascular, renal, hepatic, non-insulin dependent diabetes mellitus (NIDDM), hypothyroidism defects, as well as the etiology of thyroid medullary tumors.

Characteristics of calcitonin degradation in vitro produced by incubation with procine thyroid tissue

The possibility of calcitonin (CT) degradation by thyroid tissue in vitro was investigated. Tissue slices or a thyroid supernatant solution was incubated with 131I-labeled porcine CT for 30--60 min at 37 degrees C. Calcitonin degradation and deiodination were determined by chromatoelectrophoresis and/or quso adsorption. Degradation of ]131I]CT occurred rapidly in the presence of 50--200 mg porcine thyroid tissues; however, little or no hormone deiodination was observed. Porcine liver, kidney and lung slices also degraded [131I]CT. The data indicate that a nonspecific proteolytic enzyme, capable of rapid CT degradation, is liberated from thyroid tissue slices in vitro. This substance, which is found in the cytosol fraction of thyroid tissue, is heat labile, has a molecular weight between 25,000 and 100,000, displays an acidic pH maximum for CT degradation and is inhibited by sulfhydryl enzyme inhibitors. The liberation of this substance from thyroid slices may explain the relative unresponsiveness of CT secretion observed in previous in vitro studies.