T4 accumulation in lysosomes of rat thyroid remnants after subtotal thyroidectomy

In chronically stimulated rat thyroids after subtotal thyroidectomy, lysosomes increased in number and volume. They contained iodocompounds and did not appear in iodine-deficient animals. In this study, we analyzed the subcellular localization and the nature of these intracellular iodocompounds. Classical subcellular fractions were isolated from homogenates of rat thyroids and remnants 14 weeks after sham-operation or subtotal thyroidectomy. Two lysosome subpopulations of increasing density, a light fraction, lysosomes 2 (L2, density 1.065-1.08 g/ml) and a dense fraction, lysosomes 1 (L1, density > 1.08 g/ml) were separated from crude lysosomal particulate fractions (ML) by centrifugation in Percoll gradients. Results obtained with thyroids of normal rats were used as controls. In TSH-stimulated thyroid remnants, total activities of three lysosomal enzymes and iodine concentration were increased by 1.6-fold compared with thyroids of sham-operated rats. Total iodoprotein-derived T3 and T4 concentrations, measured after pronase hydrolysis, were slightly decreased. Thyroglobulin (Tg) concentration in the supernatant was reduced by 50%. Iodine, T3 and T4 contents of Tg were not modified. After differential centrifugation, the iodine excess of remnants sedimented with subcellular particulate fractions. The concentration of iodine in dense lysosomes (L1) was 6 times that in sham L1. Intact Tg did not accumulate in L1. Two thirds of the iodine in L1 was soluble in methanol, double the normal proportion, with twice as much iodine included in hydrophobic peptides eluted after T4 by reverse-phase HPLC. Although iodoprotein-derived T4 and T3 concentrations were decreased in the remnant homogenate, they were increased in particles, particularly in L1 where they were increased by 8 and 4-fold, respectively. In contrast, specific activities of lysosomal enzymes in ML and L1 remained unchanged. It is concluded that the chronic TSH stimulation of thyroid remnants in subthyroidectomized rats receiving a normal iodine supply induces the endocytosis of a normal Tg with iodine kept in dense lysosomes. The expansion of the lysosomal compartment resulted from a limitation in iodopeptides degradation as though secondary lysosomes would be overloaded with Tg. The accumulation in L1 of hydrophobic iodopeptides and of more iodoprotein-derived T4 than T3 suggests that exopeptidases involved in the liberation of T4 become rate-limiting.