Expression, localization, and thyrotropin regulation of cathepsin D in human thyroid tissues

Enzymatic activity and isoform expression of cathepsin D (cath D) were studied in 107 cytosols from various human thyroid tissues including 21 normal tissues, 12 cold benign nodules, 17 toxic adenomas, 22 samples from Graves' disease patients, and 35 thyroid carcinomas. Cath D assay was optimized for human thyroid tissues. We found that mean cath D specific activities, expressed as units per milligrams protein minus thyroglobulin, were higher in carcinomas (P = 0.0001), toxic adenomas (P = 0.0001), and specimens from Graves' disease patients (P = 0.0001) than in normal thyroid tissues. Mean cath D activity in carcinomas was also significantly different from that in cold benign nodules (P < 0.001) and Graves' disease tissues (P < 0.05) but not from that of toxic adenomas. To determine possible mechanisms by which the observed increase in cath D activity might be regulated, we used Western blotting to measure relative amounts of cath D isoforms in the various thyroid tissues. We found that the 31-kDa major processing form of cath D was significantly increased in carcinomas and toxic adenomas compared with normal tissues (P < 0.01), cold benign nodules (P < 0.05), and Graves' disease tissues (P < 0.05). A positive correlation of cath D activity with relative expression of the 31-kDa form (r = 0.67, P = 0.0001) was observed in 104 thyroid cytosols. These data demonstrate a deregulation at the protein level, with resulting increases in cath D activity. Immunogold labeling of cath D showed particle concentration in lysosomes or phagosomes in both normal follicles and papillary carcinoma cells, indicating that cath D localization was not altered by malignant transformation in human thyroid cells. TSH induced cath D synthesis and secretion in extracellular fluid of normal human thyroid cells in primary culture; TSH had little effect on intracellular cath D level. In conclusion, TSH-induced cath D synthesis may explain high cath D levels in Graves' disease tissues and toxic adenomas, because these tissues possess a permanently stimulated cAMP transduction pathway. Furthermore, the overexpression of cath D in thyroid carcinomas in comparison with normal controls adds further arguments for the potential role of cath D in tumor growth and metastasis.