Hermansky-Pudlak Syndrome (HPS) is a genetically heterogeneous disorder in which mutations in one of several genes interrupts biogenesis of melanosomes, platelet dense bodies, and lysosomes. Affected patients have oculocutaneous albinism, a bleeding diathesis, and sometimes develop granulomatous colitis or pulmonary fibrosis. In order to assess the role of HPS genes in melanosome biogenesis, melanocytes cultured from patients with HPS subtypes 1, 2, or 3 were assessed for the localization of various melanocyte proteins. Tyrosinase, Tyrp1, and Dct/Tyrp2 were atypically and distinctly expressed in HPS-1 and HPS-3 melanocytes, whereas only tyrosinase showed an atypical distribution in HPS-2 melanocytes. The HPS1 and AP3B1 (i.e., HPS-2) gene products showed no expression in HPS-1 and HPS-2 melanocytes, respectively, whereas HPS-3 melanocytes exhibited normal expression for both proteins. In normal human melanocytes, the HPS1 protein was expressed as an approximately 80 kDa molecule with both granular and reticular intracellular profiles. In HPS-1, lysosome associated membrane protein 1 (LAMP1), and LAMP3 were localized to abnormal large granules; in HPS-2, all LAMPs exhibited a normal granular expression; and in HPS-3, LAMP1, and LAMP3 exhibited a distinct less granular and more floccular pattern. In contrast, the expressions of Rab 27, transferrin, and cKit were unaffected in all three HPS genotypes. These data demonstrate that the three initially identified subtypes of human HPS exhibit distinct defects in the trafficking of various melanocyte-specific proteins.

To determine whether thyroid cell apoptosis observed in autoimmune thyroid disease could be related to activation of the Fas pathway, we examined the expression and function of Fas on thyroid follicular cells in vitro. Fas messenger RNA was found to be present using two different techniques and was expressed at equal levels in thyrocytes cultured either in the presence or absence of TSH. Fas antigen protein expression was demonstrated by Western blot of thyroid cell lysates and by immunohistochemical staining of thyrocytes, and the amount of Fas protein present did not appear to vary regardless of culture conditions. Despite expressing substantial amounts of Fas protein, thyrocytes treated with anti-Fas monoclonal antibody failed to undergo apoptosis. The addition of either interferon-gamma or interleukin-1beta to the anti-Fas-treated cell cultures also did not promote apoptotic signaling through this pathway. In contrast, the concomitant administration of cycloheximide allowed the induction of apoptosis through the activation of Fas in thyrocytes. These results suggest that Fas is constitutively expressed in thyrocytes, but that the induction of apoptosis through the Fas pathway is blocked by a labile protein inhibitor.

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