Ultrastructural analysis of human leukemia U-937 cells after apoptosis induction: Localization of proteasomes and perichromatin fibers

We studied the ultrastructure of human histiocytic lymphoma U-937cells after apoptosis induction with two external agents, hypertonic shock and etoposide. Appearance of aggregates of particles of nuclear origin within the nuclei and cytoplasm of the induced cells was the first and the most prominent morphological sign of apoptosis. These aggregates were not coated by a membrane, had variable shape, density and size. Two types of particles dominated in the aggregates: perichromatin fibers (PFs) and proteasomes (PRs). PFs represent a morphological expression of transcriptional and co-transcriptional processing of pre-mRNA (Biggiogera et al., 2008), PRs are involved in hydrolysis of proteins and nucleoproteins, and participate in regulation of apoptosis (Ciechanover, 1998; Liu et al., 2007). We examined the ultrastructure and localization of PFs and PRs, and confirmed the proteasome nature of the latter by immunoelectron microscopy. We traced the formation and migration of the aggregates along the nucleus and their exit into the cytoplasm across the nuclear pores. Finally, we demonstrated degradation of the aggregates and relocating their content into exosomes at the terminal stages of apoptosis with aid of exosomes. We suggest that proteasomes function as morphologically definite and independent intracellular organelles. Alongside with proteasomes, autophagic vacuoles were revealed in apoptotic cells. Occurrence of autophagic vacuoles in apoptotic cells may suggest that both proteolytic pathways, autophagy and proteasome degradation, are coordinated with each other along the programmed cell death pathway.