Pronounced up-regulation of the PA28α/β proteasome regulator but little increase in the steady-state content of immunoproteasome during dendritic cell maturation

Dendritic cells (DC) are professional antigen-presenting cells that activate CTL by presenting MHC class I-restricted peptides that are processed through the proteasome pathway. Previously, we reported that upon DC maturation the synthesis is switched towards the exclusive production of immunoproteasomes containing the active site subunits LMP2, LMP7 and MECL-1. In this study we investigated the mechanism by which proteasome assembly is regulated in mature DC. Quantitative analysis of mRNA expression showed very limited transcriptional induction of LMP7, MECL-1 and UMP1 in mature DC and a moderate mRNA increment for LMP2 and PA28 alpha and beta. We investigated a role of PA28 alpha/beta in regulating proteasome assembly in DC. PA28 alpha/beta coprecipitated with 13S/16S proteasome precursor complexes but associated with mature constitutive and immunoproteasomes to the same extent. Furthermore, we determined the steady-state proteasome subunit composition in DC. Replacement of constitutive proteasomes by immunoproteasomes in maturing DC was very slow and occurred only to a minor extent. Our data suggest that the limited turnover of 20S proteasomes in mature DC probably contributes little to recently reported marked differences in antigen presentation between immature and mature DC and that alternative mechanisms may be responsible for this phenomenon.