ISHL10 Abstract P036

Tightly regulated FOXO3 expression is critical for the maintenance of cHL

FOXO (forkhead box O) transcription factors regulate genes involved in cell cycle arrest, apoptosis, or oxidative stress response. Recently, we have shown that FOXO1 is specifically repressed in classical Hodgkin lymphoma (cHL) and thus acts as a tumor suppressor in this tumor entity. In contrast, FOXO3 was reported to be markedly expressed in the malignant Hodgkin and Reed-Sternberg (HRS) cells of cHL, but not in other B-cell lymphoma subtypes. We confirmed a high FOXO3/FOXO1 ratio in cHL by analysing publically available gene expression profiling (GEP) data and found that this ratio is similiar to those of plasma cells (PC) and multiple myeloma (MM). Of note, cHL FOXO3 expression was higher than in non-HL, but still less than in terminally differentiated MM cell lines. These intermediate FOXO3 levels might reflect the „abortive PC differentiation“ phenotype of cHL. Moreover, analysis of GEP data of microdissected HRS cells revealed a positive correlation between FOXO3 and PRDM1 expression, the master regulator of PC differentiation and a tumor suppressor in cHL. This is similar to our previously published findings showing that FOXO1 regulates PRDM1α, the active isoform of PRDM1. Using ChIP, we next proved that FOXO3, too, directly binds to the PRDM1α promoter in cHL. To further investigate this interaction, we overexpressed a constitutively active variant of FOXO3 (FOXO3(A3)ER) in cHL cell lines. FOXO3 activation increased PRDM1α expression levels and inhibited proliferation of cHL cell lines. So far, frequent genomic deletions are known to restrict FOXO3 levels in cHL. Aiming to identify additional factors involved in the tight regulation of FOXO3 in cHL, we found that miR-155 negatively influences FOXO3 expression. Paradoxically, knockdown of FOXO3 with the help of a specific shRNA also showed an antitumor effect in two of three cHL cell lines, whereas FOXO1 downregulation was not toxic. Our data suggest that intermediate levels of FOXO3 contribute to abortive PC differentiation of cHL and that tightly regulated FOXO3 expression might be critical for the oncogenic program of cHL.