SOX11 Regulates Angiogenesis In Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive tumor but recent studies have identified a subtype of this lymphoma with an indolent clinical behavior and long survival of the patients even without chemotherapy that may correspond to a different clinical and biological subtype of the disease. These indolent MCLs differ from conventional MCLs in a particular gene signature that lacks the expression of some transcription factors of the High Mobility Group (HMG). SOX11 is one of the best discriminatory genes between these conventional and indolent MCL tumors. We have recently demonstrated the oncogenic implication of SOX11 expression in the aggressive behavior of MCL as SOX11-knockdown derived tumors display a significant reduction on tumor growth compared to SOX11-control tumors in subcutaneous MCL xenograft experiments (Vegliante MC, Palomero J et al. Blood. 2013; 121:2175-85). To uncover SOX11 regulated genes and transcriptional programs in MCL, we performed an integrative analysis coupling data from human genome-wide promoter analysis by SOX11 ChIP-chip experiments and gene expression profiling (GEP) upon SOX11 silencing in MCL cell lines. In these studies we initially identified the role of SOX11 in blocking the terminal B-cell differentiation by the direct positive regulation of PAX5. However, how SOX11 may enhance tumor growth is still not well known.

To further characterize the potential oncogenic mechanisms regulated by SOX11, we have integrated our ChIP-chip data with further analyses including GEP derived from the xenografted SOX11-positive and silenced tumors. The gene ontology term analysis of the genome-wide promoter study revealed “blood vessel development” as one of the most significant biological processes overrepresented among the SOX11-bound genes. Concordantly, a gene set enrichment analysis (GSEA) of the differentially expressed genes in the xenografted tumors showed a significant higher enrichment of gene signatures related to tumor angiogenesis in the SOX11-positive tumors. To validate these results in vivo, we first investigated protein extracts of these tumors using an angiogenesis proteome profiler antibody array and discovered a significant higher expression of 20 pro-angiogenic factors in the SOX11-positive tumors. We have then investigated the angiogenic development in tissue sections from four Z-138 SOX11-positive and nine SOX11-silenced xenografted tumors. The microvascular density area recognized by CD31 staining was significantly larger in SOX11-positive than knockdown tumors (90%±µ²m vs 15%±µ²m, respectively, p<1x10^-4). SOX11-silenced tumors had larger necrotic areas than positive tumors (20%±µ²m vs 1%±µ²m, respectively, p<1x10^-4), suggesting that vascular paucity could contribute to the significant smaller volume of these tumors.

To determine whether angiogenic development was also differentially represented in primary human MCL we have studied the GEP of 16 SOX11-positive and 22 SOX11-negative MCL. A GSEA revealed that SOX11 expressing tumors were enriched in signatures related to tumor vasculature, angiogenesis and vasculature development. Concordantly, the CD34 staining of an independent series of primary SOX11-positive (n=8) and negative (n=9) human MCL showed higher microvascular density in SOX11 expressing tumors (30%±µ²m vs 5%±µ²m, respectively, p<1x10^-4).

In conclusion, these findings indicate that SOX11 promotes angiogenesis in experimental and primary human MCL and this may be an oncogenic mechanism contributing to the aggressiveness of these tumors.