Interaction of Mantle Cell Lymphoma with the Microenvironment Induces Phospho Akt-Mediated Resistance Against BTK Inhibition and Can be Overcome By Co-Treatment with a Specific Akt Inhibitor (MK-2206)

Mantle cell lymphoma (MCL) is a distinct lymphoma subtype representing 6-8% of non-Hodgkin's lymphoma (NHL). Although with current standard therapy high initial response rates can be achieved, early relapses and rapid disease progression determine the clinical course of most MCL patients. Recently, Bruton´s tyrosine Kinase (BTK) inhibitors have been introduced with highly promising clinical activity. Nevertheless, interindividual responsiveness is heterogenous and primary and secondary resistance has been reported. However, molecular mechanisms driving resistance to BTK inhibition are not well understood yet. Among other factors, interactions between the tumor and its microenvironment have been proposed to play an important role in response to targeted therapy.

In this study, we investigated the influence of tumor cell interaction with its microenvironment on sensitivity to the BTK inhibitor CC292 in vitro. MCL cell lines JeKo-1, Z-138 and Granta-519 were treated with 5 µM of CC292 alone or in co-culture with human bone marrow stromal cells (HS-5) and cell death induction and proliferation were assessed. Expression of proteins involved in BCR signaling and other tumor-promoting pathways was analyzed by Western Blot. Co-cultured MCL cells settled within the stromal cell layer were separated using MACS Feeder removal microbeads prior to Western Blot analysis.

In all cell lines, direct interaction with the microenvironment markedly reduced sensitivity towards CC292 treatment (by 22% (JeKo-1), 33% (Granta) and 64 % (Z-138)). Importantly, cell-cell contact was shown to play a crucial role for mediating resistance to CC292 as only those MCL cells settled within the stromal cell layer proved to be significantly less vulnerable to the inhibitor compared to MCL cells co-cultured with HS-5 but separated by a transwell insert. Western Blot analysis showed a reduction of protein levels of phBTK upon treatment with CC292 in both, mono- and co-cultured cells. Interestingly, direct interaction of MCL cells with the microenvironment strongly induced protein expression of phAkt. Accordingly, phosphorylation (inactivation) of the pro-apoptotic FoxO1, a downstream-target of phAkt, was increased and its translocation to the nucleus was decreased in those cells. We could show that the effect of microenvironment interaction on sensitivity towards CC292 is mediated by Akt as knockdown of Akt using siRNA restored sensitivity to the drug. Furthermore, co-treatment of MCL cells with CC292 and the specific Akt inhibitor MK-2206 hampered upregluation of phAkt in co-cultivated cells and prevented Akt-mediated sequestration of FoxO1 in the cytoplasm, resulting in translocation of FoxO1 to the nucleus. Thus, combination with MK-2206 could significantly overcome microenvironment-mediated protection from growth inhibition and apoptosis induction upon CC292 treatment. Moreover, combination of the BTK inhibitor CC292 and the Akt inhibitor MK-2206 proved to act synergistically in MCL cells in all dose combinations tested (Combination index 0,73-0,93 in Z-138; 0,47-0,78 in JeKo-1).

Taken together, cell-cell-interaction of MCL cells with their microenvironment protected them from CC292-induced cell death. This effect was mediated by increased phAkt expression resulting in inhibition of pro-apoptotic signaling and could effectively be overcome by combination with the specific Akt inhibitor MK-2206. Furthermore, CC292 and MK-2206 acted synergistically in MCL cells. Our results indicate that co-targeting the PI3K/Akt-pathway might be a promising strategy to overcome resistance to BTK inhibition mediated by interaction with the microenvironment.