MAPK-Erk1/2 Pathway Activation and Survival Of Chronic Lymphocytic Leukemia (CLL) Cells Is Induced By In Vitro Modeling Of The Tumor Microenvironment and Can Be Effectively Inhibited Using The MEK1 Inhibitor MEK162

Infiltration of chronic lymphocytic leukemia (CLL) cells into the lymph node and bone marrow microenvironments promotes leukemic cell survival, proliferation and ultimately drives disease progression. Successful treatment of CLL relies on targeting the cells within these niches.

Activity of the MAPK-Erk1/2 pathway is considered to be crucial for the survival of a diverse range of cancer cell types, including CLL cells. However, to date pre-clinical studies of inhibitors of MEK1/2, a key component in this pathway, have demonstrated little efficacy against CLL cells.

We have recently shown that MAPK-Erk1/2 pathway activity is attenuated during ex vivo cell processing and subsequent culture of primary CLL cells; an observation which suggests that factors exist in vivo that maintain Erk1/2 activity and promote cell survival.

Here we present data that show that in vitro modeling of the effects of the microenvironment on primary CLL cells, using recombinant stroma-derived factor CXCL12 or media conditioned by the HS5 stromal cell line (HS5-CM), results in a significant increase in MAPK-Erk1/2 pathway activity. Treatment with the MEK1/2 inhibitor MEK162 (Novartis) blocked the effects of CXCL12 and HS5-CM on Erk1/2 activation in CLL cells. In the presence of CXCL12 or HS5-CM, MEK162 induced apoptosis in a significantly (P< 0.05) greater proportion of cells compared to treatment of cells cultured in media alone (media: 30.14 ± 5.74%, CXCL12: 78.34 ± 9.06%, HS5 CM: 65.79 ± 12.88%). Furthermore, MEK162 completely abrogated the survival advantage conferred by the presence of HS5-CM and reduced cell migration through a permeable support towards HS5-CM by an average of 35.64 +/- 8.48%.

In conclusion, our work suggests that MAPK-Erk1/2 signaling is abrogated in CLL cells during ex vivo processing and subsequent culture. This observation provides a rational explanation for the current lack of in vitro data concerning any efficacy of MEK inhibitors against CLL cells. Our data show that by introducing purified stroma-derived factor (SDF-1a/CXCL12) or media conditioned by the HS5 stromal cell line it is possible to recapitulate the activity of Erk1/2 and increase CLL-cell sensitivity to the MEK1/2 inhibitor MEK162. These observations highlight the importance of modeling the influence of the microenvironment in all pre-clinical drug-efficacy studies in CLL and suggest that inhibition of MEK1/2 may represent a novel treatment strategy for CLL.