Beschreibung:
<jats:p> 501 </jats:p><jats:p> Background: RAS mutations are negative predictors of efficacy of anti-EGFR antibodies in patients with colorectal cancer (CRC). Oncogenic RAS activates the MAPK- and PI3K/AKT pathways, which are considered the main effectors of resistance. However, the relative impact of these pathways and their downstream effectors in RAS-mutant CRC is less defined. To this end we conducted functional studies using transgenic cancer models in vitro and in vivo. In addition, we analyzed pathway activation in primary tumor samples from metastatic CRC patients. </jats:p><jats:p> Methods: Anti-EGFR-antibody sensitive cancer cell lines were retrovirally transduced to stably express (i) oncogenic RAS, (ii) conditionally active mutant RAF-1 or (iii) AKT. The impact of oncogene expression on cell proliferation, induction of apoptosis, clonal survival and tumor growth in NOD/SCID mice was analyzed in relation to treatment with cetuximab or panitumumab. Cell lines with endogenous KRAS or PI3KCA mutations were used as additional controls. Primary tumor samples from 47 patients with metastatic CRC were immunohistochemically analyzed for activation of the MAPK and PI3K/AKT pathways in relation to KRAS status. </jats:p><jats:p> Results: Transgenic RAS, RAF-1 and AKT protected anti-EGFR-sensitive CRC cells against anti-proliferative and pro-apoptotic effects of the antibodies in vitro and in vivo. This was in line with immunohistochemical analyses of primary tumour samples, which revealed a strong correlation of MAPK and PI3K/AKT pathway activation and mutant KRAS (regression 0.921, p<0.0001). Sustained activation of both pathways caused upregulation of anti-apoptotic BCL-2 proteins. Transgenic expression of these BCL-2 proteins could substitute oncogenic RAS in mediating antibody resistance. Cotreatment with BH3 mimetics neutralizing anti-apoptotic BCL-XL and BCL-2 sensitized RAS-mutant cancer cells to anti-EGFR therapy. </jats:p><jats:p> Conclusions: Activation of the MAPK and PI3K/AKT pathways mediates resistance to anti-EGFR therapies in cancer models in vitro and in vivo. Simultaneous or cross activation of both pathways is observed in KRAS mutant primary CRC. These results may guide clinical strategies to overcome RAS-mediated antibody resistance. </jats:p>