The increased EGFR phosphorylation in primary mouse lung tumors bearing Kras mutations that we previously reported could result from IGF 1R EGFR coupling and trans activation after IGF 1 stimulation, Muta tions in EGFR and KRAS are mutually exclusive in both human and murine NSCLC, and EGF stimulation would not be expected drive Kras mutant models of lung can cer, A requirement for the IGF 1 receptor in mediating lung cancer growth is consistent with other reports that IGF 1 stimulates rapid anchorage indepen dent growth in vitro, while IGF 1R inhibition slows tumor growth in both animal xenograft studies and human clinical trials, IGF 1R signals through numerous downstream path ways in which the intracellular kinases Erk1 2 and Akt are frequently activated, We have previously determined that MEK inhibition induces a potent G1 phase arrest in neoplastic lung cell cycle progression in vitro, and others have determined that blocking both MEK and PI3K slows lung tumor growth in vivo, We show herein that M CM stimulated neoplastic proliferation significantly increases cyclin D1 expression, which is abrogated by the combined inhibition of both MEK and PI3K.
Sole inhibition of either MEK or PI3K partially limits macrophage stimulation of LM2 and JF32 growth to slightly different extents. While M CM modestly increases Erk1 2 and Akt activity, long term MEK and PI3K inhibition strikingly stimulates both kinases in an additive manner with conditioned media treatment.<additional reading br> This increased kinase activity resulting from MEK and PI3K inhibition, however, is no longer asso peptide synthesis companies ciated with changes in cyclin D1, as combined inhibition resulted in the highest levels of Akt activity, but lowest levels of cyclin D1 expression, Compensatory Akt or Erk activation in response to upstream kinase inhibition is consistent with the exten sive cross talk that exists among MAPK pathways, where inhibition of any single mediator results in exag gerated and or sustained signaling through an alternate pathway, Indeed, when the MEK pathway was inhibited in LM2 cells, early p Akt activity increased, while PI3K inhibition increased p Erk1 2, Akt is hyper phosphorylated with 24 hrs of treatment with either MEK or PI3K inhibitor, and this hyper activated Akt sustains 5 10 higher levels of p GSK 3b and p cRaf for at least 48 hrs. Erk1 2 phosphor ylation is also stimulated by drug treatment, which peaks at 24 hrs and rapidly declines by 48 hrs. Consis tent with our observations, continuous hyper activation of Akt or Erk1 2 induces cytostasis or even apoptosis in some tissues, while more modest Erk1 2 activation drives Kras mutant tumor cell proliferation, While our studies demonstrate that M CM and IGF 1 stimulated neoplastic growth is affected similarly by MEK and PI3K inhibition, further studies in genetically silenced or kinase mutant cell lines are required to determine the discrete cellular mechanisms necessary for growth factor stimulated neoplastic proliferation.