HS 1 erythroleukemic cells engineered to overexpress the SPHK 1 protein exhibited growth proliferative advantage , increased clonogenicity , and resistance to apoptosis in reduced serum level by a mechanism involving activation of the extracellular signal related kinases 1 / 2 ( ERK1 / 2 ) and phosphatidylinositol 3 kinase ( PI3K ) / AKT pathways . ^^^