We developed a macrospin model describing the storage layer magnetization dynamics under spin-transfer-torque (STT) in in-plane magnetized magnetic tunnel junctions comprising a perpendicular, out-of-plane magnetized, polarizer. Two regimes were observed in the storage layer magnetization dynamics under the influence of the two STT contributions originating from the perpendicular polarizer and in-plane reference layer: (i) a regime of steady precession under DC current when the STT from the perpendicular polarizer dominates and (ii) a regime of bipolar switching in a direction determined by the current direction when the STT from the in-plane reference layer dominates.