We report tunnel magnetoresistance (TMR) measurements on $\mathrm{Co}/{\mathrm{SrTiO}}_{3}/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ junctions. The half-metallic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ electrode is used as a spin analyzer. The large $(\ensuremath{-}50%)$ inverse TMR indicates a negative spin polarization of Co, in agreement with the density of states (DOS) of the $d$ band in Co. The bias dependence of the TMR, with a maximum inverse TMR at $\ensuremath{-}0.4\mathrm{V}$ and a crossover to normal TMR above $+0.8\mathrm{V}$, reflects the structure of this DOS. Our results demonstrate that the choice of the insulating barrier can strongly influence and even reverse the spin polarization of tunneling electrons.