Bottom shelf of cabinet, support weight of itself?
Knowing nothing about the construction of your cabinetry, I would not trust the bottom shelf to carry the weight of the cabinet, the cabinet's contents, and all the children and dogs likely to be on top of it all. That shelf is designed to carry load applied downward. 

Instead, get some heavy angle brackets and fasten them to the side panels of the cabinet using backer plates of 3/4" plywood, which you'd mount with construction adhesive and 1-1/4" screws. Mount your casters to the angle brackets.