Here "Car" etc are the "fields", not the "subfields" (which would mean something like: DataBase.Vehicle.Car).
DataBase.Car = 1;
DataBase.Bus = 2;
DataBase.Motorbike = 3;
List = fieldnames(DataBase);
uicontrol('Style', 'listbox', 'String', List);
Keeping the listbox "updated" when the struct is changed, is very complicated. You need an object oriented approach to start a callback, when the data are changed. But you can fake it by not adding further fields manually, but when you use a dedicated function for this:
function S = myAddField(S, Field, Data)
S.(Field) = Data;
updateGui(S); % <-- Implementation depends on your GUI
