Custom cache region: Invalidating mechanism when updating values in HMC

So, I have a Custom cache region, and I have my own itemtype values cached (CustomMessages)

Now my problem is: When I update the value of this itemtype in the HMC, for example I change one Custom message, the cache for that item doesn't get invalidated, and therefore the values do not change on the front end. How do I make sure that if the value for that object is changed, the cache for that object gets invalidated, so that new values could be used.