If I set "incompatible" transaction isolation level and lock mode, which takes precedence?

Question

Both (a) transaction isolation levels and (b) JPA lock modes are enforced using DBMS' locks on tables/rows. What will happen if I set (a) not to check any locks and (b) to check some, or vice versa? Which takes precedence?

Example (using Spring):

@Transactional(isolation = Isolation.READ_UNCOMMITTED)
public void incrementVal(int primaryKey) {      
    SomeEntity e = entityManager.find(
        SomeEntity.class, primaryKey, LockModeType.PESSIMISTIC_WRITE);
    int val = e.getVal();
    e.setVal(val + 1);
}

Or, reversing the constraints:

@Transactional(isolation = Isolation.SERIALIZABLE)
public void incrementVal(int primaryKey) {        
    SomeEntity e = entityManager.find(
        SomeEntity.class, primaryKey, LockModeType.NONE);
    int val = e.getVal();
    e.setVal(val + 1);
}

This is obviously not a real problem, but I'm trying to understand how these work, and their interplay. Thanks!