LONDON, June 15 (Reuters) - Moody's Investors Servicecut its credit rating on Tribune Co. (TRB.N: Quote, Profile, Research) one notch on Thursday, sending its debt to "junk", because of the U.S. publisher's plans to increase its debt load to return cash to shareholders. The publisher and broadcaster's plan to buy back stock had already prompted downgrades from Moody's, and rivals Standard & Poor's and Fitch Ratings to the lowest level of investment grade at the end of May. In a statement, Moody's said it cut its rating on Tribune to Ba1 from Baa3 and assigned a stable outlook. The downgrade...