Take context from the input text and decide on the accurate relationship between 'AmEx' and 'American Express' from the options provided.
Options: operator, owner of, owned by, employer

Sam’s Clubs to Accept AmEx Cards Earlier this year, retailer Costco cut ties with American Express after 16-year relationship By lisa.beilfuss@wsj.com Updated Sept. 10, 2015 5:41 p.m.
operator