BUZZ-American Express Co: To end U.S. credit card partnership with Costco
** World's largest credit card issuer's shares down 2.8 pct at $83.57 premarket
** AmEx to stop issuing credit cards co-branded with Costco Wholesale Corp in the U.S. from March 31, 2016
** "We were unable to reach terms that would have made economic sense for our company and shareholders," AmEx CEO Kenneth Chenault says
** Costco has already replaced AmEx with Capital One Financial Corp in Canada starting this year
** COF stock up 066 pct at $77.83 premarket
** Through Wednesday, AmEx's stock is off about 3.5 pct since September when the end of the Canadian partnership was announced
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