Spanish website Ciclo21 reported in July that police in Spain were scrutinising Lopez's connections to Marcos Maynar of the University of Extremadura.
Lopez was suspended by his team before he was provisionally reinstated in August.
"Astana Qazaqstan Team discovered new elements showing Miguel Angel Lopez's probable connection with Dr Marcos Maynar," the team said in a statement.
"Accordingly, the team had no other solution than to end the contract between team and rider, based on breaches of said contract and internal team rules, with immediate effect."
Lopez said his contract had been extended in November for the 2023 season and that Astana's decision was "without just cause".
"The rider rejects any allegation that could damage his name and honour as professional rider, and reminds that he has never tested positive for any drugs or doping, nor has he been investigated by any authority," he said on Twitter.
Lopez has previously won stages on the Tour de France and Vuelta a Espana and is a past winner of the Tour de Suisse and the Volta a Catalunya.