COPENHAGEN (Reuters) - Danish diabetes drug maker Novo Nordisk on Friday defended its pricing of diabetes drugs after facing allegations of pricing collusion by U.S. senators.

"In order to get on the (product lists), which receive subsidies by the different insurance companies, we need to offer rebates," Mike Rulis, senior vice president of corporate communications told Reuters.

"Those rebates have increased over the years, as well as list prices."

The world's largest insulin maker gets around half its revenue in the United States, but prices have been squeezed by pharmacy benefit managers (PBMs) who administer drug benefits and negotiate rebates for employers and health plans.

U.S. Senator Bernie Sanders and Representative Elijah Cummings called on antitrust regulators on Thursday to investigate whether insulin and diabetes drugs makers such as Sanofi SA, Eli Lilly and Co and Novo Nordisk have colluded to set drug prices on the U.S. market.

Last year Novo Nordisk offered an average rebate of 55 percent on its products in the United States, Rulis told Reuters, but could not elaborate further due to trade confidentiality.

Some U.S. consumers have experienced price increases, while others have not, he said.

Novo Nordisk shares were down 3.0 percent at 1047 GMT on the Copenhagen main index.

(Reporting by Jacob Gronholt-Pedersen; writing by Nikolaj Skydsgaard; editing by Jason Neely)