Google Inc. set aside $500 million related to the possible resolution of a U.S. Justice Department investigation of its advertising business, resulting in lower first-quarter profits. The expense trimmed net income to $1.8 billion, or $5.51 a share, in the period, Google said yesterday in a regulatory filing. The company had reported a first-quarter profit of $2.3 billion, or $7.04 per share, on April 14. The Justice Department is investigating the use of Google ads by “certain advertisers,” Google said in the filing. Aaron Zamost, a spokesman for the company, declined to comment on the disclosure. Google gets almost all its revenue from online advertising, which runs on its search engine and other sites, such as YouTube.