TOKYO, June 20 (Reuters) - Japan's broad Topix index fell for a second straight session on Tuesday as investors locked in recent gains, while trading companies surged after billionaire Warren Buffett raised his stakes in the sector.

The Topix index fell 0.29% to close at 2,283.85. The benchmark Nikkei index was in the red for most of the day, but rallied in the afternoon to finish the session up 0.06% at 33,388.91.

Tokio Marine Holdings slid 5.09% after the Jiji news service said the company and three other insurers were ordered by regulators to report on an alleged formation of a cartel. Mizuho Financial Group fell 1.59% amid a broad sell-off of banking shares.

Mitsubishi Corp jumped 3.72%, leading trading houses higher after Buffett's Berkshire Hathaway said after Monday's market close that it added to holdings in the five biggest players in the sector.

Mitsui & Co and Marubeni - also in Buffett's portfolio - rose 3.31% and 1.87% respectively.

The trading house sector added 2.06% to become the best performer among the Tokyo Stock Exchange's 33 industry sub-indexes.

Trading cues were scarce following a market holiday in the United States and ahead of data on U.S. housing starts due later on Tuesday.

"Technical indicators continue to show signs of short-term overheating," said Maki Sawada, a strategist at Nomura Securities.

"Today, in the absence of any special inputs, we are likely to see some selling to lock in profits, especially in sectors and stocks that have been rising conspicuously."

Among Nikkei constituents, 154 companies fell, while 69 gained. Insurers led decliners among Topix groups, losing 3.84%

SoftBank Group, whose shares have surged recently amid euphoria over artificial intelligence, jumped 2.84% after its chairman Masayoshi Son said he is speaking "almost everyday" to OpenAI CEO Sam Altman.

McDonald's Holdings Company Japan rose 0.87% after the hamburger chain said it would raise prices at its urban locations to contend with higher costs for labour and rent. (Reporting by Rocky Swift; Editing by Janane Venkatraman and Varun H K)