TOKYO, June 13 (Reuters) - NipponKoa Insurance Co Ltd (8754.T: Quote, Profile, Research) said on Friday it had taken steps to boost investor returns, rejecting criticisms from its largest shareholder, U.S. investment fund Southeastern Asset Management.

Southeastern Asset Management said this week it would vote against the re-election of chief executive Makoto Hyodo, saying he had failed to "articulate and implement plans to enhance NipponKoa's value to all stakeholders." [ID:nT262370]

The fund's announcement came as many Japanese companies prepare for their annual shareholders' meetings, which are attracting attention after U.S. hedge fund Steel Partners recently helped oust the president of wig maker Aderans Holdings (8170.T: Quote, Profile, Research).

NipponKoa, a non-life insurer, said on Friday it is trying to reward investors. The firm spent 37.7 billion yen ($349.5 million) last July to buy back a 4.1 percent of its outstanding stock in its largest share buyback, according to a NipponKoa statement.

NipponKoa also said the company is improving its governance, and will seek shareholders' approval to add two more outside directors at the shareholders meeting held on June 26. That would give the company four outside directors out of 10, it said.

According to a filing with authorities in March 2007, Southeastern had an 18.3 percent stake in NipponKoa. The insurer said its foreign shareholder ownership stood at 43.1 percent as of the end of March 2008.

Nipponkoa shares fell 0.7 percent on Friday versus a 0.2 percent decline for the broader Nikkei 225 index .N225. ($1=107.87 Yen) (Reporting by Junko Fujita; Editing by Kim Coghill)