Can kind of believe it, but it's still kind of sad news.
I thought I would contribute to the information gathering and translate the news from yahoo (with info from sanspo sports news). I think some folks may have already posted a summary of the announcement. I translated it. There may be a few mistakes but I think it's all there. I have included the original article link at the bottom.
7月17日7時52分配信 (サ ンケイスポーツ)
While working solo in the U.S., KAT-TUN's Akanishi Jin (26) on the 16th made know that he would formally be withdrawing from the group KAT-TUN. Johnny's Jimusho's president Johnny Kitagawa made the announcement. It will be decided officially this fall, after Akanishi completes a 7 city solo tour in the U.S. but, Johnny decided "He is trying to make it in America. At the present time, he will not be returning to the group."
Akanishi wants to be allowed to stand on his own. Johnny finally agreed to let him pursue his dream of trying to be successful in the U.S.
On June 19th and 20th in LA he was able to realize his heart's desire for a solo performance, and in September and October, in places like NY and Vegas, Akanishi will do a 7 city concert tour.
On the other hand, even in his absence KAT-TUN began their Asia Tour at the Tokyo Dome on the 16th. There, Johnny Kitagawa stated to the press that Akanishi was planning solo work in the U.S. for next year as well.
After that he said, "Now he is only thinking of becoming a success in the U.S. We don't want to drag it out for KAT-TUN or the fans." After getting through this fall's U.S. performances, he will formally be withdrawing form the group.
He went on to say, "If he failed in the U.S., it wouldn't be fair to let him come back to KAT-TUN (as if nothing happened). Because he's not a cute child that just went on a trip. He must be able to follow his own path with all his might."
Akanishi, in October of 2006, left the group for language study in LA, and returned to KAT-TUN in April of 2007 after all that time. Kitagawa-san said "The fan's probably won't be so understanding if the same thing were to happen again," and considering the feelings of the fans he made his decision.
He said, KAT-TUN is not breaking up, the group's name that comes from the first initial of each name, the 'A' rather than have it left out (when Akanishi leaves) will be attached to Kamenashi Kazuya's 'K' to become 'KA'.
Formed in 2001, the group officially debuted in 2006, having a popular member leave is a new situation for them but, Kitagawa-san said "(Even in Akanishi's absence, the 5 members) they are moving forward, and doing just fine."
The same chief, spent his youth trying to get in to the American entertainment business, after establishing his production company still had strong relationships with the likes of Micheal Jackson. 'If Akanishi is successful in the U.S., this is the only way I can be happy."