PARIS (Reuters) - AXA (>> AXA) Chief Executive Henri de Castries will step down in September after nearly 17 years at the top of Europe's second-largest insurer, fuelling speculation that his next role could be as HSBC (>> HSBC Holdings plc) chairman.

Castries, who joined AXA in 1989, has spent the past five years expanding the group's business into emerging markets with a 5.1 billion euro ($5.7 billion) acquisition spree in Asia, Latin America and Africa.

In a letter to staff announcing his departure, Castries, 61, said "it is only natural that a new team launches and manages our new strategic plan to be announced in June 2016".

Thomas Buberl, 42, who heads AXA's German business, has been appointed deputy chief executive and will take over from Castries when he leaves in September.

AXA is also splitting the chairman and CEO roles. The current deputy CEO in charge of finance, Denis Duverne, 62, will replace Castries as chairman.

Castries' departure is the latest in a series of leadership changes at European insurers, with Prudential (>> Prudential plc), Zurich (>> Zurich Insurance Group Ltd), Old Mutual (>> Old Mutual plc) and Munich Re (>> Muenchener Rueckversicherungs-Ges. AG) all announcing new CEOs in the past year.

Under a five-year strategic plan completed last year, AXA consolidated its position as Europe's second-biggest insurer after Germany's Allianz (>> Allianz SE) by trimming its exposure to mature markets and increasingly focusing on faster-growing emerging markets where insurance coverage remains low.

HSBC SPECULATION

Castries is seen as a front-runner to take over from Douglas Flint as HSBC chairman, having joined the bank's board last year. HSBC said last week that it had begun to look for a replacement for Flint and expected to nominate someone in 2017.

"I think Henri de Castries is quite likely to replace Flint, (I) have been hearing that for the last nine months," one HSBC shareholder told Reuters, on condition of anonymity.

"He is in a learning phase now and could step up next year".

HSBC said it did not comment on speculation.

Castries tried to dampen the speculation, saying people should not read too much into the timing of the announcement of management changes at AXA and HSBC.

"I know there is some sort of fantasy that I saw this morning regarding HSBC," Henri de Castries told journalists.

"I haven't taken part in a single board meeting at HSBC yet, I will do that next month...I'm a board member. I'm proud of it, that's it".

GERMAN-BORN BOSS

Incoming CEO Buberl is expected to focus on shareholder returns and changes in digital technology, a London-based analyst said.

"AXA is in very good shape at the moment, with a strong balance sheet...The first challenge is to keep what we have today," Buberl told journalists.

Buberl would be a rare foreign boss among leading French companies. He was born in the German town of Wuppertal and educated in Germany, Britain, and Switzerland.

Since joining AXA in 2012, he has led the group's life and savings business, and health line. Previously he worked at the Winterthur group, which was acquired by AXA in 2006, and Zurich Insurance Group.

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(Additional reporting by James Regan, Leigh Thomas in Paris and Sinead Cruise in London; Editing by Andrew Callus and Keith Weir)

By Maya Nikolaeva