Japanese insurer Dai-ichi Life Co is in advanced talks to buy Protective Life Corp of the United States in a deal that could be worth over $5 billion, a source with direct knowledge of the matter said on Monday. The deal would be the biggest so far in a string of overseas acquisitions by Japanese insurers, led by Dai-ichi Life, seeking out higher-growth markets to offset weak long-term prospects at home as the country’s population ages rapidly.