We're starting to get an idea of how many millions have been lost in the stock market by investors of all stripes, including big-shot chief executives. Latest case in point: Arthur Coppola, CEO of Santa Monica-based shopping mall owner Macerich. Last week, Coppola had to unload almost half of his stock in the company in order to cover a collateral requirement on his line of credit - otherwise known as a margin call. An SEC filing discloses that he sold 345,173 Macerich shares on Thursday and Friday for nearly $13.9 million. In August and September, he sold nearly 500,000 shares for $29.5 million. Here's how the WSJ explains a margin call:

A broker issues a margin call when the value of stock bought with borrowed money declines below a certain point, requiring the borrower either to sell stock or put cash into the account to bring the loan back in line with the stock's value.

Coppola still has 445,119 Macerich shares and nearly 1.5 million operating units. He also has a balance of $13 million on his line of credit, so his remaining stock may be subject “to future involuntary sales," according to the filing. By the way, Macerich owns a bunch of local malls, including Westside Pavilion, Santa Monica Place, Lakewood Center and Panorama Mall.