Columbia Steel Casting Co., a Portland metal casting company that dates back more than a century, told state regulators Wednesday it plans to shut down operations at its North Portland foundry and lay off most of its workforce. The company said 225 employees, many represented by three different unions, would be laid off beginning in October. The filing indicated the closure would be permanent, though a letter to employees indicated that the company was in talks with “various companies” about buying the foundry and maintaining operations.