Reuse alias that shares an existing column name
For that you need user defined variables

SELECT
  item_id,
  @sum1 := value_a + value_b as value_sum,
  CONCAT('VALUE SUM: ', @sum1 + 5) as label,
  @sum2 := value_a + value_b as value_sum2,
  CONCAT('VALUE SUM: ', @sum2) as label2
 FROM item i
   LEFT JOIN item_sums isu USING (item_id);

Result
item_id     value_sum   label           value_sum2  label2
1           4           VALUE SUM: 9    4           VALUE SUM: 4
2           5           VALUE SUM: 10   5           VALUE SUM: 5
3           11          VALUE SUM: 16   11          VALUE SUM: 11

sample
  SELECT 
      item_id,
      value_sum,
      CONCAT('VALUE SUM: ', value_sum + 5) as label,
      value_sum2,
      CONCAT('VALUE SUM: ', value_sum2) as label2
  FROM
  (SELECT
      item_id,
      value_a + value_b as value_sum,
      value_a + value_b as value_sum2
   FROM item i
       LEFT JOIN item_sums isu USING (item_id)) t1;

sample2