SPHK1 and posterior cortical atrophy: •Levels of circulating S1P were significantly higher in healthy subjects and patients with BPH than in patients with PCa•Died patients’ circulating S1P levels were significantly lower than in the surviving patients•Circulating S1P levels were an early marker of PCa progression to hormonal unresponsiveness and correlated with PSA levels and lymph node metastasis•The decrease in circulating S1P during PCa progression may stem from a highly significant downregulation of erythrocyte SPHK1 activity