Acid ceramidase induces sphingosine kinase 1/S1P receptor 2-mediated activation of oncogenic Akt signaling.

Acid ceramidase (AC) is overexpressed in most prostate tumors and confers oncogenic phenotypes to prostate cancer cells. AC modulates the cellular balance between ceramide, sphingosine and sphingosine 1-phosphate (S1P). These bioactive sphingolipids have diverse, powerful and often oppositional impacts on cell signaling, including the activation status of the oncogenic kinase Akt. Our studies show that AC expression correlates with phosphorylation of Akt in human prostate tumors, and elevation of phosphorylated Akt in tumor versus patient-matched benign tissue is contingent upon AC elevation. Investigation of the mechanism for AC-induced Akt activation revealed that AC activates Akt through sphingosine kinase 1 (SphK1)-derived generation of S1P. This signaling pathway proceeds through S1P receptor 2 (S1PR2)-dependent stimulation of PI3K. Functionally, AC-overexpressing cells are insensitive to cytotoxic chemotherapy, however, these cells are more susceptible to targeted inhibition of Akt. AC-overexpressing cells proliferate more rapidly than control cells and form more colonies in soft agar; however, these effects are profoundly sensitive to Akt inhibition, demonstrating increased dependence on Akt signaling for the oncogenic phenotypes of AC-overexpressing cells. These observations may have clinical implications for targeted therapy as PI3K and Akt inhibitors emerge from clinical trials.

Autophagy is increased in prostate cancer cells overexpressing acid ceramidase and enhances resistance to C6 ceramide.

Acid ceramidase (AC) overexpression has been observed in prostate cancer cell lines and primary tumors, and contributes to resistance to chemotherapy and radiation. The consequence of AC overexpression is the ability to convert ceramide, which is often produced as a proapoptotic response to stress, to sphingosine, which can then be converted to the prosurvival molecule sphingosine-1-phosphate. In addition to their ability to metabolize ceramide produced in response to stress, we show here that prostate cancer cell lines overexpressing AC also have increased lysosomal density and increased levels of autophagy. Furthermore, pretreatment with 3-methyladenine restores sensitivity of these cells to treatment with C(6) ceramide. We also observed increased expression of the lysosomal stabilizing protein KIF5B and increased sensitivity to the lysosomotropic agent LCL385. Thus, we conclude that AC overexpression increases autophagy in prostate cancer cells, and that increased autophagy enhances resistance to ceramide.