Many processes are deregulated in
melanoma cells and one of those is
protein production. Although much is known about
protein synthesis in
cancer cells, effective ways of therapeutically targeting this process remain an understudied area of research. A process that is upregulated in
melanoma compared with normal melanocytes is
proline biosynthesis, which has been linked to both oncogene and
tumor suppressor pathways, suggesting an important convergent point for therapeutic intervention. Therefore, an RNAi screen of a
kinase library was undertaken, identifying
aldehyde dehydrogenase 18 family, member A1 (ALDH18A1) as a critically important gene in regulating
melanoma cell growth through
proline biosynthesis. Inhibition of ALDH18A1, the gene encoding
pyrroline-5-carboxylate synthase (P5CS), significantly decreased cultured
melanoma cell viability and
tumor growth. Knockdown of P5CS using
siRNA had no effect on apoptosis, autophagy, or the cell cycle but cell-doubling time increased dramatically suggesting that there was a general slowdown in cellular metabolism. Mechanistically, targeting ALDH18A1 activated the
serine/threonine protein kinase GCN2 (general control nonderepressible 2) to inhibit
protein synthesis, which could be reversed with
proline supplementation. Thus, targeting ALDH18A1 in
melanoma can be used to disrupt
proline biosynthesis to limit cell metabolism thereby increasing the cellular doubling time mediated through the GCN2 pathway.
IMPLICATIONS: This study demonstrates that
melanoma cells are sensitive to disruption of
proline synthesis and provides a proof-of-concept that the
proline synthesis pathway can be therapeutically targeted in
melanoma tumors for
tumor inhibitory efficacy.