The integrity of cholesterol-enriched microdomains is essential for the constitutive high activity of protein kinase B in tumour cells.
A deregulated activity of PKB/Akt (where PKB stands for protein kinase B) renders tumour cells resistant to a variety of apoptosis-inducing stimuli. Elucidation of the mechanisms responsible for this deregulation is of prime importance for the development of novel anti-cancer drugs. Results of the present study demonstrate that the constitutive activity of PKB/Akt in B16BL6 melanoma cells depends on the integrity of cholesterol-enriched membrane microdomains, since the exposure of cells to cholesterol-depleting agents decreases the phosphorylation of this enzyme, with no change in its total protein level. Inhibitors of Hsp90 (heat-shock protein 90) decreased phosphorylation of PKB/Akt with a similar pattern. Dephosphorylation of the enzyme, as a consequence of raft disintegration, could be precluded by inhibition of serine/threonine (but not tyrosine) phosphatases. Our results imply that destabilization of lipid rafts seemingly affects the association of Hsp90 with the respective serine/threonine phosphatases, thereby increasing the accessibility to PKB/Akt to deactivating phosphatases. We have found recently that reconstituted expression of H-2K class I glycoproteins in class I-deficient B16BL6 cells also decreases the phosphorylation of PKB/Akt. Therefore it is possible that raft-associated regulation of this important enzyme involves both H-2K glycoproteins and Hsp90.
Elhyany
S
S
Department of Immunology and Microbiology, University Cancer Research Center, Ben-Gurion University of the Negev, Beer-Sheva, Israel.
Assa-Kunik
E
E
Tsory
S
S
Muller
T
T
Fedida
S
S
Segal
S
S
Fishman
D
D
D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D002784:Cholesterol / Q000378:metabolism*; D006023:Glycoproteins / Q000737:chemistry; D018841:HSP90 Heat-Shock Proteins / Q000737:chemistry; D008546:Melanoma, Experimental; D021962:Membrane Microdomains / Q000737:chemistry*; D051379:Mice; D009369:Neoplasms / Q000378:metabolism*; D009675:Novobiocin / Q000737:chemistry; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis* / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis* / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt
D000970:Antineoplastic Agents; D006023:Glycoproteins; D018841:HSP90 Heat-Shock Proteins; D011518:Proto-Oncogene Proteins; D009675:Novobiocin; D002784:Cholesterol; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt