Differing roles of Akt and serum- and glucocorticoid-regulated kinase in glucose metabolism, DNA synthesis, and oncogenic activity.
Serum- and glucocorticoid-regulated kinase (SGK) is a serine kinase that has a catalytic domain homologous to that of Akt, but lacks the pleckstrin homology domain present in Akt. Akt reportedly plays a key role in various cellular actions, including glucose transport, glycogen synthesis, DNA synthesis, anti-apoptotic activity, and cell proliferation. In this study, we attempted to reveal the different roles of SGK and Akt by overexpressing active mutants of Akt and SGK. We found that adenovirus-mediated overexpression of myristoylated (myr-) forms of Akt resulted in high glucose transport activity in 3T3-L1 adipocytes, phosphorylated glycogen synthase kinase-3 (GSK3) and enhanced glycogen synthase activity in hepatocytes, and the promotion of DNA synthesis in interleukin-3-dependent 32D cells. In addition, stable transfection of myr-Akt in NIH3T3 cells induced an oncogenic transformation in soft agar assays. The active mutant of SGK (D-SGK, substitution of Ser422 with Asp) and myr-SGK were shown to phosphorylate GSK3 and to enhance glycogen synthase activity in hepatocytes in a manner very similar to that observed for myr-Akt. However, despite the comparable degree of GSK3 phosphorylation between myr-Akt and d-SGK or myr-SGK, d-SGK and myr-SGK failed to enhance glucose transport activity in 3T3-L1 cells, DNA synthesis in 32D cells, and oncogenic transformation in NIH3T3 cells. Therefore, the different roles of SGK and Akt cannot be attributed to ability or inability to translocate to the membrane thorough the pleckstrin homology domain, but rather must be attributable to differences in the relatively narrow substrate specificities of these kinases. In addition, our observations strongly suggest that phosphorylation of GSK3 is either not involved in or not sufficient for GLUT4 translocation, DNA synthesis, or oncogenic transformation. Thus, the identification of substrates selectively phosphorylated by Akt, but by not SGK, may provide clues to clarifying the pathway leading from Akt activation to these cellular activities.
Institute for Adult Diseases, Asahi Life Foundation, 1-9-14 Nishishinjuku, Shinjuku-ku, Tokyo 116, Japan.
Hideyuki
H
Sakoda
Yukiko
Y
Gotoh
Hideki
H
Katagiri
Mineo
M
Kurokawa
Hiraku
H
Ono
Yukiko
Y
Onishi
Motonobu
M
Anai
Takehide
T
Ogihara
Midori
M
Fujishiro
Yasushi
Y
Fukushima
Miho
M
Abe
Nobuhiro
N
Shojima
Masatoshi
M
Kikuchi
Yoshitomo
Y
Oka
Hisamaru
H
Hirai
Tomoichiro
T
Asano
D016475:3T3 Cells; D000256:Adenoviridae / Q000235:genetics / Q000378:metabolism; D017667:Adipocytes / Q000378:metabolism; D000362:Agar / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D015153:Blotting, Western; D020134:Catalytic Domain; D002455:Cell Division; D002460:Cell Line; D002471:Cell Transformation, Neoplastic; D004247:DNA / Q000096:biosynthesis* / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D018014:Gene Transfer Techniques; D005947:Glucose / Q000378:metabolism*; D051275:Glucose Transporter Type 4; D006006:Glycogen Synthase / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism; D017874:Immediate-Early Proteins; D015151:Immunoblotting; D007377:Interleukin-3 / Q000378:metabolism; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D009124:Muscle Proteins; D009154:Mutation; D009369:Neoplasms / Q000378:metabolism*; D009687:Nuclear Proteins; D010766:Phosphorylation; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D017434:Protein Structure, Tertiary; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D013997:Time Factors; D014162:Transfection; D000099184:Serum-Glucocorticoid Regulated Kinases
D000362:Agar; D004247:DNA; D005947:Glucose; D051275:Glucose Transporter Type 4; D006006:Glycogen Synthase; D017874:Immediate-Early Proteins; D007377:Interleukin-3; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D000099184:Serum-Glucocorticoid Regulated Kinases; C495693:Slc2a4 protein, mouse; C495694:Slc2a4 protein, rat; C494920:Akt1 protein, rat