Glycogen Synthase Kinase-3α Reduces Cardiac Growth and Pressure Overload-induced Cardiac Hypertrophy by Inhibition of Extracellular Signal-regulated Kinases

Abstract

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase having multiple functions and consisting of two isoforms, GSK-3␣ and GSK-3␤. Pressure overload increases expression of GSK-3␣ but not GSK-3␤. Despite our wealth of knowledge about GSK-3␤, the function of GSK-3␣ in the heart is not well understood. To address this issue, we made cardiac-specific GSK-3␣ transgenic mice (Tg). Left ventricular weight and cardiac myocyte size were significantly smaller in Tg than in non-Tg (NTg) mice, indicating that GSK-3␣ inhibits cardiac growth. After 4 weeks of aortic banding (transverse aortic constriction (TAC)), increases in left ventricular weight and myocyte size were significantly smaller in Tg than in NTg, indicating that GSK-3␣ inhibits cardiac hypertrophy. More severe cardiac dysfunction developed in Tg after TAC. Increases in fibrosis and apoptosis were greater in Tg than in NTg after TAC. Among signaling molecules screened, ERK phosphorylation was decreased in Tg. Adenovirus-mediated overexpression of GSK-3␣, but not GSK-3␤, inhibited ERK in cultured cardiac myocytes. Knockdown of GSK-3␣ increased ERK phosphorylation, an effect that was inhibited by PD98059, rottlerin, and protein kinase C⑀ (PKC⑀) inhibitor peptide, suggesting that GSK-3␣ inhibits ERK through PKC-MEK-dependent mechanisms. Knockdown of GSK-3␣ increased protein content and reduced apoptosis, effects that were abolished by PD98059, indicating that inhibition of ERK plays a major role in the modulation of cardiac growth and apoptosis by GSK-3␣. In conclusion, up-regulation of GSK-3␣ inhibits cardiac growth and pressure overload-induced cardiac hypertrophy but increases fibrosis and apoptosis in the heart. The anti-hypertrophic and pro-apoptotic effect of GSK-3␣ is mediated through inhibition of ERK. GSK-33 is a proline-directed serine/threonine kinase that is ubiquitously expressed. It has versatile biological functions, including regulation of metabolism, cell growth/death, development, cytoskeletal organization, transcription, and protein translation (1-3). GSK-3 remains active at resting state and is inactivated by a variety of mitogens, many protein kinases including protein kinase B/Akt, and by the Wnt signaling pathway. Many targets of GSK-3 are negatively regulated by it, and inactivation of GSK-3 stimulates cellular functions by removing the repression (1-3).GSK-3 has two isoforms, GSK-3␣ and GSK-3␤. The molecular mass of GSK-3␣ is 51 kDa, and that of GSK-3␤ is 47 kDa (4). Structurally, the two isoforms have 97% sequence homology within their kinase domains, but GSK-3␣ has an extended N-terminal glycine-rich tail (2), and the two kinases share only 36% in the last 76 C-terminal residues (1). Although both isoforms share substrates, their expression patterns, substrate preferences, and cellular functions are not identical (for review, see Ref. 5). GSK-3␣ is not phosphorylated/inhibited by protein kinase C (PKC), whereas the activity of GSK-3␤ is decreased by about 50% when phosphorylated by some PKC isotypes, including PKC-␣, -␤1, and -...

GSK-3α directly regulates β-adrenergic signaling and the response of the heart to hemodynamic stress in mice

“…Prior studies examining effects of GSK-3s on TAC-induced LV dysfunction have suggested that increased activity of GSK-3α, by either transgenesis or knockin, led to declines in LV function, suggesting GSK-3α signaling is detrimental (13,14). In distinct contrast, our lossof-function studies identify a clear and critical role for GSK-3α in preserving LV function in the setting of pressure stress.…”

“…Findings with GSK-3α have been even less clear. Whereas a constitutively active transgenic repressed the hypertrophic response to TAC, the same mutant expressed in a knockin model showed an increased hypertrophic response (13,14). These disparate and confusing findings, which led Sugden et al to state "we just do not know whether to inhibit or activate GSK-3, or simply not to interfere" (29), highlight the need to examine loss-of-function approaches that do not rely on transgenesis.…”

“…The role of GSK-3β in cardiac hypertrophy has been examined extensively by several groups, and the vast majority of studies have found that expression of either WT or a constitutively active S9A mutant repressed hypertrophy (8)(9)(10)(11)(12)(13)(14). However, based on studies with overexpression of a dominant inhibitory mutant of GSK-3β, which showed no enhancement of hypertrophy following TAC, it was not clear what role, if any, inhibition of GSK-3β played in the response (12).…”

“…This is due to the fact that all studies to date have used transgenesis, knockin of activated mutants, or nonselective small molecule inhibitors (e.g., BIO) (8)(9)(10)(11)(12)(13)(14). None of these strategies allow one to define the true biology of GSK-3s or an understanding of isoform-specific effects.…”

“…For example, work from the same laboratory alternately reported that constitutively active GSK-3α suppressed pressure overload-induced hypertrophy in a transgenic model but enhanced hypertrophy in a knockin model (13,14). Further complicating understanding, the strategy used in these models (mutation of the inhibitory serine 21 phosphorylation site of GSK-3α and serine 9 phosphorylation site of GSK-3β to alanine residues) blocks only 1 mechanism by which GSK-3α can be inhibited (i.e., PI3-kinase-mediated activation of Akt) and does not affect Wntmediated or p38-MAPK-mediated inhibition (15)(16)(17).…”