Tài liệu Báo cáo khoa học:Insulin-like growth factor 1 signaling regulates cytosolic sialidase Neu2

Insulin-like growth factor 1 signaling regulates cytosolic

sialidase Neu2 expression during myoblast differentiation

and hypertrophy

Alessandro Fanzani, Francesca Colombo, Roberta Giuliani, Augusto Preti and Sergio Marchesini

Department of Biomedical Sciences and Biotechnology, Unit of Biochemistry, University of Brescia, Italy

Skeletal muscle hypertrophy plays an important role

during postnatal development and occurs in response

to physical exercise [1], resulting in an increase in fiber

size accompanied by the increased expression of

insulin-like growth factor 1 (IGF-1) [2,3]. Since IGF-1

overexpression in the skeletal muscle of transgenic

mice triggers an increase in muscle size [4–6], the emer￾ging idea is that IGF-1 is sufficient to induce muscle

hypertrophy. Administration of IGF-1 to cultured

muscle cells elicits a biphasic response, first promoting

cell proliferation and then enhancing myogenic differ￾entiation [7,8], reproducing the events occurring during

Keywords

AKT; IGF-1; myoblast; Neu2 sialidase;

gangliosides

Correspondence

A. Fanzani, University of Brescia,

Department of Biomedical Sciences and

Biotechnology, viale Europa 11,

25123 Brescia, Italy

Fax: +39 030 3701157

Tel: +39 030 3717568

E-mail: [email protected]

(Received 5 May 2006, revised 12 June

2006, accepted 13 June 2006)

doi:10.1111/j.1742-4658.2006.05380.x

Cytosolic sialidase (neuraminidase 2; Neu2) is an enzyme whose expression

increases during myoblast differentiation. Here we show that insulin-like

growth factor 1 (IGF1)-induced hypertrophy of myoblasts notably increa￾ses Neu2 synthesis by activation of the phosphatidylinositol 3-kinase/AKT/

mammalian target of rapamycin (P13K/AKT/mTOR) pathway, whereas

the proliferative effect mediated by activation of the extracellular regulated

kinase 1 ⁄ 2 (ERK1 ⁄ 2) pathway negatively contributed to Neu2 activity.

Accordingly, the differentiation L6MLC ⁄IGF-1 cell line, in which the

forced postmitotic expression of insulin-like growth factor 1 stimulates a

dramatic hypertrophy, was accompanied by a stronger Neu2 increase.

Indeed, the hypertrophy induced by transfection of a constitutively activa￾ted form of AKT was able to induce high Neu2 activity in C2C12 cells,

whereas the transfection of a kinase-inactive form of AKT prevented myo￾tube formation, triggering Neu2 downregulation. Neu2 expression was

strictly correlated with IGF-1 signaling also in C2 myoblasts overexpressing

the insulin-like growth factor 1 binding protein 5 and therefore not

responding to endogenously produced insulin-like growth factor 1.

Although Neu2-transfected myoblasts exhibited stronger differentiation, we

demonstrated that Neu2 overexpression does not override the block of dif￾ferentiation mediated by PI3 kinase and mTOR inhibitors. Finally, Neu2

overexpression did not modify the ganglioside pattern of C2C12 cells, sug￾gesting that glycoproteins might be the target of Neu2 activity. Taken

together, our data demonstrate that IGF-1-induced differentiation and

hypertrophy are driven, at least in part, by Neu2 upregulation and further

support the significant role of cytosolic sialidase in myoblasts.

Abbreviations

AKT or PKB, protein kinase B; caAKT, constitutively active form of AKT; DM, differentiating medium; GM, growth medium; IGF-1, insulin-like

growth factor 1; IGFBP5, insulin-like growth factor 1-binding protein; IRS-1, insulin receptor substrate 1; kiAKT, kinase-inactive form of AKT;

LY, LY294002; Neu2, neuraminidase 2; HS, horse serum; MAP kinase, mitogen-activated protein kinase; mTOR, mammalian target of

rapamycin; PD, PD098059.

FEBS Journal 273 (2006) 3709–3721 ª 2006 The Authors Journal compilation ª 2006 FEBS 3709