The type 1 insulin-like growth factor receptor (IGF-1R), activated by its ligands, protects several cell types from a variety of apoptotic injuries. The main signaling pathway for IGF-1R-mediated protection from apoptosis has been previously elucidated and rests on the activation of phosphatidylinositol 3-kinase, Akt/protein kinase B, and the phosphorylation and inactivation of BAD, a member of the Bcl-2 family of proteins. In 32D cells (a murine hemopoietic cell line devoid of insulin receptor substrate 1 [IRS-1]), the IGF-1R activates alternative pathways for protection from apoptosis induced by withdrawal of interleukin-3. One of these pathways leads to the activation of mitogen-activated protein kinase, while a third pathway results in the mitochondrial translocation of Raf and depends on the integrity of a group of serines in the C terminus of the receptor that are known to interact with 14.3.3 proteins. All three pathways, however, result in BAD phosphorylation. The presence of multiple antiapoptotic pathways may explain the remarkable efficacy of the IGF-1R in protecting cells from apoptosis.

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The insulin receptor substrate-1 (IRS-1), a docking protein for both the type 1 insulin-like growth factor receptor (IGF-IR) and the insulin receptor, is known to send a mitogenic, anti-apoptotic, and anti-differentiation signal. Several micro RNAs (miRs) are suggested by the data base as possible candidates for targeting IRS-1. We show here that one of the miRs predicted by the data base, miR145, whether transfected as a synthetic oligonucleotide or expressed from a plasmid, causes down-regulation of IRS-1 in human colon cancer cells. IRS-1 mRNA is not decreased by miR145, while it is down-regulated by an siRNA targeting IRS-1. Targeting of the IRS-1 3-untranslated region (UTR) by miR145 was confirmed using a reporter gene (luciferase) expressing the miR145 binding sites of the IRS-1 3-UTR. In agreement with the role of IRS-1 in cell proliferation, we show that treatment of human colon cancer cells with miR145 causes growth arrest comparable to the use of an siRNA against IRS-1. Taken together, these results identify miR145 as a micro RNA that down-regulates the IRS-1 protein, and inhibits the growth of human cancer cells.The insulin receptor substrate-1 (IRS-1) 2 is one of the major substrates of both the type 1 insulin-like growth factor receptor (IGF-IR) and the insulin receptor (InR). IRS-1 plays an important role in cell growth and cell proliferation (1). IRS-1, especially when activated by the IGF-IR, sends an unambiguous mitogenic, anti-apoptotic, and anti-differentiation signal (2, 3). IRS-1 levels are often increased in human cancer (4), and they are low or even absent in differentiating cells (1,5,6). Overexpression of IRS-1 causes cell transformation, including the ability to form colonies in soft agar and tumors in mice (7,8). Transgenic expression of IRS-1 in the mammary gland of mice causes mammary hyperplasia, tumorigenicity, and metastases (9). Conversely, down-regulation of IRS-1 (by antisense or siRNA procedures) reverses the transformed phenotype (10 -12). The IRS proteins are conserved during evolution, and a gene described in Drosophila, called chico, is the equivalent of IRS-1 to IRS-4 in mammalian cells. IRS proteins play an important role in cell size. Deletion of chico reduces fly weight by 65% in females and 55% in males (13). Mice with a targeted disruption of the IRS-1 genes are also smaller than their wild-type littermates (14), and ectopic expression of IRS-1 increases rRNA synthesis and doubles cell size in cells in culture (7,15). Thus, IRS-1 seems to play important roles in cell growth (cell size), cell proliferation, and differentiation.Micro RNA (miRs) are RNAs of ϳ22-nucleotides long, that arise from one arm of longer endogenous hairpin transcripts. The characteristics of miRs have been summarized in several reviews (16 -19). Briefly, miRs are cleaved from one arm of a longer endogenous double-stranded precursor (70 -100 nt in length) by Drosha and Dicer enzymes (RNase III family). They are transcribed by RNA polymerase II (20) as long primary transcripts (pri-miRNAs), which ...