EFFECTS OF CHRONIC EXPOSURE TO SIMULATED MICROGRAVITY ON SKELETAL MUSCLE CELL PROLIFERATION AND DIFFERENTIATION

Cell culture models that mimic long-term exposure to microgravity provide important insights into the cellular biological adaptations of human skeletal muscle to long-term residence in space. We developed insert scaffolding for the NASA-designed rotating cell culture system (RCCS) in order to study the effects of time-averaged microgravity on the proliferation and differentiation of anchorage-dependent skeletal muscle myocytes. We hypothesized that prolonged microgravity exposure would result in the retardation of myocyte differentiation. Microgravity exposure in the RCCS resulted in increased cellular proliferation. Despite shifting to media conditions promoting cellular differentiation, 5 d later, there was an increase in cell number of approximately 62%, increases in total cellular protein (52%), and cellular proliferating cell nuclear antigen (PCNA) content (2.7 times control), and only a modest (insignificant) decrease (10%) in sarcomeric myosin protein expression. We grew cells in an inverted orientation on membrane inserts. Changes in cell number and PCNA content were the converse to those observed for cells in the RCCS. We also grew cells on inserts at unit gravity with constant mixing. Mixing accounted for part, but not all, of the effects of microgravity exposure on skeletal muscle cell cultures (53% of the RCCS effect on PCNA at 4-6 d). In summary, the mechanical effects of simulated microgravity exposure in the RCCS resulted in the maintenance of cellular proliferation, manifested as increases in cell number and expression of PCNA relative to control conditions, with only a modest reciprocal inhibition of cellular differentiation. Therefore, this model provides conditions wherein cellular differentiation and proliferation appear to be uncoupled.

Regulation of the insulin-like growth factor axis by increasing cell number in intestinal epithelial (IEC-6) cells

Insulin-like growth factor binding protein-2 (IGFBP-2) production as a function of cell number by intestinal epithelial cells (IEC-6) was regulated such that the IGFBP-2 concentration in 24-h conditioned medium reached a maximum, which was maintained despite increasing cell number. Northern blot analysis revealed that this effect could largely be attributed to decreasing IGFBP-2 mRNA. In contrast to IGFBP-2, secretion of IGF-II and accumulation of IGF-II mRNA by IEC-6 cells correlated positively with cell number. The highest level of IGF-II protein detected by immunoblotting of conditioned medium occurred in post-confluent cells. IGF-I stimulated the cells to grow to a high cellular density and inhibited IGFBP-2 secretion in a concentration-dependent fashion. We conclude that expression of IGF-II and IGFBP-2 are regulated in IEC-6 cells by cellular density, and IGF-II may act as a survival factor at high cell density.

An insulin-like growth factor II (IGF-II) affinity-enhancing domain localized within extracytoplasmic repeat 13 of the IGF-II/mannose 6-phosphate receptor

Insulin-like growth factor II (IGF-II) and phosphomannosylated glycoproteins bind to distinct sites on the same receptor, the IGF-II/mannose 6-phosphate receptor (IGF2R). Analysis of truncated receptors (minireceptors) has been used to map the IGF-II binding site within the receptor's extracytoplasmic domain, which consists of 15 homologous repeats. A minireceptor consisting of repeat 11 contained the minimal elements for binding IGF-II, but with 5- to 10-fold lower relative binding affinity than the full-length receptor. We hypothesized that the complete, high-affinity IGF-II binding site is formed by interaction between the primary site in repeat 11 and a putative affinity-enhancing domain. To determine the minimum portion of the IGF2R's extracytoplasmic domain needed for expression of high-affinity IGF-II binding, a nested set of FLAG epitope-tagged minireceptors encompassing repeats 11 through 15 was prepared and transiently expressed in 293T cells. Minireceptors containing repeats 11-13 or 11-15 exhibited high affinity, comparable to the full-length receptor (IC50 = 1-2 nM), whereas constructs containing repeat 11 only or repeats 11-12 did not (IC50 = 10-20 nM). These data suggested that the affinity-enhancing domain is located within repeat 13, which contains a unique 43-residue insert that has approximately 50% sequence identity to the type II repeat of fibronectin. Although a repeat 13 minireceptor did not bind IGF-II on its own, an 11-13 minireceptor containing a deletion of the 43-residue insert exhibited low IGF-II binding affinity (IC50 = 10-20 nM). Expression of mutant receptors from a full-length IGF2R construct bearing a deletion of the 43-residue insert was very low relative to wild type. Depletion assays using IGF-II-Sepharose showed that the mutant receptor had lower affinity for IGF-II than the wild-type receptor. This study reveals that two independent receptor domains are involved in the formation of a high-affinity binding site for IGF-II, and that a complete repeat 13 is required for high-affinity IGF-II binding.

Truncated forms of the insulin-like growth factor II (IGF-II)/mannose 6-phosphate receptor encompassing the IGF-II binding site: characterization of a point mutation that abolishes IGF-II binding

Complete understanding of the functional significance of insulin-like growth factor II (IGF-II) binding by the IGF-II/mannose-6-phosphate (Man-6-P) receptor requires mapping and ultimately mutational analysis of the receptor's IGF-II binding domain. Recent advances have localized the IGF-II binding site to extracytoplasmic repeats 10-11. To improve resolution of the binding site map, a nested set of epitope-tagged, truncated forms of the human IGF-II/Man-6-P receptor were transiently expressed in COS-7 cells. The IGF-II binding properties of truncated receptors immunoprecipitated from cell lysates and conditioned media were determined by affinity cross-linking. From the largest truncated receptor, encompassing extracytoplasmic repeats 8-11 (M(r) 68 K), through the smallest, comprised primarily of repeat 11 (M(r) 23 K), all were able to bind and cross-link to IGF-II. As a group, the truncated receptors had similar affinities for IGF-II, but with relative binding affinities 5-to 10-fold lower than those of full-length receptors. A point mutation substituting threonine for isoleucine at residue 1572, located in the NH2-terminal half of repeat 11, completely abolished IGF-II binding. We conclude that repeat 11 of the IGF-II/Man-6-P receptor's extracytoplasmic domain contains the minimal elements required for binding and cross-linking to IGF-II, and that lle1572 and other residues within the NH2-terminal half of repeat 11 are particularly important for IGF-II interaction.

Expression of insulin-like growth factor-II and insulin-like growth factor binding proteins during Caco-2 cell proliferation and differentiation

The components of the insulin-like growth factor (IGF) axis and their roles in regulating proliferation and differentiation of the human colon adenocarcinoma cell line, Caco-2, have been investigated. Caco-2 cells proliferated in serum-free medium at 75% the rate observed in medium containing 10% fetal bovine serum. IGF-I (10 nM) increased Caco-2 cell growth in serum-free medium, but not to the rate seen with serum. Multiple IGF-II mRNA species were produced by Caco-2 cells, but IGF-I mRNA was undetectable. Secretion of radioimmunoassayable IGF-II corresponded with steady-state levels of IGF-II mRNA, neither of which was observed to change markedly over the course of 16 days of Caco-2 cell differentiation. Levels of sucrase-isomaltase mRNA, a marker for enterocytic differentiation, increased 12-fold between days 5 and 16 of culture. Northern blotting of total RNA and ligand blot and immunoblot analyses of serum-free conditioned medium revealed that Caco-2 cells produce several IGF binding proteins (IGFBPs), including IGFBP-2, -3, and -4, as well as a 31,000 M(r) species that was not identified. The pattern of IGFBP secretion changed dramatically during Caco-2 cell differentiation: IGFBP-3 and IGFBP-2 increased 8.5-fold and 5-fold, respectively, whereas IGFBP-4 and the 31,000 M(r) species decreased 43% and 90%. Caco-2 cell clones stably transfected with a human IGFBP-4 cDNA construct exhibited a 60% increase in steady-state level of IGFBP-4 mRNA, and secreted twice as much IGFBP-4 protein as controls. Moreover, IGFBP-4-overexpressing cells proliferated at only 25% the rate of control cells in serum-free medium, in conjunction with a 70% increase in expression of sucrase-isomaltase. In summary, these studies indicate that a complex IGF axis is involved in autocrine regulation of Caco-2 cell proliferation and differentiation.

Growth stimulation by transfection of intestinal epithelial cells with an antisense insulin-like growth factor binding protein-2 construct

IEC-6 cells, an intestinal epithelial cell line, produce insulin-like growth factor (IGF)-II and IGF-binding protein-2 (IGFBP-2). Exogenous IGFs stimulate and IGFBP-2 attenuates DNA synthesis. To investigate whether endogenously secreted IGFBP-2 inhibits proliferation, IEC-6 cells were transfected with a full-length rat IGFBP-2 cDNA antisense expression construct. The steady-state level of IGFBP-2 mRNA decreased by 54% in the antisense cDNA-transfected cells (denoted revBP2) compared with vector-transfected controls. Moreover, revBP2 cells secreted less IGFBP-2 than controls (maximally a 68% decrease). In serum-containing medium, revBP2 cells exhibited a 35% increase in log-phase proliferation rate and growth-arrested at a maximum density 14% higher than controls. We conclude that endogenous IGFBP-2 inhibits proliferation of IEC-6 cells, probably by sequestering IGFs.