Insulin-like growth factor II overexpression does not affect sorting of lysosomal enzymes in NIH-3T3 cells

Does the overexpression of pro-insulin-like growth factor-II in transfected human embryonic kidney fibroblasts increase the secretion of lysosomal enzymes?

Insulin-like growth factor-II (IGF-II) and lysosomal enzymes bearing the mannose 6-phosphate (Man6P) recognition marker, bind to two distinct binding sites of the IGF-II/M6P receptor. The two classes of ligands reciprocally modulate the binding of the other class of ligand to the receptor [Kiess, W., Thomas, C. L., Greenstein, L., Lee, L., Sklar, M. M., Rechler, M. M., Sahagian, G. G. & Nissley, S. P. (1989) J. Biol. Chem. 264, 4710-4714]. We asked whether or not overexpression of pro-IGF-II by cells in culture leads to missorting of lysosomal enzymes. Human embryonal kidney fibroblasts were transfected with the full-length human IGF-II cDNA or a control cDNA. Solution hybridization/RNase protection experiments using a human IGF-II riboprobe showed that two transfectants expressed large quantities of IGF-II mRNA, whereas the non-transfected cells did not. The analysis of conditioned media revealed that these cells secrete approximately 0.15 micrograms and 1.0 micrograms immunoreactive IGF-II/ml and 22 x 10(6) cells and 24 x 10(6) cells within 24 hours. Immunoreactive IGF-II was shown by Western blotting to represent 17-kDa pro-IGF-II. The amount of the lysosomal enzyme, beta-hexosaminidase, was approximately twofold increased in the conditioned media from pro-IGF-II overexpressing cells compared with control media, as shown by Western-blot analysis and immunoprecipitation of media extracts of metabolically labeled cells. The synthesis rate of beta-hexosaminidase was not affected by pro-IGF-II overexpression. In addition, the basal amount of another newly synthesized lysosomal enzyme, the cathepsin D precursor, was also twofold higher in pro-IGF-II overexpressing cells than in control cells. In contrast, the surface binding and cellular uptake rate of a Man6P-containing neoglycoprotein did not differ between the cell lines. The results indicate that the overexpression of pro-IGF-II doubles the secretion and/or reduces the re-uptake of beta-hexosaminidase and cathepsin D to approximately 20% of the total synthesized enzymes in human embryonal kidney fibroblasts compared to control cells. We hypothesize that, in cells synthesizing high amounts of pro-IGF-II, the growth factor may modulate the targeting of a portion of lysosomal enzymes, mainly by partially enhancing the secretion of newly synthesized enzymes and, in addition, possibly by affecting the re-uptake mechanism.

Mannose 6-phosphate receptors in sorting and transport of lysosomal enzymes

Mannose 6-phosphate receptors have been intensively studied with regard to their genomic organization, protein structure, ligand binding properties, intracellular trafficking and sorting functions. That their main function is sorting of newly synthesized lysosomal enzymes is commonly accepted, but much more remains to be learned about their precise recycling pathways and the mechanisms which regulate their vesicular transport. Additional functions have been reported, e.g., export of newly synthesized lysosomal enzymes from the cell by MPR 46 or a--probably indirect--participation in growth factor-mediated signal transduction by MPR 300. To understand the physiological relevance of these observations will be a challenge for future research.

Escape from negative regulation of growth by transforming growth factor beta and from the induction of apoptosis by the dietary agent sodium butyrate may be important in colorectal carcinogenesis

There are a number of lines of evidence suggesting that transforming growth factor beta (TGF beta) has an important role in the control of intestinal growth and differentiation. In vivo localization studies show that TGF beta expression occurs predominantly in the differentiated non proliferating cells of the intestinal epithelium. The use of an antisense expression vector for TGF beta resulted in an increased tumorigenicity in an antisense-transfected cancer cell line. In vitro proliferation studies showed colorectal premalignant adenoma cells to be more sensitive to the growth inhibitory effects of TGF beta than colorectal cancer cells. Furthermore the conversion of an adenoma to a carcinoma was accompanied by a reduced response to the inhibitory effects of TGF beta. The acquisition of partial or complete resistance to the inhibitory effects of TGF beta may be an important late event in colorectal carcinogenesis. Of further interest is the possibility that clonal selection could occur even more rapidly in colorectal tumour cells which not only had lost response to TGF beta inhibition but produced TGF beta and were growth stimulated by it. This could have the advantage of not only inhibiting the growth of surrounding less malignantly advanced cells but of also escaping from their potential growth suppressive influence. Carcinogenesis is not, however, simply losing response to negative regulators of growth; the fully malignant cell has to acquire new characteristics of invasiveness and metastatic potential. Growth factors including TGF beta may have a role in the complex cascade of events leading to the activation of proteolytic enzymes which are involved in progression to an invasive phenotype. Cell proliferation in the large bowel, as well as being under the control of endogenous growth factors, is also under the influence of dietary components in the lumen such as the naturally occurring fatty acid sodium butyrate. Sodium butyrate at physiological concentrations induces apoptosis (programmed cell death) in colonic tumour cell lines. Since sodium butyrate occurs naturally in the colorectum, being produced by bacterial fermentation of dietary fibre, it may be involved in the control of cell death in human colorectal epithelium. This could, in part, explain the apparent protective effects of dietary fibre. Clonal evolution and tumour progression in colorectal carcinogenesis could therefore involve loss of response to endogenous growth factors such as TGF beta and an escape from the induction of programmed cell death by dietary factors.

Developmental expression of the IGF-II/mannose 6-phosphate receptor

The first indication that the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/M6PR) is developmentally regulated came from studies of the serum form of the receptor in the rat. By immunoblotting, the circulating form of the receptor, which was 10 kDa smaller than the tissue receptor, was high in 19 day fetal and 3, 10, and 20 day postnatal sera and then declined sharply. We next used quantitative immunoblotting to measure the total tissue IGF-II/M6PR in the rat. The receptor levels were high in fetal tissues and in most tissues declined dramatically in late gestation and/or in the early postnatal period. The rank order of receptor expression was heart > placenta > lung = intestine > muscle = kidney > liver > brain. In heart, the receptor was 1.7% of total protein in the extract. More recently, we have examined the expression of IGF-II/M6PR mRNA using Northern blotting and a solution hybridization/RNase protection assay. The rank order of receptor mRNA concentration among fetal tissues agreed with the rank order of receptor protein. The concentration of receptor mRNA was significantly lower in postnatal tissue than in fetal tissue. Thus IGF-II/M6PR mRNA concentration is an important determinant of receptor protein in most tissues. What is the function of the IGF-II/M6PR in embryonic and fetal tissues? The M6PR in birds and frogs does not bind IGF-II. It is intriguing that the rat IGF-II/M6PR is prominent during the embryonic and fetal periods, times at which the differences between mammals, on the one hand, and frogs and birds, on the other, are most striking.(ABSTRACT TRUNCATED AT 250 WORDS)