IGF-binding protein-3 fragments in plasma of a child with acute renal failure

Involvement of pregnancy-associated plasma protein-A2 in insulin-like growth factor (IGF) binding protein-5 proteolysis during pregnancy: a potential mechanism for increasing IGF bioavailability

CONTEXT

During pregnancy, circulating IGF binding protein-5 (IGFBP-5) undergoes substantial molecular redistribution from ternary complexes to either binary complexes or the uncomplexed protein.

OBJECTIVE

This study aimed to characterize the proteolysis of circulating IGFBP-5 during pregnancy and to determine whether it can increase IGF bioavailability.

DESIGN

Biochemical methods were used to purify and characterize IGFBP-5 fragments and IGFBP-5-specific proteolytic activity from pregnancy plasma.

RESULTS

Circulating IGFBP-5 was fully proteolyzed at all stages of pregnancy. Cleavage after either Ser143 or Lys144 resulted in two complementary fragments. Of two pools of proteolytic activity (>150 kDa and approximately 40 kDa) identified in pregnancy plasma, only the greater than 150-kDa proteolytic activity was specific to pregnancy. The approximately 40-kDa proteolytic activity, also present in nonpregnancy plasma, appeared largely inactive against IGF-I-complexed IGFBP-5. The greater than 150-kDa proteolytic activity was inhibited by alpha-PAPP-A2 but not alpha-PAPP-A1 antibody, cleaved recombinant IGFBP-5 at Ser143-Lys144 similar to PAPP-A2, and was inactive against IGFBP-5 (Ala128), a PAPP-A2-resistant analog. Compared to nonpregnancy plasma, incubation with pregnancy plasma resulted in release of more bioactive IGF-I from IGF-I-IGFBP-5 complexes as measured by stimulation of IGF-I receptor phosphorylation.

CONCLUSIONS

Circulating IGFBP-5 is proteolyzed by PAPP-A2 during pregnancy, resulting in increased IGF bioavailability, which may have important consequences for the development of the fetus and/or the well-being of the mother.

IGFBP-1 protease activity and IGFBP-1 fragments in a patient with multiple myeloma

OBJECTIVE

Cleavage of IGFBPs by proteases results in IGFBP fragments that have altered IGF-binding affinity, and IGF-independent roles. We have previously purified a specific IGFBP-1 protease activity from the urine of an individual with multiple myeloma and dermatitis. The aim of this study was to determine whether IGFBP-1 protease activity and/or IGFBP-1 fragments were present in the circulation of this patient.

METHODS

The size of immunoreactive IGFBP-1 in serum samples was determined after Superose 12 chromatography. Intact IGFBP-1 and IGFBP-1 fragments were characterized in four RIAs and after SDS-PAGE.

RESULTS

Specific proteolysis of IGFBP-1 generated an N-terminal fragment (IGFBP-1(1-130)) with a predicted molecular mass of 13kDa but an apparent mass of 21kDa on SDS-PAGE. A C-terminal fragment (IGFBP-1(131-234)) produced in vitro migrated at 11.4kDa, close to its predicted size. However a C-terminal fragment of cleaved IGFBP-1 (IGFBP-1(142-234)) migrated at 14kDa on SDS-PAGE. Serum from the patient inhibited IGFBP-1 protease activity. Immunoreactive IGFBP-1 in patient serum was present at molecular masses consistent with IGFBP-1 fragments, in addition to intact IGFBP-1.

CONCLUSIONS

Specific cleavage of IGFBP-1 occurs at the tissue level and not in the circulation in a patient with multiple myeloma and dermatitis. The fragments that are generated may have endocrine roles.

Activity of human pregnancy insulin-like growth factor binding protein-3: determination by reconstituting recombinant complexes

During pregnancy, IGF binding protein-3 (IGFBP-3) is completely proteolyzed to fragments with low affinities for IGFs but appears to transport IGFs normally in high-molecular-mass complexes. We previously reported that synthetic isolated amino- and carboxyl-terminal domains of IGFBP-3 cooperate to bind IGFs, and we investigated whether this is the mechanism whereby proteolyzed IGFBP-3 fragments bind IGFs normally in pregnancy serum. Two fragments of IGFBP-3 have been isolated from pregnancy serum, one with the same N-terminal sequence as intact IGFBP-3 (GASSG) and the other with an N-terminal sequence (160)KVDYE. Recombinant forms of these proteins, IGFBP-3(1-159) and IGFBP-3(160-264), have been synthesized and characterized, demonstrating that although the fragments individually have greatly reduced affinity for IGF complex formation, when combined they cooperate to form complexes with IGF with or without the acid-labile subunit, inhibit IGF transport across endothelial cell monolayers and inhibit IGF-I-induced IGF type I receptor phosphorylation. It is proposed that proteolysis of IGFBP-3 into two discrete complementary fragments does not significantly increase IGF bioavailability, consistent with previous findings that proteolyzed IGFBP-3 in pregnancy serum is functionally normal and circulates as part of the IGF ternary complexes.