Mitogenic properties of insulin and insulin analogues mediated by the insulin receptor.

Insulin has traditionally been considered as a hormone essential for metabolic regulation, while the insulin-like growth factors (IGF-I and IGF-II) are postulated to be more specifically involved in growth regulation. The conventional wisdom is that they share each other's effects only at high concentrations, due to their weak affinity for the heterologous receptor. We discuss here the evidence that in the proper cellular context, insulin can be mitogenic at physiologic concentrations through its own receptor. We studied the insulin and IGF-I binding characteristics of a new model suitable for analysing insulin receptor mediated mitogenesis; that is, a T-cell lymphoma line that depends on insulin for growth, but is unresponsive to IGFs. The cells showed no specific binding of 125I-IGF-I and furthermore, no IGF-I receptor mRNA was detected by RNAse protection assay in the LB cells, in contrast with mouse brain and thymus. The cells bound at saturation about 3000 insulin molecules to receptors that had normal characteristics in terms of affinity, kinetics, pH dependence and negative co-operativity. A series of insulin analogues competed for 125I-insulin binding with relative potencies comparable to those observed in other insulin target cells. The full sequence of the insulin receptor cDNA was determined and found to be identical to the published sequence of the murine insulin receptor cDNA. The LB cell line is therefore an ideal model with which to investigate insulin mitogenic signalling without interference from the IGF-I receptor. Using this model, we have started approaching the molecular basis of insulin-induced mitogenesis, in particular the role of signalling kinetics in choosing between mitogenic and metabolic pathways.

Release of ecto-protein kinases by the protozoan parasite Leishmania major.

Leishmania major promastigotes have externally oriented ecto-protein kinases (PK) that are capable of phosphorylating both endogenous membrane substrates and foreign proteins. Live parasites phosphorylate protamine sulfate, casein, and phosvitin but not bovine serum albumin. Addition of exogenous PK substrates, such as phosvitin or casein, induced the shedding of ecto-PK that are capable of phosphorylating protamine sulfate. No phosphorylation of protamine sulfate was seen when cell-free supernatants from promastigotes incubated with either buffer alone or bovine serum albumin were used. A second enzyme, a constitutively released PK that phosphorylates casein or phosvitin and not protamine sulfate or mixed histones, was identified and characterized. This PK is inhibited by 5 microM staurosporine, 50 microg/ml heparin, and 75 microM CKI-7, concentrations typical of the IC50 found for other eukaryotic casein kinases (CK). The constitutively shed ecto-PK specifically phosphorylated a peptide substrate for CK1 but not for CK2, suggesting that this shed PK is similar to CK1.