Insulin inhibition of protein degradation in cells expressing wild-type and mutant insulin receptors

Gene expression of vitamin D metabolic enzymes at baseline and in response to vitamin D treatment in thyroid cancer cell lines

The association between vitamin D and thyroid cancer is unclear. It is unknown if CYP27A1 or CYP2R1 are present in normal thyroid or cancer cells and there is limited information regarding response to treatment with vitamin D. SV40 immortalized follicular cells (N-thy) and six thyroid cancer cell lines were treated with 10 µM vitamin D(3), 0.1 µM 1,25(OH)(2)D(3) or vehicle × 24 h. CYP27A1, CYP2R1, CYP27B1 and CYP24A1 mRNA were measured using quantitative real-time-PCR before and after treatment. Cell proliferation was also evaluated in TPC1 and C643 cells after treatment with D(3), 25(OH)D(3) and 1,25(OH)(2)D(3). Baseline CYP27A1 and CYP27B1 mRNA were present in all cells, CYP2R1 was higher and CYP24A1 mRNA was lower in cancer cell lines versus N-thy. TPC1 cells had increased CYP24A1 mRNA levels when treated with both D(3) (3.49, p < 0.001) and 1,25(OH)(2)D(3) (5.05, p < 0.001). C643 cells showed increased CYP24A1 mRNA expression when treated with 1,25(OH)(2)D(3) (5.36, p < 0.001). D(3), 25(OH)D(3) and 1,25(OH)(2)D(3) all significantly decreased cell proliferation in TPC1 and C643 cells. Overall, both cancerous and N-thy cell lines express CYP27A1 and CYP2R1 in addition to CYP27B1, establishing the potential to metabolize D(3) to 1,25(OH)(2)D(3). Additionally, vitamin D(3), 25(OH)D(3) and 1,25(OH)(2)D(3) all had an antiproliferative effect on two thyroid cancer cell lines.

Insulin stimulation of PKCδ triggers its rapid degradation via the ubiquitin-proteasome pathway

Insulin rapidly upregulates protein levels of PKCδ in classical insulin target tissues skeletal muscle and liver. Insulin induces both a rapid increase in de novo synthesis of PKCδ protein. In this study we examined the possibility that insulin may also inhibit degradation of PKCδ. Experiments were performed on L6 skeletal muscle myoblasts or myotubes in culture. Phorbol ester (PMA)- and insulin-induced degradation of PKCδ were abrogated by proteasome inhibition. Both PMA and insulin induced ubiquitination of PKCδ, but not of that PKCα or PKCε and increased proteasome activity within 5 min. We examined the role of tyrosine phosphorylation of PKCδ in targeting PKCδ for degradation by the ubiquitin-proteasome pathway. Transfection of cells with PKCδY(311)F, which is not phosphorylated, resulted in abolition of insulin-induced ubiquitination of PKCδ and increase in proteasome activity. We conclude that insulin induces degradation of PKCδ via the ubiquitin-proteasome system, and that this effect requires phosphorylation on specific tyrosine residues for targeting PKCδ for degradation by the ubiquitin-proteasome pathway. These studies provide additional evidence for unique effects of insulin on regulation of PKCδ protein levels.

Aspartyl, arginyl and alanyl aminopeptidase activities in the hippocampus and hypothalamus of streptozotocin-induced diabetic rats

Acid (aspartyl), basic (arginyl) and neutral (alanyl) aminopeptidases degrade angiotensins, vasopressin, oxytocin, bradykinin and enkephalins. These peptides regulate memory, energy homeostasis, water-salt balance and blood pressure, functions that are mainly exerted in the hippocampus and hypothalamus, and that can be affected by diabetes mellitus. To evaluate the relationship between the diabetes mellitus and processing and inactivation roles of these representative aminopeptidases, we measured their activities in both brain structures of control and streptozotocin-diabetic rats. Hypothalamic soluble aspartyl and arginyl aminopeptidases presented significant decreased activity levels in diabetic rats, which were mitigated by insulin therapy. In addition to membrane-bound puromycin sensitive and insensitive alanyl aminopeptidases, its soluble puromycin sensitive form did not differ between diabetic and control rats in both brain structures. Glucose and/or insulin did not seem to alter in vitro the hypothalamic activities of soluble aspartyl and arginyl aminopeptidases. The implied hypothalamic control of regulatory peptide activity by aspartyl and arginyl aminopeptidases supports the hypothesis that the hydrolytic ability of these enzyme types could be a common link for the disruptions of water-salt balance, blood pressure and energy homeostasis in diabetes mellitus.

Prolyl, cystyl and pyroglutamyl peptidase activities in the hippocampus and hypothalamus of streptozotocin-induced diabetic rats

Prolyl, cystyl and pyroglutamyl peptidases are emerging targets for diabetes and cognitive deficit therapies. The present study is focused on the influence of diabetes mellitus induced by streptozotocin on levels of representative hydrolytic activities of these enzymes in the rat hypothalamus and hippocampus. Streptozotocin-diabetic rats presented about 348mg glucose/dL blood, and a slightly increased hematocrit and plasma osmolality. The activities of soluble and membrane-bound dipeptidyl-peptidase IV, and soluble cystyl aminopeptidase did not differ between diabetic and control rats in both brain areas. Hippocampal soluble prolyl oligopeptidase presented similar activities between diabetic and controls. Increased activities in diabetics were observed for soluble prolyl oligopeptidase (1.78-fold) and membrane-bound cystyl aminopeptidase (2.55-fold) in the hypothalamus, and for membrane-bound cystyl aminopeptidase (5.14-fold) in the hippocampus. In both brain areas, the activities of membrane-bound and soluble pyroglutamyl aminopeptidase were slightly lower (<0.7-fold) in diabetics. All modifications (except hematocrit) observed in streptozotocin-treated rats were mitigated by the administration of insulin. Glucose and/or insulin were shown to alter in vitro the hypothalamic activities of soluble pyroglutamyl aminopeptidase and prolyl oligopeptidase, as well as membrane-bound cystyl aminopeptidase. These data provide the first evidence that diabetes mellitus generates direct and indirect effects on the activity levels of brain peptidases. The implied regional control of regulatory peptide activity by these peptidases suggests novel potential approaches to understand certain disruptions on mediator and modulatory functions in diabetes mellitus.