Effects of estrogen on parkin, UCH-L1, and uncoupling protein (UCP) 2, -4, and -5 on MPP+-induced apoptosis in human neuroblastoma

Abstract

OBJECTIVE: To explore the role of Parkin, UCH-L1 and uncoupling proteins (UCP2, 4 and 5) in estrogenic neuroprotection in MPP-induced cell death. BACKGROUND: Parkin and UCH-L1 are crucial enzymes in the ubiquitin-proteasome proteolytic system, and may be neuroprotective, and their activities are lost in juvenile-onset autosomal parkinsonism. Uncoupling Un-coupling proteins (UCPs) are mitochondrial transporter proteins involved in metabolic and thermoregulation. UCP2, 4 and 5 are abundantly expressed in the brain but their functions are unclear. UCP2 has neuroprotective properties, but whether UCP4 and 5 have similar properties is unknown. We explored whether neuroprotective effects of estrogen involve modulation of Parkin, UCH-L1, and UCPs to alleviate MPP-induced toxicity. METHODS: We used the MPP+ concentration that caused apoptosis as measured by caspase 3 activity (from total cell protein) and significant cell death as indicated by LDH release (from charcoal-stripped culture medium). Differentiated SK-N-SH cells were incubated for 24 hr in 17β- estradiol (E2; 1 µM) before MPP exposure for another 0, 24, 48 and 72 hr. Total RNA was digested with DNase I before spectrophotometric quantification. Corresponding mRNA levels of Parkin, UCH-L1, UCP2, 4 and 5 were measured using real-time RT-PCR. Ribosomal-18S RNA was used to normalize RNA amount. RESULTS: MPP (0.5 mM) exposure caused significant LDH release after 48 and 72 hr by 65% and 68%, respectively (P < 0.05), and increased caspase 3 activity by 15% after 24 hr and 36% after 48 hr (P < 0.01), and back to control level after 72 hr. E2 (1 µM) prevented apoptosis, and reduced MPP+-induced caspase 3 activity by 11% at 48 hr, although no significant differences were found at 24 or 72 hr. MPP+ increased UCP2 (0% at 24 hr, 16% at 48 hr, 38% at 72 hr) and UCP5 (33% at 24 hr, 55% at 48 hr, 65% at 72 hr), but not Parkin, UCH-L1, and UCP4. E2 pretreatment did not affect Parkin, UCH-L1, and UCP2, -4, -5 gene expression in MPP+-treated cells. CONCLUSION: Estrogen can protect against MPP-induced toxicity but it did not appear to involve modulation of Parkin, UCH-L1, UCP2, -4, and -5. However, the increase in UCP2 and -5 in MPP+ toxicity may indicate their possible roles in neuroprotection.