Uncoupling of ER-mitochondrial calcium communication by transforming growth factor-β

Transforming growth factor-β (TGF-β) has been implicated as a key factor in mediating many cellular processes germane to disease pathogenesis, including diabetic vascular complications. TGF-β alters cytosolic [Ca ²⁺] ([Ca²⁺]_c) signals, which in some cases may result from the downregulation of the IP₃ receptor Ca²⁺ channels (IP3R). Ca²⁺ released by IP3Rs is effectively transferred from endoplasmic reticulum (ER) to the mitochondria to stimulate ATP production and to allow feedback control of the Ca²⁺ mobilization. To assess the effect of TGF-β on the ER-mitochondrial Ca²⁺ transfer, we first studied the [Ca²⁺]_c and mitochondrial matrix Ca ²⁺ ([Ca²⁺]_m) signals in single preglomerular afferent arteriolar smooth muscle cells (PGASMC). TGF-β pretreatment (24 h) decreased both the [Ca²⁺]_c and [Ca²⁺] _m responses evoked by angiotensin II or endothelin. Strikingly, the [Ca²⁺]_m signal was more depressed than the [Ca ²⁺]_c signal and was delayed. In permeabilized cells, TGF-β pretreatment attenuated the rate but not the magnitude of the IP ₃-induced [Ca²⁺]_c rise, yet caused massive depression of the [Ca²⁺]_m responses. ER Ca²⁺ storage and mitochondrial uptake of added Ca²⁺ were not affected by TGF-β. Also, TGF-β had no effect on mitochondrial distribution and on the ER-mitochondrial contacts assessed by two-photon NAD(P)H imaging and electron microscopy. Downregulation of both IP3R1 and IP3R3 was found in TGF-β-treated PGASMC. Thus, TGF-β causes uncoupling of mitochondria from the ER Ca²⁺ release. The sole source of this would be suppression of the IP3R-mediated Ca²⁺ efflux, indicating that the ER-mitochondrial Ca²⁺ transfer depends on the maximal rate of Ca ²⁺ release. The impaired ER-mitochondrial coupling may contribute to the vascular pathophysiology associated with TGF-β production.