Control of myofibroblast differentiation by microtubule dynamics through a regulated localization of mDia2.

Myofibroblast differentiation plays a critical role in wound healing and in the pathogenesis of fibrosis. We have previously shown that myofibroblast differentiation is mediated by the activity of serum response factor (SRF), which is tightly controlled by the actin polymerization state. In this study, we investigated the role of the microtubule cytoskeleton in modulating myofibroblast phenotype. Treatment of human lung fibroblasts with the microtubule-destabilizing agent, colchicine, resulted in a formation of numerous stress fibers and expression of myofibroblast differentiation marker proteins. These effects of colchicine were independent of Smad signaling but were mediated by Rho signaling and SRF, as they were attenuated by the Rho kinase inhibitor, Y27632, or by the SRF inhibitor, CCG-1423. TGF-ß-induced myofibroblast differentiation was not accompanied by gross changes in the microtubule polymerization state. However, microtubule stabilization by paclitaxel attenuated TGF-ß-induced myofibroblast differentiation. Paclitaxel had no effect on TGF-ß-induced Smad activation and Smad-dependent gene transcription but inhibited actin polymerization, nuclear accumulation of megakaryoblastic leukemia-1 protein, and SRF activation. The microtubule-associated formin, mDIA2, localized to actin stress fibers upon treatment with TGF-ß, and paclitaxel prevented this localization. Treatment with the formin inhibitor, SMI formin homology 2 domain, inhibited stress fiber formation and myofibroblast differentiation induced by TGF-ß, without affecting Smad-phosphorylation or microtubule polymerization. Together, these data suggest that (a) TGF-ß promotes association of mDia2 with actin stress fibers, which further drives stress fiber formation and myofibroblast differentiation, and (b) microtubule polymerization state controls myofibroblast differentiation through the regulation of mDia2 localization.

Regulation of Prostate Androgens by Megalin and 25-hydroxyvitamin D Status: Mechanism for High Prostate Androgens in African American Men.

Vitamin D deficiency is associated with an increased risk of prostate cancer mortality and is hypothesized to contribute to prostate cancer aggressiveness and disparities in African American populations. The prostate epithelium was recently shown to express megalin, an endocytic receptor that internalizes circulating globulin-bound hormones, which suggests regulation of intracellular prostate hormone levels. This contrasts with passive diffusion of hormones that is posited by the free hormone hypothesis. Here, we demonstrate that megalin imports testosterone bound to sex hormone-binding globulin into prostate cells. Prostatic loss of Lrp2 (megalin) in a mouse model resulted in reduced prostate testosterone and dihydrotestosterone levels. Megalin expression was regulated and suppressed by 25-hydroxyvitamin D (25D) in cell lines, patient-derived prostate epithelial cells, and prostate tissue explants. In patients, the relationships between hormones support this regulatory mechanism, as prostatic DHT levels are higher in African American men and are inversely correlated with serum 25D status. Megalin levels are reduced in localized prostate cancer by Gleason grade. Our findings suggest that the free hormone hypothesis should be revisited for testosterone and highlight the impact of vitamin D deficiency on prostate androgen levels, which is a known driver of prostate cancer. Thus, we revealed a mechanistic link between vitamin D and prostate cancer disparities observed in African Americans. Significance: These findings link vitamin D deficiency and the megalin protein to increased levels of prostate androgens, which may underpin the disparity in lethal prostate cancer in African America men.