Glycosphingolipids are required for sorting melanosomal proteins in the Golgi complex

A;lthough glycosphingolipids are ubiquitously expressed and essential for multicellular organisms, surprisingly little is known about their intracellular functions. To explore the role of glycosphingolipids in membrane transport, we used the glycosphingolipid-deficient GM95 mouse melanoma cell line. We found that GM95 cells do not make melanin pigment because tyrosinase, the first and rate-limiting enzyme in melanin synthesis, was not targeted to melanosomes but accumulated in the Golgi complex. However, tyrosinase-related protein 1 still reached melanosomal structures via the plasma membrane instead of the direct pathway from the Golgi. Delivery of lysosomal enzymes from the Golgi complex to endosomes was normal, suggesting that this pathway is not affected by the absence of glycosphingolipids. Loss of pigmentation was due to tyrosinase mislocalization, since transfection of tyrosinase with an extended transmembrane domain, which bypassed the transport block, restored pigmentation. Transfection of ceramide glucosyltransferase or addition of glucosylsphingosine restored tyrosinase transport and pigmentation. We conclude that protein transport from Golgi to melanosomes via the direct pathway requires glycosphingolipids

What's new in Birt-Hogg-Dubé syndrome?

Birt-Hogg-Dubé syndrome (BHD) is a rare inherited condition, which predisposes to the development of benign hair follicle tumors called fibrofolliculomas, pneumothorax and kidney cancer. Lung and kidney cysts, respectively, are thought to cause the latter symptoms. The causative gene codes for a highly conserved protein called folliculin. Its function is still unknown, although recent data hint at a pervasive function in cellular signaling, affecting hypoxia responses and growth pathways. Because folliculin's role in the cell is unclear, BHD symptoms are not well understood. Treatment, therefore, is still empirical. In this review, the authors summarize the current state of knowledge and report some of the most recent findings. The authors discuss the implications for pathogenesis and treatment of the cutaneous manifestations in BHD

Birt-Hogg-Dubé syndrome and the skin

Birt-Hogg-Dubé syndrome (MIM #135150) is characterized by the development of benign skin tumours called fibrofolliculomas, pulmonary cysts that may lead to pneumothorax and a high risk of developing kidney cancer. BHD is caused by mutations affecting the highly conserved protein folliculin (FLCN), which probably has a role in intracellular transport. Most of the research effort directed towards BHD has focused on understanding how loss of FLCN causes kidney cancer. The cutaneous manifestations have received comparatively little attention. Although understandable, it is unfortunate, as the fibrofolliculomas are highly accessible and thus potentially are an excellent system for trying to understand the basic pathobiology of BHD. Also, patients can be very much burdened by the cosmetic consequences of having hundreds of facial skin tumours. Our lack of insight into what drives fibrofolliculoma growth translates into a very limited therapeutic arsenal. Thus, paying attention to fibrofolliculomas has both basic science and practical benefits. In this review, we will discuss the state of the art regarding our understanding of fibrofolliculoma pathogenesis and indicate future directions for research

Neuroendocrine carcinoma in a patient with Birt-Hogg-Dube syndrome

Background. A patient with Birt-Hogg-Dube syndrome (BHD) presented with gross hematuria of 6 months' duration. Imaging revealed the presence of a mass in the left prostatic lobe, in addition to a previously observed renal mass. Prostate biopsy and imaging findings indicated an inflammatory etiology, and the patient was discharged. 5 months later, the patient presented once again with urinary retention. During transurethral resection of the prostate, a mass adjacent to the bladder was observed. Postoperative imaging revealed a large pelvic mass, a second mass impinging on the rectum, and extensive lymphadenopathy. The patient died 2 weeks later. Investigations. CT and MRI, physical examination, measurement of serum markers, urinalysis, transrectal prostate biopsy, histopathological and genetic examination of tumor specimens, postmortem immunohistochemical analysis. Diagnosis. Neuroendocrine carcinoma of prostate or bladder origin. Management. The patient died before planned chemotherapy or radiation therapy could be implemented. More-frequent monitoring of the patient might have led to earlier diagnosis and allowed treatment to be started before widespread tumor metastasis and invasion

FLCN, a novel autophagy component, interacts with GABARAP and is regulated by ULK1 phosphorylation

Birt-Hogg-Dubé (BHD) syndrome is a rare autosomal dominant condition caused by mutations in the FLCN gene and characterized by benign hair follicle tumors, pneumothorax, and renal cancer. Folliculin (FLCN), the protein product of the FLCN gene, is a poorly characterized tumor suppressor protein, currently linked to multiple cellular pathways. Autophagy maintains cellular homeostasis by removing damaged organelles and macromolecules. Although the autophagy kinase ULK1 drives autophagy, the underlying mechanisms are still being unraveled and few ULK1 substrates have been identified to date. Here, we identify that loss of FLCN moderately impairs basal autophagic flux, while re-expression of FLCN rescues autophagy. We reveal that the FLCN complex is regulated by ULK1 and elucidate 3 novel phosphorylation sites (Ser406, Ser537, and Ser542) within FLCN, which are induced by ULK1 overexpression. In addition, our findings demonstrate that FLCN interacts with a second integral component of the autophagy machinery, GABA(A) receptor-associated protein (GABARAP). The FLCN-GABARAP association is modulated by the presence of either folliculin-interacting protein (FNIP)-1 or FNIP2 and further regulated by ULK1. As observed by elevation of GABARAP, sequestome 1 (SQSTM1) and microtubule-associated protein 1 light chain 3 (MAP1LC3B) in chromophobe and clear cell tumors from a BHD patient, we found that autophagy is impaired in BHD-associated renal tumors. Consequently, this work reveals a novel facet of autophagy regulation by ULK1 and substantially contributes to our understanding of FLCN function by linking it directly to autophagy through GABARAP and ULK1

Birt-Hogg-Dubé syndrome is a novel ciliopathy

Birt-Hogg-Dubé (BHD) syndromeis an autosomal dominant disorder where patients are predisposed to kidney cancer, lung and kidney cysts and benign skin tumors. BHD is caused by heterozygous mutations affecting folliculin (FLCN), a conserved protein that is considered a tumor suppressor. Previous research has uncovered multiple roles for FLCN in cellular physiology, yet it remains unclear how these translate to BHD lesions. Since BHD manifests hallmark characteristics of ciliopathies, we speculated that FLCN might also have a ciliary role. Our data indicate that FLCN localizes to motile and non-motile cilia, centrosomes and the mitotic spindle. Alteration of FLCN levels can cause changes to the onset of ciliogenesis, without abrogating it. In threedimensional culture, abnormal expression of FLCN disrupts polarized growth of kidney cells and deregulates canonical Wnt signalling. Our findings further suggest that BHD-causing FLCN mutants may retain partial functionality. Thus, several BHD symptoms may be due to abnormal levels of FLCN rather than its complete loss and accordingly, we show expression of mutant FLCN in a BHD-associated renal carcinoma. We propose that BHD is a novel ciliopathy, its symptoms at least partly due to abnormal ciliogenesis and canonical Wnt signalling