Pathogenesis of Acne Vulgaris – Sebum: Factors Regulating Sebum Production
The sebaceous glands exude lipids by disintegration of entire cells, a process known as holocrine secretion. The life span of a sebocyte from cell division to holocrine secretion is approximately 21 to 25 days (1). Because of the constant state of renewal and secretion of the sebaceous gland, individual cells within the same gland are engaged in different metabolic activities dependent on their differentiation state (2). The stages of this process are evident in the histology of the gland (3). The outermost cells, basal cell layer membrane, are small, nucleated, and devoid of lipid droplets.
This layer contains the dividing cells that replenish the gland as cells are lost in the process of lipid excretion. As cells are displaced into the center of the gland, they begin to produce lipid, which accumulates in droplets. Eventually, the cells become greatly distended with lipid droplets with the nuclei and other subcellular structures disappearing. As the cells approach the sebaceous duct, they disintegrate and release their con-tents. Only neutral lipids reach the skin surface. Proteins, nucleic acids, and the membrane phospholipids are digested and most likely recycled during the dis-integration of the cells.
LIPID COMPOSITION OF SEBUM
Human sebum, as it leaves the sebaceous gland, contains squalene, cholesterol, cholesterol esters, wax esters, and triglycerides. During passage of sebum through the hair canal, bacterial lipases from Propionibacterium acnes hydrolyze some of the triglycerides, so that the lipid mixture reaching the skin surface contains free fatty acids (FFA) and small proportions of mono- and diglycerides in addition to the original components. The wax esters and squalene
distinguish sebum from the lipids of human internal organs, which contain no wax esters and little squalene. Human sebaceous glands, however, appear to be unable to cyclize squalene to sterols such as cholesterol. The unsaturation pat-terns of the fatty acids in the triglycerides, wax esters, and cholesterol esters also distinguish human sebum from the lipids of other organs. The “normal” mam-malian pathway of desaturation involves inserting a double bond between the ninth and tenth carbons of stearic acid (18:0) to form oleic acid (18:1A9). However, in human sebaceous glands, the predominant pattern is the insertion of a A6 double bond into palmitic acid (16:0). The resulting sapienic acid (16:1A6) is the major fatty acid of adult human sebum. Elongation of the chain by two carbons and insertion of another double bond gives sebaleic acid (18:2A5,8), a fatty acid thought to be unique to human sebum (4).
Sebaceous fatty acids and alcohols are also distinguished by chain branching. Methyl branches can occur on the next to last (penultimate) carbon of a fatty acid chain (iso branching), on the third from the last (antepenultimate) carbon (anteiso branching), or on any even-numbered carbon (internal branching).
FUNCTION OF SEBUM
The precise function of sebum in humans is unknown. Cunliffe and Shuster proposed that sebum’s solitary role is to cause acne (6). Another theory suggests that sebum reduces water loss from the skin’s surface and functions to keep skin soft and smooth. The sebaceous gland-deficient mouse (Asebia) model provides evidence that glycerol derived from triglyceride hydrolysis in sebum is critical for maintaining stratum corneum hydration (7), but there is no evidence for this in humans as stratum corneum hydration is normal during periods, such as, childhood when the gland is fairly quiescent. Similarly, vitamin E delivery to the upper layers of the skin protects the skin and its surface lipids from oxidation; thus, sebum flow to the surface of the skin may provide the transit mechanism necessary for vitamin E to function (8).
Recent evidence suggests that sebaceous glands and sebum play a role in the skin’s innate immune defense mechanism. Initial investigations showed that sebum has mild antibacterial action, presumably due to the presence of immu-noglobulin A, which is secreted from most exocrine glands (9). Recent studies show that FFA in human sebum is bactericidal against gram-positive organisms as a result of its ability to increase antimicrobial peptide, R-defensin 2 (HBD2) expression (10). Additional antimicrobial peptides including cathelicidin, psoriasin, R-defensin 1, and R-defensin 2 are expressed within the sebaceous gland. Functional cathelicidin peptides have direct antimicrobial activity against P. acnes but also initiate cytokine production and inflammation in the host organism (11,12). Innate immune Toll-like receptors 2 and 4 (TLR2, TLR4) and CD1d and CD14 molecules are also expressed in sebaceous glands and immortalized human sebocytes (13). The antibacterial activities of sebum itself and the expression of innate immune receptors and antibacterial peptides within the sebaceous gland provide compelling evidence that the sebaceous gland may play an important role in pathogen recognition and protection of the skin surface.
FACTORS REGULATING SEBUM PRODUCTION
The exact mechanisms underlying the regulation of human sebum production are not fully defined. A variety of experimental models are used to study the factors involved in sebaceous gland regulation including cell culture of isolated human sebaceous glands, primary sebocytes, and immortalized sebocyte cell lines; as well as mouse and hamster animal models. Results from these investigations clearly indicate that sebaceous glands are regulated by androgens and retinoids. Recent evidence suggests that peroxisome proliferator activated receptors, melanocortins, corticotropin-releasing hormone, and fibroblast growth factor receptors play a role as well.
Androgen receptors are located in both the keratinocytes of the outer root sheath of hair follicles as well as the basal layer of the sebaceous gland (14). Individuals with a genetic deficiency of androgen receptors (complete androgen insensitivity) have no detectable sebum secretion and do not develop acne (15). Conversely, addition of testosterone and dihydroepiandrosterone increases the size and secretion of sebaceous glands (16), although which androgen is physiologically significant is still debated. The most potent androgens are testosterone and dihydrotestosterone (DHT); however, levels of testosterone do not parallel the patterns of sebaceous gland activity. Sebum secretion starts to increase in children (5 6 years of age) during adrenarche, although the levels of androgens are very low at this time (17). Testosterone levels are significantly higher in males than in females, with no overlap between the sexes. However, the average rates of sebum secretion are only slightly higher in males than in females, with considerable overlap between the sexes (18,19). The majority of females with acne have serum androgen levels that, although higher, are within normal limits, and it has been hypothesized that locally produced androgens within the sebaceous gland may contribute to acne (20,21).
The weak adrenal androgen, dehydroepiandrosterone sulfate (DHEAS), may regulate sebaceous gland activity through its conversion to testosterone and DHT within the sebaceous gland. The enzymes required to convert DHEAS to more potent androgens are present within sebaceous glands (22). The predominant isozymes in the sebaceous gland include the type 1 30-hydroxysteroid dehydrogenase (30-HSD), the type 2 170-HSD, and the type 1 5oc-reductase (23 25). Investigations into the influence of locally produced androgens indicated that the activities of 5oc-reductase and 170-HSD enzymes within the sebaceous gland are not higher in male or female patients with acne compared with patient controls with no acne. Because of the small sample size, the influence of local androgen synthesis cannot be ruled out (19). Clearly, androgens influence sebaceous glands and sebum production, although which androgens are important and the mechanism of their influence are not known.
Isotretinoin [13-cis retinoic acid (13-cis RA), Accutane®] is the most potent pharmacologic inhibitor of sebum secretion. Significant reductions in sebum production can be observed as early as two weeks after use (26). Histologically, sebaceous glands are markedly reduced in size, and individual sebocytes appear undifferentiated, lacking the characteristic cytoplasmic accumulation of seba-ceous lipids.
Isotretinoin does not interact with any of the known retinoid receptors. It may serve as a prodrug for the synthesis of all-trans RA or 9-cis RA, which does interact with retinoid receptors; however, it has greater sebosuppressive action than do all-trans or 9-cis RA (27). The mechanism by which 13-cis RA lowers sebum secretion is currently under investigation. Experimental evidence shows that 13-cis RA inhibits the 3oc-hydroxysteroid activity of retinol dehydrogenase leading to decreased androgen synthesis (28). In addition, isotretinoin triggers cell cycle arrest in human sebocytes and immortalized cell culture models of human sebocytes (SZ95 and SEB-1), as well as induces apoptosis in SEB-1 sebocytes in part due to an increase in the neutrophil gelatinase associated lipocalin (NGAL) (29 32). Inhibition of androgen synthesis, cell cycle arrest, and apoptosis by 13-cis RA may explain the reduction of sebaceous gland size after treatment.
Peroxisome Proliferator–Activated Receptors
Peroxisome proliferator activated receptors (PPARs) are orphan nuclear recep-tors that are similar to retinoid receptors in many ways. Each of these receptors forms heterodimers with retinoid X receptors to regulate the transcription of genes involved in a variety of processes, including lipid metabolism and cellular proliferation and differentiation (33 36). Rat preputial cells serve as a model for human sebocytes in the laboratory (37). In rat preputial cells, agonists of the PPAR-oc and PPAR-,y receptors induced lipid droplet formation in preputial sebocytes but not in epidermal cells, while linoleic acid (PPAR-R/S agonist) induced lipid formation in both preputial sebocytes and epidermal cells (38). On the basis of the results from their studies, Rosenfield et al. propose that PPAR-oc activation plays a role in the beginning stages of lipogenesis, PPAR-R/S acti-vation enhances the lipogenesis, and PPAR-,y activation controls the transition to a more differentiated state complete with more lipid droplets within the cells, clearly identifying PPARs as a key player in sebocyte differentiation (38). Within human sebocytes, PPAR-oc, -R/S, and –,y receptor subtypes are expressed in basal sebocytes. PPAR-,y is also present in differentiated sebocytes (39 41). In patients receiving fibrates (PPAR-oc agonists) for hyperlipidemia or thiazolidi-nediones (PPAR-,y agonists) for diabetes, sebum secretion rates are increased (41), indicating that PPARs do play a role in sebocyte differentiation and mat-uration in humans.
Melanocortins include melanocyte-stimulating hormone (MSH) and adreno-corticotropic hormone (ACTH). In rodents, melanocortins increase sebum pro-duction. Human primary sebocyte cultures treated with MSH have increased numbers of cytoplasmic lipid droplets (42). Both melanocortin 1 receptor (MC1R) and melanocortin 5 receptor (MC5R) are expressed within human sebaceous glands (43,44). MC1R expression is increased in sebaceous glands of both noninvolved and involved skin of acne patients when compared with nor-mal patients. MC5R expression is only detectable within differentiated sebo-cytes, both within human skin biopsies and cell culture models (45). It is currently unknown whether MC1R or MC5R mediates the effects of melano-cortins on sebaceous glands, although transgenic mice deficient in MC5R have hypoplastic sebaceous glands and reduced sebum production (46), suggesting that receptor involvement is highly likely.
Recent evidence suggests that physiological stress plays a role in sebaceous gland regulation. The expression of corticotropin-releasing hormone (CRH), its receptors (CRH-R1, CRH-R2), as well as CRH-binding protein (CRHBP) has been detected within human sebaceous glands by immunohistochemistry (47). Immortalized sebocyte cells (SZ95) also express CRH, CRHBP, and the receptors. Furthermore, treatment with CRH increased lipid synthesis within SZ95 sebocytes (48). The presence and functionality of this pathway in seba-ceous glands suggest that sebaceous glands and sebum production may be influenced by neuroendocrine mechanisms.
Fibroblast Growth Factor Receptors
Fibroblast growth factor receptors 1 and 2 (FGFR1 and 2) are expressed in the epidermis and skin appendages. Expression of FGFR3 and FGFR4 is localized to dermal vessels and microvessels and is notably absent in epidermis and appendages (49). FGFR2 plays an important role during embryogenesis in skin formation (50). Germline mutations in FGFR2 lead to Apert’s syndrome, which is commonly associated with acne. In addition, somatic mutations in the same location can lead to acne, but how this receptor is involved in sebaceous gland development and how its mutation leads to acne are unknown (51,52).
SEBUM IN THE PATHOGENESIS OF ACNE VULGARIS
The role of sebum in the pathogenesis of acne is closely associated with the activity of P. acnes. The microenvironment within the sebaceous gland is anaerobic, therefore favoring the survival of P. acnes bacteria over others (i.e., Staphylococcus epidermidis). The P. acnes bacterium relies on sebaceous lipids as a nutrient source and breaks down triglycerides into FFA (53). FFA within sebum can be irritating and contribute to the inflammatory response (54). Furthermore, experiments demonstrated that P. acnes is capable of stimulating the production of both proinflammatory cytokines/chemokines and antimicrobial peptides from keratinocytes and cultured sebocytes (SZ95), indicating that keratinocytes and sebocytes themselves may play a role in the inflammatory aspects of acne (55,56).