Ever hear of humanin? At the Paul K Glenn Symposium on Aging yesterday at Harvard, Dr. Pinchas Cohen gave a talk on The New World of Mitochondrial Proteins featuring humanin and other closely related proteins. The subject is important because it points to a new and important function of mitochondria – generating protective proteins. This blog post draws on material from that talk heavily augmented with materials from a number of published sources.
Humanin is a mitochondria-derived peptide. That is, humanin is expressed by mitochondrial genes in the 16s ribosomal RNA coding region. The evidence suggests that gene transcription takes place in the mitochondria but translation into protein form takes place in the cytoplasm. Humanin was independently cloned by three different groups in 2001. It is one of some seven health-related peptides encoded by mitochondrial genes.
Several things have been learned about humanin:
· Humanin is created by an evolutionary-conserved gene sequence and homolog versions of this sequence can be found in humans, mice, horses, nematodes, zebrafish and no doubt in many other species.
· Humanin suppresses apoptosis by interfering with Bax activation (ref). “HN prevents the translocation of Bax from cytosol to mitochondria. Conversely, reducing HN expression by small interfering RNAs sensitizes cells to Bax and increases Bax translocation to membranes. HN peptides also block Bax association with isolated mitochondria, and suppress cytochrome c release in vitro. Notably, the mitochondrial genome contains an identical open reading frame, and the mitochondrial version of HN can also bind and suppress Bax. We speculate therefore that HN arose from mitochondria and transferred to the nuclear genome, providing a mechanism for protecting these organelles from Bax(ref).” “The anti-apoptotic potential of HN appears to be dependent upon the formation of homodimers, as interfering with this process completely blocks its ability to suppress cell death . Once dimerized, HN directly interacts with a variety of pro-apoptotic proteins, including Bax-related proteins  and insulin-like growth factor binding protein-3 (IGFBP-3) (ref).”
· Tyrosine kinases and STAT3 in are involved in humanin-mediated neuroprotection(ref), particularly in the brain.
· Humanin suppresses hepaptic glucose production, its main site of action being the hypothalamus(ref).
· Humanin also works through STAT3 to exercise metabolic effects(ref).
· Expression of humanin declines with age. “Based upon the link of HN with two age-related diseases (AD and diabetes), we examined if there were age associated changes in HN levels. Indeed, the amount of detectable HN in hypothalamus, skeletal muscle, and cortex was decreased with age in rodents, and circulating levels of HN were decreased with age in humans and mice. — We conclude that the decline in HN with age could play a role in the pathogenesis of age-related diseases including AD and T2DM(ref).”
· Humanin appears in multiple tissue types “Endogenous HN is both an intracellular and secreted protein and has been detected in normal mouse testis and colon at specific stages of development . In addition to brain, colon and testis, we have shown the presence of HN by western blot in rodent heart, ovary, pancreas and kidney (unpublished data). In addition, our group has demonstrated the presence of HN in cerebral spinal fluid (CSF), seminal fluid and plasma, with levels in the biologically active range (Cohen & Hwang, unpublished data)(ref).”
· Finally, the mitochondrial genes that make humanin may also transmigrate out of mitochondria into cell nucleuses and be incorporated into chromosomal DNA. So, some of the humanin observed in tissues may result from ordinary protein-generating mechanisms.
Humanin, neural protection and Alzheimer’s disease
· Humanin was from the start recognized to be a powerful neuroprotective substance. Humanin blocks the process through which beta-amyloid causes neuronal death and was thought to be possibly useful for prevention or treatment of Alzheimer’s disease. As outlined in one of the initial 2001 papers Mechanisms of Neuroprotection by a Novel Rescue Factor Humanin from Swedish Mutant Amyloid Precursor Protein: “We report a novel gene, designated Humanin (HN) cDNA, that suppresses neuronal cell death by K595N/M596L-APP (NL-APP), a mutant causing familial Alzheimer’s disease (FAD), termed Swedish mutant. — Therefore, HN suppressed neuronal cell death by NL-APP not through inhibition of AÎ²42 secretion, but with two targets for its inhibitory action: (i) the intracellular toxic mechanism directly triggered by NL-APP and (ii) neurotoxicity by AÎ². HN will contribute to the development of curative therapy of AD, especially as a novel reagent that could mechanistically supplement AÎ²-production inhibitors.”
· Humanin rescues cortical neurons treated with beta-amaloid in a concentration-dependent manner(ref).
· The 2001 publication Detailed characterization of neuroprotection by a rescue factor humanin against various Alzheimer’s disease-relevant insults stated “A novel factor, termed Humanin (HN), antagonizes against neurotoxicity by various types of familial Alzheimer’s disease (AD) genes [V642I and K595N/M596L (NL) mutants of amyloid precursor protein (APP), M146L-presenilin (PS) 1, and N141I-PS2] and by Abeta1-43 with clear action specificity ineffective on neurotoxicity by polyglutamine repeat Q79 or superoxide dismutase 1 mutants. Here we report that HN can also inhibit neurotoxicity by other AD-relevant insults: other familial AD genes (A617G-APP, L648P-APP, A246E-PS1, L286V-PS1, C410Y-PS1, and H163R-PS1), APP stimulation by anti-APP antibody, and other Abeta peptides (Abeta1-42 and Abeta25-35).” However, it was not clear how it worked to do that.
· The 2008 publication A rescue factor for Alzheimer’s diseases: discovery, activity, structure, and mechanism states the practical case for humanin in AD. “While understanding the mechanism of AD and the involvement of key players should lead to rational drug discovery against this disease, a traditional screening approach should also work for identifying drugs using AD models. We have used a cellular AD model, in which a cell death was induced by AD-causing neurotoxicities, and then screened the genes, which rescued the cells from the cell death. This resulted in isolation of a gene encoding a novel 24-amino acid long peptide, termed Humanin (HN), which protected neuronal cells at approximately microM level. Surprisingly, these gene products and the synthetic peptides not only protected neurons from cell death induced by Abeta-related neurotoxicities, but also Abeta-unrelated neurotoxicities. While a broad range of activities of HN against AD-related insults is discovered, the detailed mechanism of its action is still obscure.” In other words, “we don’t know how it works, but it works.”
· A 2009 publication suggests a mechanism of action: Humanin inhibits neuronal cell death by interacting with a cytokine receptor complex or complexes involving CNTF receptor alpha/WSX-1/gp130. “Together, these results indicate that HN protects neurons by binding to a complex or complexes involving CNTFR/WSX-1/gp130.”
Humanin and diabetes
Humanin appears to have several salutary effects with respect to diabetes
· Humanin is an insulin secretalog. Humanin is A Novel Central Regulator of Peripheral Insulin Action. “Decline in insulin action is a metabolic feature of aging and is involved in the development of age-related diseases including Type 2 Diabetes Mellitus (T2DM) and Alzheimer’s disease (AD). A novel mitochondria-associated peptide, Humanin (HN), has a neuroprotective role against AD-related neurotoxicity. Considering the association between insulin resistance and AD, we investigated if HN influences insulin sensitivity. — Using state of the art clamp technology, we examined the role of central and peripheral HN on insulin action. — HN represents a novel link between T2DM and neurodegeneration and along with its analogues offers a potential therapeutic tool to improve insulin action and treat T2DM.”
· HNG-F6a, an analog of humanin, has a powerful capability to suppress blood sugar in Zucker rats(ref).
· In non-obese diabetic mice, humanin both prevents and treats Type 1 Diabetes. According to the 2009 paper The neurosurvival factor Humanin inhibits Î²-cell apoptosis via signal transducer and activator of transcription 3 activation and delays and ameliorates diabetes in nonobese diabetic mice “Humanin normalized glucose tolerance in NOD mice treated for 6 weeks, and their pancreata revealed decreased lymphocyte infiltration and severity. In addition, Humanin delayed/prevented the onset of diabetes in NOD mice treated for 20 weeks. In summary, Humanin treatment decreases cytokine-induced apoptosis in Î²-cells in vitro and improved glucose tolerance and onset of diabetes in NOD mice in vivo. This indicates that Humanin may be useful for islet protection and survival in a spectrum of diabetes-related therapeutics.”
Humanin and vascular processes
· The 2010 publication Humanin is Expressed in Human Vascular Walls and Has a Cytoprotective Effect against Oxidized LDL-Induced Oxidative Stress suggests other roles for humanin in vascular and possibly cardiovascular health and disease processes. “The current study demonstrates for the first time the expression of Humanin in the endothelial cell layer of human blood vessels. Exogenous addition of Humanin to endothelial cell cultures was shown to be effective against Ox-LDL-induced apoptosis. These findings suggest that Humanin may play a role and may have protective effect in early atherosclerosis in humans.”
Other SHLPs – SHLP6
There are six additional small humanin-like mitochondria-derived peptides (SHLPs) that Dr. Cohen talked about. Four of these are somewhat similar in their effects to Humanin but one, SHLP6 is very different and interesting in its effects. SHLP1 through SHLP5 line humanin are anti-apoptosis pro cell-survival substances, although they have varied neuro-protective and other effects. I will not discuss those here. SHLP6, however, is strongly pro-apoptosis, inhibits cancer cell growth in-vitro, inhibits VEGF, inhibits tumor growth and angiogenesis in vivo. Its levels are reduced in prostate cancer – pretty much the opposite profile of humanin and the first five SHLPs. One important difference is that levels of humanin and the first 5 SHLPs in the brain are reduced to half or less in the process of aging, but the level of SHLP6 may actually increase. In blood plasma, the level of humanin drops to less than half with age but the level of SHLP6 remains nearly the same. SHLP6 is also a highly conserved 20 amino-acid peptide. It could be that, evolutionary-speaking, there is an anti-cancer survival advantage to high levels of SHLP6 with age. It is effective in inducing apoptosis in prostate, breast and other cancer cell lines. Potentially, SHLP6 could provide a basis for new cancer prevention or treatment approaches. To sum it up, mitochondria-derived peptides, humanin and its cousins, appear to define a new and largely unexploited field in biology with particular implications related to two important diseases of the elderly – Alzheimer’s disease and diabetes. Humanin and some of its cousins seem to have similar health and longevity effects as SIRT1 and certain heat-shock proteins involved in hormesis responses(ref)(ref). As time goes on I plan to explore possible relationships between these pro-life proteins and report on them in this blog.Somebody reading this blog is bound to ask me “Where can I get humanin or humanin-promoting dietary substances or supplements?” I don’t know, except to suggest that keeping mitochondria healthy is probably the best way to keep humanin levels up with aging. So, please see the Mitochondrial Damage Firewall in my treatise.